Abstract:
A semiconductor device comprises a substrate extending in a horizontal direction and a vertical transistor on the substrate. The vertical transistor comprises: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the channel region; and a gate electrode at a sidewall of, and insulated from, the channel region. A horizontal transistor is positioned on the substrate, the horizontal transistor comprising: a first diffusion region and a second diffusion region on the substrate and spaced apart from each other; a channel region on the substrate between the first diffusion region and the second diffusion region; and a gate electrode on the channel region and isolated from the channel region. A portion of a gate electrode of the vertical transistor and a portion of the gate electrode of the horizontal transistor are at a same vertical position in the vertical direction relative to the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. non-provisional patent application is a divisional of U.S. patent application Ser. No. 13/412,760, filed Mar. 6, 2012, which claims benefit under 35 U.S.C. §119 to Korean Patent Application No. 10-2011-0058623 filed on Jun. 16, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments of the inventive concepts relate to semiconductor devices having a vertical transistor and a non-vertical transistor and methods of forming the same. 
     2. Description of Related Art 
     A vast amount of research has been conducted on various methods for embodying low-power semiconductor devices. With the growing tendency for MOSFETs to have channel lengths of about 100 nm or less, the fabrication of semiconductor devices having both a high driving current and a low off-leakage current has become increasingly difficult due to a phenomenon known in the industry as the short-channel effect. To overcome these limitations, fabrication techniques have been employed whereby devices having different threshold voltages are formed on the same semiconductor substrate by controlling the doping profile of a channel region. However, as the operating voltage of devices becomes about 1 V or lower, the leakage current of a low threshold voltage (V T ) device may greatly increase, leading to unreliable and inefficient operation. 
     SUMMARY 
     Embodiments of the inventive concepts provide semiconductor devices suitable for increasing integration density and reducing power consumption, and methods of forming the same. 
     Other embodiments of the inventive concepts provide a static random access memory (SRAM) cell, suitable for increased integration density and reduced power consumption. 
     Aspects of the inventive concepts are not limited by the above description, and other unmentioned aspects will be clearly understood by one of ordinary skill in the art from example embodiments described herein. 
     In one aspect, a semiconductor device comprises: a substrate extending in a horizontal direction; a vertical transistor on the substrate, the vertical transistor comprising: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the channel region; and a gate electrode at a sidewall of, and insulated from, the channel region; and a horizontal transistor on the substrate, the horizontal transistor comprising: a first diffusion region and a second diffusion region on the substrate and spaced apart from each other; a channel region on the substrate between the first diffusion region and the second diffusion region; and a gate electrode on the channel region and isolated from the channel region; wherein a portion of a gate electrode of the vertical transistor and a portion of the gate electrode of the horizontal transistor are at a same vertical position in the vertical direction relative to the substrate. 
     In one embodiment, the semiconductor device further comprises a layer of material on the horizontal transistor and the vertical transistor, the gate electrode of the vertical transistor and the gate electrode of the horizontal transistor both in direct contact with the layer of material. 
     In one embodiment, the layer of material comprises an etch stop layer. 
     In one embodiment, the layer of material comprises an insulating layer. 
     In one embodiment, the gate electrode of the vertical transistor and the gate electrode of the horizontal transistor comprise portions of a same layer of material. 
     In one embodiment, the first diffusion region of the horizontal transistor is contiguous with the first diffusion region of the vertical transistor. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that is higher in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that is lower in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that has a same vertical position as a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the vertical transistor comprises a drain of the vertical transistor; the second diffusion region of the vertical transistor comprises a source of the vertical transistor; the first diffusion region of the horizontal transistor comprises one of a drain and source of the horizontal transistor; the second diffusion region of the horizontal transistor comprises the other of the drain and source of the horizontal transistor. 
     In one embodiment, the first diffusion region of the vertical transistor and the first diffusion region and second diffusion region of the horizontal transistor lie at a same vertical position relative to the substrate. 
     In one embodiment, the first diffusion region of the vertical transistor includes a vertical protrusion extending in the vertical direction, and wherein the vertical channel region is on the vertical protrusion. 
     In one embodiment, the vertical transistor further comprises a silicide region on the second diffusion region. 
     In one embodiment, the vertical transistor further comprises a metal pattern on the silicide region. 
     In one embodiment, the second diffusion region of the vertical transistor comprises a silicide region in direct contact with the vertical channel region of the vertical transistor. 
     In one embodiment, the first diffusion region of the horizontal transistor and the first diffusion region of the vertical transistors both have a silicide region thereon. 
     In one embodiment, the semiconductor device further comprises an insulating spacer on sidewalls of the gate electrode of the vertical transistor and on sidewalls of the gate electrode of the horizontal transistor. 
     In one embodiment, the semiconductor device further comprises a silicide region on the gate electrode of the vertical transistor and on the gate electrode of the horizontal transistor. 
     In one embodiment, the second diffusion region of the vertical transistor has a width in the horizontal direction that is greater than a width of the channel region of the vertical transistor in the horizontal direction. 
     In one embodiment, the gate electrode of the horizontal transistor has a bottom that is at a position that is lower than a lower boundary of the first and second diffusion regions of the horizontal transistor. 
     In one embodiment, the semiconductor device further comprises an interlayer via in direct contact with a top of the second diffusion region of the vertical transistor. 
     In one embodiment, the semiconductor device further comprises a buried oxide layer on the substrate and wherein the vertical transistor and the horizontal transistor are on the buried oxide layer. 
     In one embodiment, the channel region of the vertical transistor comprises single-crystal material. 
     In one embodiment, the vertical transistor comprises a first vertical transistor, and further comprising: a second vertical transistor on the substrate, the second vertical transistor comprising: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the first vertical channel region; and a gate electrode at a sidewall of, and insulated from, the vertical channel region. 
     In one embodiment, the first vertical transistor and second vertical transistor comprise an inverter pair. 
     In one embodiment, the first vertical transistor comprises one of a p-channel and re-channel transistor and wherein the second vertical transistor comprise the other of a p-channel and n-channel transistor. 
     In one embodiment, the substrate comprises one of a bulk substrate and a silicon-on-insulator (SOI) substrate. 
     In another aspect, a semiconductor device comprises: a substrate extending in a horizontal direction; a vertical transistor on the substrate, the vertical transistor comprising: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the channel region; and a gate electrode at a sidewall of, and insulated from, the channel region; a horizontal transistor on the substrate, the horizontal transistor comprising: a first diffusion region and a second diffusion region on the substrate and spaced apart from each other; a channel region on the substrate between the first diffusion region and the second diffusion region; and a gate electrode on the channel region and isolated from the channel region; and a layer of material on the horizontal transistor and the vertical transistor, the gate electrode of the vertical transistor and the gate electrode of the horizontal transistor both in direct contact with the layer of material. 
     In one embodiment, the layer of material comprises an etch stop layer 
     In one embodiment, the layer of material comprises an insulating layer. 
     In one embodiment, a portion of a gate electrode of the vertical transistor and a portion of the gate electrode of the horizontal transistor are at a same vertical position in the vertical direction relative to the substrate. 
     In one embodiment, the gate electrode of the vertical transistor and the gate electrode of the horizontal transistor comprise portions of a same layer of material. 
     In one embodiment, the first diffusion region of the horizontal transistor is contiguous with the first diffusion region of the vertical transistor. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that is higher in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that is lower in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that has a same vertical position as a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the vertical transistor comprises a drain of the vertical transistor; the second diffusion region of the vertical transistor comprises a source of the vertical transistor; the first diffusion region of the horizontal transistor comprises one of a drain and source of the horizontal transistor; the second diffusion region of the horizontal transistor comprises the other of the drain and source of the horizontal transistor. 
     In one embodiment, the first diffusion region of the vertical transistor and the first diffusion region and second diffusion region of the horizontal transistor lie at a same vertical position relative to the substrate. 
     In one embodiment, the first diffusion region of the vertical transistor includes a vertical protrusion extending in the vertical direction, and wherein the vertical channel region is on the vertical protrusion. 
     In one embodiment, the vertical transistor further comprises a silicide region on the second diffusion region. 
     In one embodiment, the vertical transistor further comprises a metal pattern on the silicide region. 
     In one embodiment, the second diffusion region of the vertical transistor comprises a silicide region in direct contact with the vertical channel region of the vertical transistor. 
     In one embodiment, the first diffusion region of the horizontal transistor and the first diffusion region of the vertical transistors both have a silicide region thereon. 
     In one embodiment, the semiconductor device further comprises an insulating spacer on sidewalls of the gate electrode of the vertical transistor and on sidewalls of the gate electrode of the horizontal transistor. 
     In one embodiment, the semiconductor device further comprises a silicide region on the gate electrode of the vertical transistor and on the gate electrode of the horizontal transistor. 
     In one embodiment, the second diffusion region of the vertical transistor has a width in the horizontal direction that is greater than a width of the channel region of the vertical transistor in the horizontal direction. 
     In one embodiment, the gate electrode of the horizontal transistor has a bottom that is at a position that is lower than a lower boundary of the first and second diffusion regions of the horizontal transistor. 
     In one embodiment, the semiconductor device further comprises an interlayer via in direct contact with a top of the second diffusion region of the vertical transistor. 
     In one embodiment, the semiconductor device further comprises a buried oxide layer on the substrate and wherein the vertical transistor and the horizontal transistor are on the buried oxide layer. 
     In one embodiment, the channel region of the vertical transistor comprises single-crystal material. 
     In one embodiment, the vertical transistor comprises a first vertical transistor, and further comprising: a second vertical transistor on the substrate, the second vertical transistor comprising: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the first vertical channel region; and a gate electrode at a sidewall of, and insulated from, the vertical channel region. 
     In one embodiment, the first vertical transistor and second vertical transistor comprise an inverter pair. 
     In one embodiment, the first vertical transistor comprises one of a p-channel and re-channel transistor and wherein the second vertical transistor comprise the other of a p-channel and n-channel transistor. 
     In one embodiment, the substrate comprises one of a bulk substrate and a silicon-on-insulator (SOI) substrate. 
     In another aspect, a semiconductor device comprises: a substrate extending in a horizontal direction; a vertical transistor on the substrate, the vertical transistor comprising: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the channel region; and a gate electrode at a sidewall of, and insulated from, the channel region; and a horizontal transistor on the substrate, the horizontal transistor comprising: a first diffusion region and a second diffusion region on the substrate and spaced apart from each other; a channel region on the substrate between the first diffusion region and the second diffusion region; and a gate electrode on the channel region and isolated from the channel region, wherein the gate electrode of the vertical transistor and the gate electrode of the horizontal transistor comprise portions of a same layer of material. 
     In one embodiment, a portion of a gate electrode of the vertical transistor and a portion of the gate electrode of the horizontal transistor are at a same vertical position in the vertical direction relative to the substrate. 
     In one embodiment, the semiconductor device further comprises a layer of material on the horizontal transistor and the vertical transistor, the gate electrode of the vertical transistor and the gate electrode of the horizontal transistor both in direct contact with the layer of material. 
     In one embodiment, the layer of material comprises an etch stop layer 
     In one embodiment, the layer of material comprises an insulating layer 
     In one embodiment, the first diffusion region of the horizontal transistor is contiguous with the first diffusion region of the vertical transistor. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that is higher in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that is lower in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the horizontal transistor that is contiguous with the first diffusion region of the vertical transistor has a lower boundary that has a same vertical position as a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of the vertical transistor comprises a drain of the vertical transistor; the second diffusion region of the vertical transistor comprises a source of the vertical transistor; the first diffusion region of the horizontal transistor comprises one of a drain and source of the horizontal transistor; the second diffusion region of the horizontal transistor comprises the other of the drain and source of the horizontal transistor. 
     In one embodiment, the first diffusion region of the vertical transistor and the first diffusion region and second diffusion region of the horizontal transistor lie at a same vertical position relative to the substrate. 
     In one embodiment, the first diffusion region of the vertical transistor includes a vertical protrusion extending in the vertical direction, and wherein the vertical channel region is on the vertical protrusion. 
     In one embodiment, the vertical transistor further comprises a silicide region on the second diffusion region. 
     In one embodiment, the vertical transistor further comprises a metal pattern on the silicide region. 
     In one embodiment, the second diffusion region of the vertical transistor comprises a silicide region in direct contact with the vertical channel region of the vertical transistor. 
     In one embodiment, first diffusion region of the horizontal transistor and the first diffusion region of the vertical transistors both have a silicide region thereon. 
     In one embodiment, the semiconductor device further comprises an insulating spacer on sidewalls of the gate electrode of the vertical transistor and on sidewalls of the gate electrode of the horizontal transistor. 
     In one embodiment, the semiconductor device further comprises a silicide region on the gate electrode of the vertical transistor and on the gate electrode of the horizontal transistor. 
     In one embodiment, the second diffusion region of the vertical transistor has a width in the horizontal direction that is greater than a width of the channel region of the vertical transistor in the horizontal direction. 
     In one embodiment, the gate electrode of the horizontal transistor has a bottom that is at a position that is lower than a lower boundary of the first and second diffusion regions of the horizontal transistor. 
     In one embodiment, the semiconductor device further comprises an interlayer via in direct contact with a top of the second diffusion region of the vertical transistor. 
     In one embodiment, the semiconductor device further comprises a buried oxide layer on the substrate and wherein the vertical transistor and the horizontal transistor are on the buried oxide layer. 
     In one embodiment, the channel region of the vertical transistor comprises single-crystal material. 
     In one embodiment, the vertical transistor comprises a first vertical transistor, and further comprising: a second vertical transistor on the substrate, the second vertical transistor comprising: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the first vertical channel region; and a gate electrode at a sidewall of, and insulated from, the vertical channel region. 
     In one embodiment, the first vertical transistor and second vertical transistor comprise an inverter pair. 
     In one embodiment, the first vertical transistor comprises one of a p-channel and re-channel transistor and wherein the second vertical transistor comprise the other of a p-channel and n-channel transistor. 
     In one embodiment, the substrate comprises one of a bulk substrate and a silicon-on-insulator (SOI) substrate. 
     In another aspect, a memory cell of a memory device comprises: a first pull-up transistor and a first pull-down transistor coupled at a first node and connected in series between a first voltage source and a second voltage source, gates of the first pull-up transistor and the first pull-down transistor coupled at a second node; a first access transistor coupled between the first node and a first bit line of the memory device, a gate of the first access transistor coupled to a word line of the memory device; a second pull-up transistor and a second pull-down transistor coupled at the second node and connected in series between the first voltage source and the second voltage source, gates of the second pull-up transistor and the second pull-down transistor coupled to the first node; a second access transistor coupled between the second node and a second bit line of the memory device, a gate of the second access transistor coupled to the word line of the memory device; wherein the first pull-up transistor, the first pull-down transistor, the second pull-up transistor and the second pull-down transistor each comprise vertical channel transistors having channel regions that extend in a vertical direction relative to a substrate of the memory device, and each comprise gate electrodes at sidewalls of the vertically extending channel regions; wherein the first access transistor and the second access transistor each comprise horizontal channel transistors having channel regions that extend in a horizontal direction of the substrate, and each comprise gate electrodes on the channel regions; and wherein the gate electrodes of the first pull-up transistor, the first pull-down transistor, the second pull-up transistor and the second pull-down transistor and the gate electrodes of the first access transistor and the second access transistor comprise portions of a same layer of material. 
     In one embodiment, the vertical channel transistors each comprise: a first diffusion region on the substrate; the channel region on the first diffusion region and extending in the vertical direction relative to the horizontal direction of extension of the substrate; a second diffusion region on the channel region; and the gate electrode at a sidewall of, and insulated from, the channel region; and wherein the horizontal channel transistors each comprise: a first diffusion region and a second diffusion region on the substrate and spaced apart from each other; the channel region on the substrate between the first diffusion region and the second diffusion region; and the gate electrode on the channel region and isolated from the channel region. 
     In one embodiment, the first diffusion region of each of the horizontal channel transistors is contiguous with the first diffusion region of one of the vertical channel transistors. 
     In one embodiment, the first diffusion region of each of the horizontal transistors that is contiguous with the first diffusion region of one of the vertical transistors has a lower boundary that is higher in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of each of the horizontal transistors that is contiguous with the first diffusion region of one of the vertical transistors has a lower boundary that is lower in vertical position than a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of each of the horizontal transistors that is contiguous with the first diffusion region of one of the vertical transistors has a lower boundary that has a same vertical position as a lower boundary of the first diffusion region of the vertical transistor, relative to an upper surface of the substrate. 
     In one embodiment, the first diffusion region of each vertical transistor comprises a drain of the vertical transistor; the second diffusion region of each vertical transistor comprises a source of the vertical transistor; the first diffusion region of each horizontal transistor comprises one of a drain and source of the horizontal transistor; the second diffusion region of each horizontal transistor comprises the other of the drain and source of the horizontal transistor. 
     In one embodiment, the first diffusion region of the vertical transistors and the first diffusion region and second diffusion regions of the horizontal transistors lie at a same vertical position relative to the substrate. 
     In one embodiment, the first diffusion regions of the vertical transistors each includes a vertical protrusion extending in the vertical direction, and wherein the vertical channel region is on the vertical protrusion. 
     In one embodiment, the vertical transistors each further comprise a silicide region on the second diffusion region. 
     In one embodiment, the vertical transistors each further comprise a metal pattern on the silicide region. 
     In one embodiment, the second diffusion region of each vertical transistor comprises a silicide region in direct contact with the vertical channel region of the vertical transistor. 
     In one embodiment, the first diffusion region of the horizontal transistors and the first diffusion region of the vertical transistors both have a silicide region thereon. 
     In one embodiment, the second diffusion region of the vertical transistors has a width in the horizontal direction that is greater than a width of the channel region of the vertical transistors in the horizontal direction. 
     In one embodiment, the gate electrodes of the horizontal transistors have a bottom that is at a position that is lower than a lower boundary of the first and second diffusion regions of the horizontal transistors. 
     In one embodiment, the memory cell further comprises an interlayer via in direct contact with a top of the second diffusion region of the vertical transistors. 
     In one embodiment, a portion of the gate electrodes of the first pull-up transistor, the first pull-down transistor, the second pull-up transistor and the second pull-down transistor and a portion of the gate electrodes of the first access transistor and the second access transistor are at a same vertical position in the vertical direction relative to the substrate. 
     In one embodiment, the memory cell further comprises a layer of material on the horizontal transistor and the vertical transistor, the gate electrodes of the first pull-up transistor, the first pull-down transistor, the second pull-up transistor and the second pull-down transistor and the gate electrodes of the first access transistor and the second access transistor both in direct contact with the layer of material. 
     In one embodiment, the layer of material comprises an etch stop layer. 
     In one embodiment, the layer of material comprises an insulating layer 
     In one embodiment, the memory cell further comprises a buried oxide layer on the substrate and wherein the vertical transistor and the horizontal transistor are on the buried oxide layer. 
     In one embodiment, the channel region of the vertical transistor comprises single-crystal material. 
     In one embodiment, the substrate comprises one of a bulk substrate and a silicon-on-insulator (SOI) substrate. 
     In another aspect, a method of forming a semiconductor device comprising: forming a first diffusion region on a substrate; forming a channel region for a vertical transistor on the first diffusion region that extends in a vertical direction relative to the substrate; and providing a vertical transistor gate electrode at sidewalls of the vertical transistor channel region and simultaneously providing a horizontal transistor gate electrode on the substrate at a position that is spaced apart from the vertical transistor. 
     In one embodiment, forming the channel region for the vertical transistor comprises: forming a first well in the substrate; forming the first diffusion region in a portion of the first well by doping the first diffusion region with a doping element of a first polarity; epitaxially growing a first channel layer on the first diffusion region; doping an upper portion of the first channel layer with a doping element of a second polarity; patterning the first channel layer to form the channel region for the vertical transistor, the channel region extending between the first diffusion region and a second diffusion region comprising the patterned upper portion of the first channel layer. 
     In one embodiment, providing a vertical transistor gate electrode at sidewalls of the vertical transistor channel region and simultaneously providing a horizontal transistor gate electrode on the substrate at a position that is spaced apart from the vertical transistor comprises: providing a gate insulating layer on the channel region of the vertical transistor and on the first well; providing a gate electrode layer on the gate insulating layer; patterning the gate electrode layer to form the vertical transistor gate electrode and to form the horizontal transistor gate electrode on a portion of the first well spaced apart from the first diffusion region 
     In one embodiment, the method further comprises forming a third diffusion region and a fourth diffusion region for a horizontal transistor in the substrate at sidewalls of the horizontal transistor gate electrode. 
     In one embodiment, the fourth diffusion region of the horizontal transistor is contiguous with the first diffusion region of the vertical transistor. 
     In one embodiment, providing the vertical transistor gate electrode and simultaneously providing a horizontal gate electrode comprises: providing a gate insulating layer on sidewalls of the vertical transistor channel region and on the substrate; providing a gate electrode layer to cover the gate insulating layer; patterning the gate electrode layer to form the vertical transistor gate electrode and simultaneously form the horizontal gate electrode. 
     In one embodiment, the method further comprises: forming a second diffusion region on the vertical transistor channel region; fainting a third diffusion region in the substrate at a side of the horizontal gate electrode opposite the vertical transistor channel region; forming a fourth diffusion region in the substrate at a side of the horizontal gate electrode opposite the third diffusion region, wherein the fourth diffusion region and the first diffusion region are contiguous with each other. 
     In one embodiment, the method further comprises forming a layer of material on and in direct contact with the gate electrode of the vertical transistor and the gate electrode of the horizontal transistor. 
     In another aspect, a method of forming a semiconductor device comprises: epitaxially forming an epitaxial layer of material on a substrate including a first region of amorphous material and a second region of single-crystal material; and etching the epitaxial layer of material to form a channel region for a vertical transistor on the second region, the channel region extending in a vertical direction relative to the substrate. 
     In one embodiment, the first region of amorphous material comprises an insulating structure present in the substrate. 
     In one embodiment, the method further comprises: forming a first diffusion region on the substrate at a position that is below the channel region of the vertical transistor, prior to formation of the channel region of the vertical transistor; forming a second diffusion region on the channel region of the vertical transistor. 
     In one embodiment, the method further comprises: providing a vertical transistor gate electrode at sidewalls of the vertical transistor channel region and simultaneously providing a horizontal transistor gate electrode on the substrate at a position that is spaced apart from the vertical transistor. 
     In another aspect, a memory system comprises: a memory controller that generates command and address signals; and a memory module comprising a plurality of memory devices, the memory module receiving the command and address signals and in response storing and retrieving data to and from at least one of the memory devices, wherein each memory device comprises: a substrate extending in a horizontal direction; a vertical transistor on the substrate, the vertical transistor comprising: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the channel region; and a gate electrode at a sidewall of, and insulated from, the channel region; and a horizontal transistor on the substrate, the horizontal transistor comprising: a first diffusion region and a second diffusion region on the substrate and spaced apart from each other; a channel region on the substrate between the first diffusion region and the second diffusion region; and a gate electrode on the channel region and isolated from the channel region; wherein a portion of a gate electrode of the vertical transistor and a portion of the gate electrode of the horizontal transistor are at a same vertical position in the vertical direction relative to the substrate. 
     In accordance with an aspect of the inventive concepts, a semiconductor device includes a first vertical transistor and a non-vertical transistor disposed on a substrate. The first vertical transistor includes a first drain region disposed on the substrate, a first vertical channel region protruding from the first drain region, a first source region disposed on the first vertical channel region, and a first gate electrode covering sidewalls of the first vertical channel region. The non-vertical transistor includes a channel region disposed on the substrate, a second gate electrode disposed on the channel region, and a non-vertical drain region and a non-vertical source region disposed adjacent to both sides of the second gate electrode. The first drain region, the non-vertical drain region, and the non-vertical source region are disposed at the same level. One of the non-vertical drain region and the non-vertical source region is in continuity with the first drain region. 
     In one embodiment, the first drain region, the channel region, the non-vertical drain region, and the non-vertical source region may include a single-crystalline semiconductor. 
     In one embodiment, the first vertical channel region may have a fin structure, a pillar structure, or a wire structure. 
     In one embodiment, the first drain region may include a protrusion, which may be aligned with the first vertical channel region. The first vertical channel region may have a horizontal width smaller than a vertical height. 
     In one embodiment, the first vertical channel region may have a first horizontal width, the first source region may have a second horizontal width, and the first horizontal width may be smaller than the second horizontal width. 
     In one embodiment, the first source region may include a metal silicide pattern. The metal silicide pattern may be in contact with the first vertical channel region. 
     In one embodiment, the non-vertical transistor may include a planar transistor or a recess channel transistor. A bottom of the second gate electrode may be at a lower level than the non-vertical drain region and the non-vertical source region. A top of the second gate electrode may be at a lower level than top surfaces of the non-vertical drain region and the non-vertical source region. 
     In one embodiment, the first and second gate electrodes may include the same material layers formed at the same time. 
     In one embodiment, the semiconductor device may further include an isolation layer disposed adjacent to the first vertical transistor and the non-vertical transistor. Top surfaces of the first drain region, the non-vertical drain region, and the non-vertical source region may be at a lower level than a top surface of the isolation layer. 
     In one embodiment, the semiconductor device may further include a first gate dielectric layer interposed between the first vertical channel region and the first gate electrode and a second gate dielectric layer interposed between the channel region and the second gate electrode. The first and second gate dielectric layers may include the same material layers formed at the same time. 
     In one embodiment, the semiconductor device may further include a second vertical transistor disposed on the substrate. The second vertical transistor may include a second drain region disposed on the substrate, a second vertical channel region protruding from the second drain region, a second source region disposed on the second vertical channel region, and a third gate electrode covering sidewalls of the second vertical channel region. The second drain region is connected to the first drain region. The second vertical channel region may have a different conductivity type from the first vertical channel region. 
     In accordance with another aspect of the inventive concept, a semiconductor device includes a buried oxide layer disposed on a substrate. A first vertical transistor, a non-vertical transistor, and a second vertical transistor are disposed on the buried oxide layer. The first vertical transistor includes an n-drain region disposed on the buried oxide layer, a p-vertical channel region disposed on the n-drain region, an n-source region disposed on the p-vertical channel region, and a first gate electrode covering sidewalls of the p-vertical channel region. The non-vertical transistor includes a channel region disposed on the buried oxide layer, a second gate electrode disposed on the channel region, and a non-vertical drain region and a non-vertical source region disposed adjacent to both sides of the second gate electrode. The second vertical transistor includes a p-drain region disposed on the buried oxide layer, an n-vertical channel region disposed on the p-drain region, a p-source region disposed on the n-vertical channel region, and a third gate electrode covering sidewalls of the n-vertical channel region. The n-drain region, the p-drain region, the non-vertical drain region, and the non-vertical source region are disposed at the same level. One of the non-vertical drain region and the non-vertical source region is in continuity with the n-drain region. The p-drain region is in contact with at least one of the n-drain region, the non-vertical drain region, and the non-vertical source region. 
     In one embodiment, each of the p-vertical channel region and the n-vertical channel region may have a fin structure, a pillar structure, or a wire structure. 
     In one embodiment, the n-drain region may include a first protrusion, which may be aligned with the p-vertical channel region. The p-drain region may include a second protrusion, which may be aligned with the n-vertical channel region. 
     In one embodiment, the n-source region may include a first metal silicide pattern, and the p-source region may include a second metal silicide pattern. The first metal silicide pattern may be in contact with the p-vertical channel region, and the second metal silicide pattern may be in contact with the n-vertical channel region. 
     In one embodiment, the semiconductor device may further include a first gate dielectric layer interposed between the p-vertical channel region and the first gate electrode, a second gate dielectric layer interposed between the channel region and the second gate electrode, and a third gate dielectric layer interposed between the n-vertical channel region and the third gate electrode. The first, second, and third gate dielectric layers may include the same material layers formed at the same time. 
     In accordance with another aspect of the inventive concept, a static random access memory (SRAM) cell includes first and second pull-up transistors disposed on a substrate, A first pull-down transistor is connected to the first pull-up transistor, and a second pull-down transistor is connected to the second pull-up transistor. A first access transistor is connected to a first bit line disposed on the substrate, and a second access transistor is connected to a second bit line disposed on the substrate. The first access transistor is connected between the first pull-up transistor and the first pull-down transistor, and the second access transistor is connected between the second pull-up transistor and the second pull-down transistor. Herein, the first pull-down transistor is a first vertical transistor, and the first access transistor is a non-vertical transistor. The first vertical transistor includes an n-drain region, a p-vertical channel region, an n-source region, and a first gate electrode disposed on the substrate. The non-vertical transistor includes a channel region, a second gate electrode, a non-vertical drain region, and a non-vertical source region disposed on the substrate. The n-drain region, the non-vertical drain region, and the non-vertical source region are disposed at the same level. One of the non-vertical drain region and the non-vertical source region is in continuity with the n-drain region. 
     In one embodiment, the first pull-up transistor may be a second vertical transistor. The second vertical transistor includes a p-drain region disposed on the substrate, an n-vertical channel region protruding from the p-drain region, a p-source region disposed on the n-vertical channel region, and a third gate electrode covering sidewalls of the n-vertical channel region. The p-drain region may be connected to the n-drain region. 
     In accordance with another aspect of the inventive concept, an SRAM includes a buried oxide layer disposed on a substrate. First and second pull-up transistors are disposed on the buried oxide layer. A first pull-down transistor is connected to the first pull-up transistor, and a second pull-down transistor is connected to the second pull-up transistor. A first access transistor is connected to a first bit line disposed on the substrate, and a second access transistor is connected to a second bit line disposed on the substrate. Herein, the first access transistor is connected between the first pull-up transistor and the first pull-down transistor, and the second access transistor is connected between the second pull-up transistor and the second pull-down transistor. The first pull-down transistor is a first vertical transistor, the first access transistor is a non-vertical transistor, and the first pull-up transistor is a second vertical transistor. The first vertical transistor includes an n-drain region, a p-vertical channel region, an n-source region, and a first gate electrode disposed on the buried oxide layer. The non-vertical transistor includes a channel region, a second gate electrode, a non-vertical drain region, and a non-vertical source region disposed on the buried oxide layer. The second vertical transistor includes a p-drain region, an n-vertical channel region, a p-source region, and a third gate electrode disposed on the buried oxide layer. The n-drain region, the p-drain region, the non-vertical drain region, and the non-vertical source region are disposed at the same level. One of the non-vertical drain region and the non-vertical source region is in continuity with the n-drain region, and the p-drain region is in contact with at least one of the n-drain region, the non-vertical drain region, and the non-vertical source region. 
     In accordance with another aspect of the inventive concept, a method of forming a semiconductor device includes forming a first vertical transistor on a substrate. The first vertical transistor includes a first drain region disposed on a substrate, a first vertical channel region protruding from the first drain region, a first source region disposed on the first vertical channel region, and a first gate electrode covering sidewalls of the first vertical channel region. A non-vertical transistor is formed on the substrate. The non-vertical transistor includes a channel region disposed on the substrate, a second gate electrode disposed on the channel region, and a non-vertical drain region and a non-vertical source region disposed adjacent to both sides of the second gate electrode. The formation of the first vertical transistor and the non-vertical transistor includes forming a semiconductor layer on the substrate using an epitaxial growth technique and forming the first vertical channel region and the channel region by patterning the semiconductor layer and the substrate. One of the non-vertical drain region and the non-vertical source region is in continuity with the first drain region. 
     In one embodiment, the first drain region, the non-vertical drain region, and the non-vertical source region may be formed at the same level. 
     In one embodiment, the method may further include forming an isolation layer adjacent to the first vertical transistor and the non-vertical transistor. Top surfaces of the first drain region, the non-vertical drain region, and the non-vertical source region may be formed at a lower level than a top surface of the isolation layer. 
     In one embodiment, the first drain region may include a protrusion, which may be aligned with the first vertical channel region. 
     In one embodiment, the first vertical channel region may have a fin structure, a pillar structure, or a wire structure. 
     In one embodiment, the method may further include forming a first gate dielectric layer between the first vertical channel region and the first gate electrode and forming a second gate dielectric layer between the channel region and the second gate electrode. The first and second gate dielectric layers may include the same material layers formed at the same time. 
     In one embodiment, the method may further include forming a second vertical transistor on the substrate. The second vertical transistor may include a second drain region disposed on the substrate, a second vertical channel region protruding from the second drain region, a second source region disposed on the second vertical channel region, and a third gate electrode covering sidewalls of the second vertical channel region. The second vertical channel region may have a different conductivity type from the first vertical channel region, and the second drain region may be connected to the first drain region. 
     In accordance with another aspect of the inventive concept, a method of forming a semiconductor device includes forming a buried oxide layer on a substrate. A first vertical transistor is formed on the buried oxide layer. The first vertical transistor includes an n-drain region disposed on the buried oxide layer, a p-vertical channel region disposed on the n-drain region, an n-source region disposed on the p-vertical channel region, and a first gate electrode covering sidewalls of the p-vertical channel region. A non-vertical transistor is formed on the buried oxide layer. The non-vertical transistor includes a channel region disposed on the buried oxide layer, a second gate electrode disposed on the channel region, and a non-vertical drain region and a non-vertical source region disposed adjacent to both sides of the second gate electrode. The second vertical transistor is formed on the buried oxide layer. The second vertical transistor includes a p-drain region disposed on the buried oxide layer, an n-vertical channel region disposed on the p-drain region, a p-source region disposed on the n-vertical channel region, and a third gate electrode covering sidewalls of the n-vertical channel region. The formation of the first vertical transistor, the non-vertical transistor, and the second vertical transistor includes forming a semiconductor layer on the substrate using an epitaxial growth technique and forming the p-vertical channel region, the channel region, and the n-vertical channel region by patterning the semiconductor layer and the substrate. One of the non-vertical drain region and the non-vertical source region is in continuity with the n-drain region. The p-drain region is in contact with at least one of the n-drain region, the non-vertical drain region, and the non-vertical source region. 
     In one embodiment, the n-drain region, the p-drain region, the non-vertical drain region, and the non-vertical source region may be formed at the same level. 
     In one embodiment, the method may further include forming an isolation layer on the buried oxide layer to define the n-drain region, the p-drain region, the channel region, the non-vertical drain region, and the non-vertical source region. Top surfaces of the n-drain region, the p-drain region, the channel region, the non-vertical drain region, and the non-vertical source region may be formed at a lower level than a top surface of the isolation layer. 
     In one embodiment, the n-drain region may include a first protrusion, which may be aligned with the p-vertical channel region, and the p-drain region may include a second protrusion, which may be aligned with the n-vertical channel region. 
     In one embodiment, the method may further include forming a first gate dielectric layer between the p-vertical channel region and the first gate electrode, forming a second gate dielectric layer between the channel region and the second gate electrode, and forming a third gate dielectric layer between the n-vertical channel region and the third gate electrode. The first, second, and third gate dielectric layers may include the same material layers formed at the same time. 
     Details of other embodiments are included in the detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages of the inventive concepts will be apparent from the more particular description of preferred embodiments of the inventive concepts, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the inventive concepts. In the drawings: 
         FIG. 1  is an equivalent circuit diagram of an electronic circuit including a complementary-metal-oxide-semiconductor (CMOS) inverter according to embodiments of the inventive concept; 
         FIG. 2  is a layout illustrating a semiconductor device according to a first embodiment of the inventive concept; 
         FIGS. 3A through 3H  are cross-sectional views taken along line I-I′ of  FIG. 2 , illustrating the semiconductor device of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a semiconductor device according to a second embodiment of the inventive concept; 
         FIG. 5  is a cross-sectional view of a semiconductor device according to a third embodiment of the inventive concept; 
         FIG. 6  is a layout illustrating a semiconductor device according to a fourth embodiment of the inventive concept; 
         FIGS. 7A and 7B  are cross-sectional views of the semiconductor device of  FIG. 6 ; 
         FIG. 8  is a layout illustrating a semiconductor device according to a fifth embodiment of the inventive concept; 
         FIGS. 9A through 9C  are cross-sectional views of the semiconductor device of  FIG. 8 ; 
         FIG. 10  is a layout illustrating a semiconductor device according to a sixth embodiment of the inventive concept; 
         FIGS. 11A through 12D  are cross-sectional views of the semiconductor device of  FIG. 10 ; 
         FIGS. 13 through 24  are cross-sectional views illustrating a method of forming a semiconductor device according to a seventh embodiment of the inventive concept; 
         FIGS. 25 through 31  are cross-sectional views illustrating a method of forming a semiconductor device according to an eighth embodiment of the inventive concept; 
         FIGS. 32 through 39  are cross-sectional views illustrating a method of forming a semiconductor device according to a ninth embodiment of the inventive concept; 
         FIGS. 40A through 43C  are cross-sectional views illustrating a method of forming a semiconductor device according to a tenth embodiment of the inventive concept; 
         FIGS. 44A and 44B  are current-voltage (IV) graphs showing drain current characteristics of Experimental Examples according to the inventive concept; 
         FIG. 45  is an equivalent circuit diagram of a CMOS static random access memory (SRAM) cell according to an eleventh embodiment of the inventive concept; and 
         FIGS. 46 and 47  are a perspective view and block diagram, respectively, of an electronic system according to a twelfth embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Various example embodiments will now be described more fully with reference to the accompanying drawings in which some example embodiments are shown. The inventive concepts may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete and fully conveys the scope of the inventive concepts to one skilled in the art. In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate or intervening layers may also be present. Like numbers refer to like elements throughout. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “top end,” “bottom end,” “top surface,” “bottom surface,” “above,” “below” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative tennis are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present inventive concepts belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Embodiment 1 
     In an ultrathin body (UTB) SOI device or in a nanowire device, which are expected to be applied in the future to sub-20 nm devices, since the dopant of a channel region has little effect on the threshold voltage V T  of the resulting device, such devices still do not solve the problem of heightened leakage current. Further, the approach of controlling the threshold voltages of devices by varying channel length is limited in viability since threshold voltage can be controlled only within a limited range and such variation in channel length is unsatisfactory in terms of integration density. 
     To obtain a low-power, high-speed circuit, the present inventive concepts provide semiconductor devices and methods of fabrication embodying multiple-threshold-voltage V T  structures which have relative low leakage current characteristics. 
       FIG. 1  is an equivalent circuit diagram of an electronic circuit including a complementary-metal-oxide-semiconductor (CMOS) inverter according to embodiments of the inventive concept.  FIG. 2  is a layout illustrating a semiconductor device according to a first embodiment of the inventive concept.  FIGS. 3A through 3H  are cross-sectional views taken along line I-I′ of  FIG. 2 , illustrating the semiconductor device of  FIG. 2 . 
     Referring to  FIG. 1 , a pull-up transistor TU, a pull-down transistor TD, and an access transistor TA may be provided. In an embodiment, the pull-up transistor TU may be a PMOS transistor, and the pull-down transistor TD and the access transistor TA may be NMOS transistors. The pull-up transistor TU and the pull-down transistor TD may be connected to each other and constitute a CMOS inverter. A source electrode of the pull-up transistor TU may be connected to a power source VDD, and a source electrode of the pull-down transistor TD may be connected to a ground GND. Gate electrodes of the pull-up transistor TU and the pull-down transistor TD may be connected to each other. Drain electrodes of the pull-up transistor TU and the pull-down transistor TD may be connected to each other and constitute a node N 1 . A selected one of source and drain electrodes of the access transistor TA may be connected to the node N 1 . A load capacitor C L  may be provided between the node N 1  and the ground GND. A gate electrode of the access transistor TA may be connected to a word line WL. 
     Each arrow (→) of  FIG. 1  refers to a direction in which current flows. As shown in  FIG. 1 , current may flow through the pull-up transistor TU and the pull-down transistor TD in one direction, or uni-directionally, while current may flow through the access transistor TA in both, opposed directions, or bi-directionally. In an optimized configuration, the pull-up transistor TU and the pull-down transistor TD may require a low-leakage current characteristic, and the access transistor TA may require a high driving current characteristic. To facilitate formation of low-power devices, the pull-up transistor TU and the pull-down transistor TD may be formed to have a lower threshold voltage V T  than the access transistor TA. 
     Referring to  FIGS. 2 and 3A , a p-well  24 , an n-well  25 , and an isolation layer  23  may be formed in a semiconductor substrate  21 . An n-drain region  26 , a first source/drain region  27 , and a second source/drain region  29  may be formed on the p-well  24 . A p-vertical channel region  31 P and an n-source region  33 S may be formed on the n-drain region  26 . The n-drain region  26  may include an n-protrusion  26 P. The n-protrusion  26 P may be disposed under the p-vertical channel region  31 P, and the n-protrusion  26 P may have sidewalls that are aligned with sidewalls of the p-vertical channel region  31 P. A first gate electrode  43 A may be formed on sidewalls of the p-vertical channel region  31 P. A first gate dielectric layer  41 A may be interposed between the first gate electrode  43 A and the p-vertical channel region  31 P and between the first gate electrode  43 A and the n-drain region  26  and n-protrusion  26 P. 
     A channel region  28  may be defined between the first source/drain region  27  and the second source/drain region  29 . A second gate electrode  43 B may be formed on the channel region  28 . A second gate dielectric layer  41 B may be interposed between the second gate electrode  43 B and the channel region  28 . 
     A p-drain region  36  may be formed on the n-well  25 . An n-vertical channel region  32 N and a p-source region  34 S may be formed on the p-drain region  36 . The p-drain region  36  may include a p-protrusion  36 P. The p-protrusion  36 P may be disposed under the n-vertical channel region  32 N, and the p-protrusion  36 P may have sidewalls that are aligned with, the n-vertical channel region  32 N. A third gate electrode  43 C may be formed on sidewalls of the n-vertical channel region  32 N. A third gate dielectric layer  41 C may be interposed between the third gate electrode  43 C and the n-vertical channel region  32 N, and between the third gate electrode  43 C and the p-drain region  36  and p-protrusion  26 P. 
     A gate pad  43 P may be formed on the isolation layer  23 . The first and third gate electrodes  43 A and  43 C may be connected to the gate pad  43 P. The gate pad  43 P, the first gate electrode  43 A, and the third gate electrode  43 C may have an integral structure. An etch stop layer  48  may be formed to cover the entire surface of the semiconductor substrate  21 . The etch stop layer  48  may function as a stress-inducing layer. An interlayer insulating layer  49  may be formed on the etch stop layer  48 . 
     A first plug  51 , a second plug  52 , a third plug  53 , a fourth plug  54 , a fifth plug  55 , and a sixth plug  56  may be formed through the interlayer insulating layer  49  and the etch stop layer  48 . First and second interconnection lines  57  and  59  may be formed on the interlayer insulating layer  49 . The first plug  51  may be connected to at least one of the n-drain region  26  and the first source/drain region  27 . The second plug  52  may be connected to the p-drain region  36 . The first interconnection line  57  may be in contact with the first and second plugs  51  and  52 . The second interconnection line  59  may be in contact with the third plug  53 . The third plug  53  may be connected to the second source/drain region  29 . The fourth plug  54  may be connected to the n-source region  33 S. The fifth plug  55  may be connected to the p-source region  34 S. The sixth plug  56  may be connected to the gate pad  43 P. 
     The n-drain region  26 , the first source/drain region  27 , the second source/drain region  29 , the channel region  28 , and the p-drain region  36  may be formed at the same level relative to the substrate  21 . Top surfaces of the n-drain region  26 , the first source/drain region  27 , the second source/drain region  29 , the channel region  28 , and the p-drain region  36  may be formed at a lower level than a top surface of the isolation layer  23 . The first source/drain region  27  may be in continuity with the n-drain region  26 . Furthermore, the first source/drain region  27  and the n-drain region  26  may have an integral structure or otherwise be contiguous with each other. The n-drain region  26  and the first source/drain region  27  may include a single-crystalline semiconductor material containing n-type impurities. Bottoms of the first source/drain region  27  and the second source/drain region  29  at a higher level than a bottom of the n-drain region  26  as shown in  FIG. 3A , or may optionally be formed at a lower level than a bottom of the n-drain region  26 , as shown in  FIG. 3B , or may optionally be formed at a same level as a bottom of the n-drain region  26 , as shown in  FIG. 3C . 
     Each of the p-vertical channel region  31 P and the n-vertical channel region  32 N may have a fin structure, a pillar structure, or a wire structure. A horizontal width of the p-vertical channel region  31 P may be less than a vertical height thereof. A horizontal width of the n-vertical channel region  32 N may be less than a vertical height thereof. In some embodiments, the p-vertical channel region  31 P may vertically protrude over the n-drain region  26 , and the n-vertical channel region  32 N may vertically protrude over the p-drain region  36 . In some embodiments, each of the p-vertical channel region  31 P and the n-vertical channel region  32 N may include a single-crystalline semiconductor material formed using an epitaxial growth technique. In some embodiments, each of horizontal widths of the p-vertical channel region  31 P and the n-vertical channel region  32 N may be 20 nm or less. 
     The n-source region  33 S may be disposed on and aligned with the p-vertical channel region  31 P and contact the p-vertical channel region  31 P. The p-source region  34 S may be disposed on and aligned with the n-vertical channel region  32 N and contact the n-vertical channel region  32 N. In some embodiments, each of the n-source region  33 S and the p-source region  34 S may include a single-crystalline semiconductor material formed using an epitaxial growth technique. 
     In some embodiments, the first, second, and third gate dielectric layers  41 A,  41 B, and  41 C may include the same material layers formed at the same time. The first through third gate dielectric layers  41 A,  41 B, and  41 C may have substantially the same thickness. The first through third gate dielectric layers  41 A,  41 B, and  41 C may include a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a high-k dielectric layer, or a combination layer thereof. 
     The first gate electrode  43 A may cover both opposite sidewalls of the p-vertical channel region  31 P. The third gate electrode  43 C may cover both opposite sidewalls of the n-vertical channel region  32 N. The first, second, and third gate electrodes  43 A,  43 B, and  43 C may include the same material layers that are formed at the same time. In various embodiments, the first through third gate electrodes  43 A,  43 B, and  43 C may include a conductive layer, such as a metal layer, a metal nitride layer, a metal silicide layer, a polysilicon (poly-Si) layer, or a combination layer thereof, or other suitable conductive material layers. 
     Referring back to  FIGS. 1, 2, and 3A , the n-drain region  26 , the p-vertical channel region  31 P, the n-source region  33 S, the first gate dielectric layer  41 A, and the first gate electrode  43 A may correspond to the pull-down transistor TD. In this case, the pull-down transistor TD may be referred to as a first vertical transistor. The fourth plug  54  may be connected to the ground GND. 
     The p-drain region  36 , the n-vertical channel region  32 N, the p-source region  34 S, the third gate dielectric layer  41 C, and the third gate electrode  43 C may correspond to the pull-up transistor TU. The pull-up transistor TU may be referred to as a second vertical transistor. The fifth plug  55  may be connected to the power source VDD. 
     The first source/drain region  27 , the second source/drain region  29 , the channel region  28 , the second gate dielectric layer  41 B, and the second gate electrode  43 B may correspond to the access transistor TA. The access transistor TA may be referred to as a planar transistor. The planar transistor may be categorized as a non-vertical or horizontal transistor. In this case, the first source/drain region  27  may be referred to as a non-vertical drain region, while the second source/drain region  29  may be referred to as a non-vertical source region. In another case, the first source/drain region  27  may be referred to as a non-vertical source region, while the second source/drain region  29  may be referred to as a non-vertical drain region. 
     The n-drain region  26 , the first plug  51 , the first interconnection line  57 , the second plug  52 , the p-drain region  36 , and the first source/drain region  27  may constitute the node N 1 . As described above, the first source/drain region  27  may be in continuity with, or contiguous with, the n-drain region  26 . Thus, an electrical resistance of the node N 1  may be greatly reduced. Furthermore, the sizes of the first source/drain region  27  and the n-drain region  26  may be minimized. That is, a structure in which the first source/drain region  27  and the n-drain region  26  are in continuity with each other at the same level may be highly advantageous to highly integrated semiconductor devices. 
     Also, it can be seen in the present embodiments of  FIGS. 3A, 3B, and 3B  that a portion of the gate electrode  43 A of the first vertical transistor and a portion of the gate electrode  43 B of the horizontal transistor are at a same vertical position in the vertical direction relative to the substrate  21 . 
     Also, in the present embodiments, the gate electrode  43 A of the first vertical transistor and the gate electrode  43 B of the horizontal transistor are formed from the same layer of material. This simplifies the number of process steps required for fabricating the resulting device. 
     The first and second vertical transistors may have a lower threshold voltage than the planar transistor. That is, a semiconductor device having various threshold voltage levels may be embodied on the same substrate, and from the same fabrication, without the requirement of additional, unnecessary, process steps. Also, the first and second vertical transistors may exhibit enhanced subthreshold characteristics and a low leakage current characteristics. Furthermore, a circuit configuration including a combination of the first and second vertical transistors and the planar transistor may remarkably reduce power consumption of the semiconductor device. 
     Referring to  FIG. 3B , in this embodiment the bottoms of the first and second source/drain regions  27  and  29  may be formed at a lower level than the bottom of the n-drain region  26 . 
     Referring to  FIG. 3C , in this embodiment, the n-drain region  26 , a first source/drain region  27 A, and a second source/drain region  29 A may be formed on a p-well  24 . Lightly doped regions  47  may be formed between the first and second source/drain regions  27 A and  29 A. A channel region  28  may be defined between the lightly doped regions  47 . A top surface of the first source/drain region  27 A may be formed at the same level as a top surface of the n-drain region  26 , while a bottom surface of the first source/drain region  27 A may be formed at the same level as a bottom surface of the n-drain region  26 . 
     Referring to  FIG. 3D , in this embodiment, a first metal silicide pattern  35 S may be formed on the n-source region  33 S, while a second metal silicide pattern  38 S may be formed on the p-source region  34 S. 
     Referring to  FIG. 3E , in this embodiment, the first metal silicide pattern  35 S may be in direct contact with a p-vertical channel region  31 P, while the second metal silicide pattern  38 S may be in direct contact with an n-vertical channel region  32 N. 
     Referring to  FIG. 3F , in this embodiment, a first metal silicide pattern  35 S and a first metal pattern  61  may be sequentially stacked on the n-source region  33 S, while a second metal silicide pattern  38 S and a second metal pattern  62  may be sequentially stacked on the p-source region  34 S. 
     In the various embodiments described herein, the first and second metal patterns  61  and  62  may comprise a material including tungsten (W), tungsten nitride (WN), titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), cobalt (Co), nickel (Ni), ruthenium (Ru), platinum (Pt), titanium aluminum nitride (TiAlN), tantalum aluminum nitride (TaAlN), titanium silicon nitride (TiSiN), tantalum silicon nitride (TaSiN), or a combination thereof. The first and second metal silicide patterns  35 S and  38 S may comprise a material including WSi, TiSi, TaSi, CoSi, NiSi, or a combination thereof. 
     Referring to  FIG. 3G  in this embodiment, insulating spacers  81 ,  82 , and  83  may be formed on sidewalls of the first, second, and third gate electrodes  43 A,  43 B, and  43 C, respectively. The first metal silicide pattern  35 S may be formed on the p-vertical channel region  31 P, the second metal silicide pattern  38 S may be formed on the n-vertical channel region  32 N, a third metal silicide pattern  35 A may be formed on the n-drain region  26  and the first source/drain region  27 , a fourth metal silicide pattern  35 B may be formed on the second source/drain region  29 , and a fifth metal silicide pattern  38 A may be formed on the p-drain region  36 . The first through fifth metal silicide patterns  35 S,  38 S,  35 A,  35 B, and  38 A may be covered with the etch stop layer  48 . The first metal silicide pattern  35 S may be in contact with the p-vertical channel region  31 P, while the second metal silicide pattern  38 S may be in contact with the n-vertical channel region  32 N. 
     Referring to  FIG. 3H , in this embodiment, the insulating spacers  81 ,  82 , and  83  may be formed on the sidewalls of the first through third gate electrodes  43 A,  43 B, and  43 C, respectively. The first metal silicide pattern  35 S may be formed on the n-source region  33 S, while the second metal silicide pattern  38 S may be formed on the p-source region  34 S. Also, the third metal silicide pattern  35 A may be formed on the n-drain region  26  and the first source/drain region  27 , the fourth metal silicide pattern  35 B may be formed on the second source/drain region  29 , and the fifth metal silicide pattern  38 A may be formed on the p-drain region  36 . Furthermore, gate silicide patterns  43 S may be formed on the first through third gate electrodes  43 A,  43 B, and  43 C. 
     In some embodiments, including those disclosed herein in connection with  FIGS. 3A-3H  described above, and with embodiments described below, including embodiments disclosed herein in connection with  FIGS. 4, 5, 7A, 7B, 9A-9C, 11A-11C, and 12A-12D , it can be seen that the gate electrodes of the horizontal transistor and the vertical transistor are both in direct contact with the same layer of material that lies on the horizontal transistor and the vertical transistor. For example, in the embodiments of  FIG. 3A , the gate electrode  43 A of the vertical transistor is in direct contact with the etch stop layer  48 . The same holds true for the gate electrode  43 B of the horizontal transistor. In various embodiments, the layer of material in contact with both the horizontal and vertical transistors can comprise an etch stop layer or an insulating layer. 
     Embodiment 2 
       FIG. 4  is a cross-sectional view of a semiconductor device according to a second embodiment of the inventive concept. 
     Referring to  FIG. 4 , in this embodiment, a p-vertical channel region  31 P and an n-source region  33 S may be formed on an n-drain region  26 . First insulating spacers  63  may be formed on sidewalls of the n-source region  33 S. The n-drain region  26  may include an n-protrusion  26 P that extends in the vertical direction. A first gate dielectric layer  41 A and a first gate electrode  43 A may be formed on sidewalls of the p-vertical channel region  31 P. 
     The p-vertical channel region  31 P may have a width in the horizontal direction that is less than that of the n-source region  33 S. The n-protrusion  26 P may have substantially the same width in the horizontal direction as that of the p-vertical channel region  31 P. 
     An n-vertical channel region  32 N and a p-source region  34 S may be formed on a p-drain region  36 . Second insulating spacers  64  may be formed on sidewalls of the p-source region  34 S. The p-drain region  36  may include a p-protrusion  36 P that extends in the vertical direction. A third gate dielectric layer  41 C and a third gate electrode  43 C may be formed on sidewalls of the n-vertical channel region  32 N. 
     The n-vertical channel region  32 N may have a width in the horizontal direction that is less than that of the p-source region  34 S. The p-protrusion  36 P may have substantially the same horizontal width in the horizontal direction as that of the n-vertical channel region  32 N. 
     Embodiment 3 
       FIG. 5  is a cross-sectional view of a semiconductor device according to a third embodiment of the inventive concept. 
     Referring to  FIG. 5 , lightly doped regions  67  may be formed under an n-drain region  26 , a first source/drain region  27 , and a second source/drain region  29 . The lightly doped impurity regions  67  may include impurities of the same conductivity type as the n-drain region  26 , the first source/drain region  27 , and the second source/drain region  29 . The lightly doped regions  67  may include n-type impurities. A second gate electrode  66  may be formed between the first and second source/drain regions  27  and  29 . A gate dielectric layer  65  may be formed between the second gate electrode  66  and a p-well  24 . A channel region  68  may be defined in the p-well  24  by the first and second source/drain regions  27  and  29 , the lightly doped regions  67 , and the second gate electrode  66 . 
     A bottom of the second gate electrode  66  may be formed at a lower level than the first and second source/drain regions  27  and  29  and the lightly doped regions  67 . A top of the second gate electrode  66  may be formed at a lower level than top surfaces of the first and second source/drain regions  27  and  29 . The second gate electrode  66 , the second gate dielectric layer  65 , the channel region  68 , the first and second source/drain regions  27  and  29 , and the lightly doped regions  67  may constitute a recess channel transistor. The recess channel transistor may be categorized as a non-vertical transistor. In this case, although the second gate electrode  66  is at a different vertical position that that of the first gate electrode  43 A, the first and second gate electrodes  43 A,  66  can still be formed of the same layer of material. Also, it can be seen that the first and second gate electrodes  43 A,  66  are both in direct contact with the same layer of material that lies on the horizontal transistor and the vertical transistor; namely etch stop layer  48 . 
     Embodiment 4 
       FIG. 6  is a layout illustrating a semiconductor device according to a fourth embodiment of the inventive concept, and  FIGS. 7A and 7B  are cross-sectional views of the semiconductor device taken along line II-II′ of  FIG. 6 . 
     Referring to  FIGS. 6 and 7A , a p-well  24 , an n-well  25 , and an isolation layer  23  may be formed in a semiconductor substrate  21 . An n-drain region  26 , a first source/drain region  27 , and a second source/drain region  29  may be formed on the p-well  24 . A p-vertical channel region  71 P and an n-source region  73 S may be formed on the n-drain region  26 . The n-drain region  26  may include an n-protrusion  26 P. A first gate dielectric layer  41 A and a first gate electrode  43 A may be formed on sidewalls of the p-vertical channel region  71 P. 
     A channel region  28  may be defined between the first and second source/drain regions  27  and  29 . A second gate electrode  43 B may be formed on the channel region  28 . A second gate dielectric layer  41 B may be interposed between the second gate electrode  43 B and the channel region  28 . 
     A p-drain region  36  may be formed on the n-well  25 . An n-vertical channel region  72 N and a p-source region  74 S may be formed on the p-drain region  36 . The p-drain region  36  may include a p-protrusion  36 P. A third gate dielectric layer  41 C and a third gate electrode  43 C may be formed on sidewalls of the n-vertical channel region  72 N. 
     A gate pad  43 P may be formed on the isolation layer  23 . The first and third gate electrodes  43 A and  43 C may be connected to the gate pad  43 P. The gate pad  43 P and the first and third gate electrodes  43 A and  43 C may have an integral structure. An etch stop layer  48  and an interlayer insulating layer  49  may be formed to cover the entire surface of the semiconductor substrate  21 . 
     A first plug  51 , a second plug  52 , a third plug  53 , a fourth plug  54 , a fifth plug  55 , and a sixth plug  56  may be formed through the interlayer insulating layer  49  and the etch stop layer  48 . First through fourth interconnection lines  57 ,  59 ,  77 , and  79  may be formed on the interlayer insulating layer  49 . The first plug  51  may be connected to at least one of the n-drain region  26  and the first source/drain region  27 . The second plug  52  may be connected to the p-drain region  36 . The first interconnection line  57  may be in contact with the first and second plugs  51  and  52 . The second interconnection line  59  may be in contact with the third plug  53 . The third plug  53  may be connected to the second source/drain region  29 . The fourth plug  54  may be connected to the n-source region  73 S. The fifth plug  55  may be connected to the p-source region  74 S. The sixth plug  56  may be connected to the gate pad  43 P. 
     In the present embodiment, each of the p-vertical channel region  71 P and the n-vertical channel region  72 N may have a pillar structure. Each of the p-vertical channel region  71 P and the n-vertical channel region  72 N may have a cylindrical shape, a square cross-section pillar shape, a rectangular cross-section pillar shape, or a polygonal cross-section pillar shape. The p-vertical channel region  71 P may protrude in a vertical direction over the n-drain region  26 , while the n-vertical channel region  72 N may protrude in a vertical direction over the p-drain region  36 . Each of the p-vertical channel region  71 P and the n-vertical channel region  72 N may comprise a single crystal semiconductor material formed using an epitaxial growth technique. 
     In other embodiments, each of the p-vertical channel region  71 P and the n-vertical channel region  72 N may include a wire structure, or a nano-wire structure. 
     The n-source region  73 S may be disposed on and have sidewalls that are aligned with those of the p-vertical channel region  71 P and contact the p-vertical channel region  71 P. The p-source region  74 S may be disposed on and have sidewalls that are aligned with those of the n-vertical channel region  72 N and contact the n-vertical channel region  72 N. Each of the n-source region  73 S and the p-source region  74 S may comprise a single crystal semiconductor material formed using an epitaxial growth technique. 
     In some embodiments, the first gate electrode  43 A may be formed to completely surround the sidewalls of the p-vertical channel region  71 P, while the third gate electrodes  43 C may be formed to completely surround the sidewalls of the n-vertical channel region  72 N. 
     Referring to  FIG. 7B , the p-vertical channel region  71 P and the n-source region  73 S may be formed on the n-drain region  26 . First insulating spacers  63  may be formed on sidewalls of the n-source region  73 S. The n-drain region  26  may include an n-protrusion  26 P. The n-protrusion  26 P may be disposed under and have sidewalls that are aligned with sidewalls of the p-vertical channel region  71 P. A first gate dielectric layer  41 P and a first gate electrode  43 A may be formed on the sidewalls of the p-vertical channel region  71 P. 
     The p-vertical channel region  71 P may have a width in the horizontal direction that is less than that of the n-source region  73 S. The n-protrusion  26 P may have a width in the horizontal direction that is substantially the same as that of the p-vertical channel region  71 P. 
     The n-vertical channel region  72 N and the p-source region  74 S may be formed on the p-drain region  36 . Second insulating spacers  64  may be formed on sidewalls of the p-source region  74 S. The p-drain region  36  may include a p-protrusion  36 P. A third gate dielectric layer  41 C and a third gate electrode  43 C may be formed on the sidewalls of the n-vertical channel region  72 N. 
     The n-vertical channel region  72 N may have a width in the horizontal direction that is less than that of the p-source region  74 S. The p-protrusion  36 P may have a width in the horizontal direction that is substantially the same as that of the n-vertical channel region  72 N. 
     Embodiment 5 
       FIG. 8  is a layout illustrating a semiconductor device according to a fifth embodiment of the inventive concept.  FIGS. 9A through 9C  are cross-sectional views of the semiconductor device taken along lines III-III′, IV-IV′, and V-V′ of  FIG. 8 , respectively. 
     Referring to  FIGS. 8 and 9A through 9C , a buried oxide layer  122  may be formed on a semiconductor substrate  121 . An isolation layer  123  may be formed on the buried oxide layer  122  to define an n-drain region  126 , a first source/drain region  127 , a second source/drain region  129 , a channel region  128 , and a p-drain region  136 . 
     A p-vertical channel region  131 P and an n-source region  133 S may be formed on the n-drain region  126 . The n-drain region  126  may include an n-protrusion  126 P. A first gate dielectric layer  141 A and a first gate electrode  143 A may be formed on sidewalls of the p-vertical channel region  131 P. 
     A second gate electrode  143 B may be formed on the channel region  128 . A second gate dielectric layer  141 B may be interposed between the second gate electrode  143 B and the channel region  128 . 
     An n-vertical channel region  132 N and a p-source region  134 S may be formed on the p-drain region  136 . The p-drain region  136  may include a p-protrusion  136 P. A third gate dielectric layer  141 C and a third gate electrode  143 C may be formed on sidewalls of the n-vertical channel region  132 N. 
     A gate pad  143 P may be formed on the isolation layer  123 . The first and third gate electrodes  143 A and  143 C may be connected to the gate pad  143 P. An etch stop layer  148  and an interlayer insulating layer  149  may be formed to cover the entire surface of the semiconductor substrate  121 . 
     A first plug  151 , a second plug  153 , a third plug  154 , a fourth plug  155 , and a fifth plug  156  may be formed through the interlayer insulating layer  149  and the etch stop layer  148 . First and second interconnection lines  157  and  159  may be formed on the interlayer insulating layer  149 . The first plug  151  may be connected to at least one of the n-drain region  126 , the p-drain region  136 , and the first source/drain region  127 . The first interconnection line  157  may be in contact with the first plug  151 . The second interconnection line  159  may be in contact with the second plug  153 . 
     The n-drain region  126 , the first source/drain region  127 , the second source/drain region  129 , the channel region  128 , and the p-drain region  136  may be formed at the same vertical level, relative to the substrate. Top surfaces of the n-drain region  126 , the first source/drain region  127 , the second source/drain region  129 , the channel region  128 , and the p-drain region  136  may be formed at a lower level than a top surface of the isolation layer  123 . The first source/drain region  127  may be in continuity with, or, in other words, contiguous with, the n-drain region  126 . Furthermore, the first source/drain region  127  and the n-drain region  126  may be integral with each other. The p-drain region  136  may be in contact with at least one of the n-drain region  126  and the first source/drain region  127 . Each of the n-drain region  126  and the first source/drain region  127  may comprise a single crystal semiconductor material having n-type impurities. The p-drain region  136  may comprise a single crystal semiconductor material having p-type impurities. 
     The n-drain region  126 , the p-drain region  136 , and the first source/drain region  127  may constitute a node (refer to N 1  in  FIG. 1 ). In some embodiments, the electric resistance of the node N 1  may be markedly reduced. The first source/drain region  127  and the n-drain region  126  may be in continuity with, or contiguous with, each other at the same vertical level relative to the substrate. Such a structure in which the p-drain region  136  is in contact with the n-drain region  126  and the first source/drain region  127  is highly advantageous in that it lends itself well to highly integrated configurations. 
     Embodiment 6 
       FIG. 10  is a layout illustrating a semiconductor device according to a sixth embodiment of the inventive concept.  FIGS. 11A, 12A, and 12D  are cross-sectional views taken along line VI-VI′ of  FIG. 10 ,  FIGS. 11B and 12B  are cross-sectional views taken along line VII-VII′ of  FIG. 10 , and  FIGS. 11C and 12C  are cross-sectional views taken along line VIII-VIII′ of  FIG. 10 . 
     Referring to  FIGS. 10, 11A, 11B, and 11C , a buried oxide layer  122  may be formed on a semiconductor substrate  121 . An isolation layer  123  may be formed on the buried oxide layer  122  to define an n-drain region  126 , a first source/drain region  127 , a second source/drain region  129 , a channel region  128 , and a p-drain region  136 . 
     A p-vertical channel region  171 P and an n-source region  173 S may be formed on the n-drain region  126 . The n-drain region  126  may include an n-protrusion  126 P. A first gate dielectric layer  141 A and a gate electrode  143 A may be formed on sidewalls of the p-vertical channel region  171 P. 
     A channel region  128  may be defined between the first and second source/drain regions  127  and  129 . A second gate electrode  143 B may be formed on the channel region  128 . A second gate dielectric layer  141 B may be interposed between the second gate electrode  143 B and the channel region  128 . 
     An n-vertical channel region  172 N and a p-source region  174 S may be formed on the p-drain region  136 . The p-drain region  136  may include a p-protrusion  136 P. A third gate dielectric layer  141 C and a third gate electrode  143 C may be formed on sidewalls of the n-vertical channel region  172 N. 
     A gate pad  143 P may be formed on the isolation layer  123 . The first and third gate electrodes  143 A and  143 C may be connected to the gate pad  143 P. An etch stop layer  148  and an interlayer insulating layer  149  may be formed to cover the entire surface of the semiconductor substrate  121 . 
     A first plug  151 , a second plug  153 , a third plug  154 , a fourth plug  155 , and a fifth plug  156  may be formed through the interlayer insulating layer  149  and the etch stop layer  148 . First through fourth interconnection lines  157 ,  159 ,  177 , and  179  may be formed on the interlayer insulating layer  149 . The first plug  151  may be connected to at least one of the n-drain region  126 , the p-drain region  136 , and the first source/drain region  127 . The first interconnection line  157  may be in contact with the first plug  151 . The second interconnection line  159  may be in contact with the second plug  153 . 
     Each of the p-vertical channel region  171 P and the n-vertical channel region  172 N may have a pillar structure. In other embodiments, each of the p-vertical channel region  171 P and the n-vertical channel region  172 N may have a wire structure, or nano-wire structure. 
     The first gate electrode  143 A may be formed to completely surround sidewalls of the p-vertical channel region  171 P, and the third gate electrode  143 C may be formed to completely surround sidewalls of the n-vertical channel region  172 N. 
     Referring to  FIGS. 10, 12A, 12B, and 12C , a p-vertical channel region  171 P and an n-source region  173 S may be formed on the n-drain region  126 . First insulating spacers  163  may be formed on sidewalls of the n-source region  173 S. The n-drain region  126  may include an n-protrusion  126 P. A first gate dielectric layer  141 A and a first gate electrode  143 A may be formed on sidewalls of the p-vertical channel region  171 P. 
     The p-vertical channel region  171 P may have a width in the horizontal direction that is less than that of the n-source region  173 S. The n-protrusion  126 P may have substantially the same width in the horizontal direction as that of the p-vertical channel region  171 P. 
     An n-vertical channel region  172 N and a p-source region  174 S may be formed on the p-drain region  136 . Second insulating spacers  164  may be formed on sidewalls of the p-source region  174 S. The p-drain region  136  may include a p-protrusion  136 P. A third gate dielectric layer  141 C and a third gate electrode  143 C may be formed on sidewalls of the n-vertical channel region  172 N. 
     The n-vertical channel region  172 N may horizontal width that is less than that of the p-source region  174 S. The p-protrusion  136 P may have substantially the same width in the horizontal direction as that of the n-vertical channel region  172 N. 
     Referring to  FIGS. 10 and 12D , impurity regions  147 A may be formed adjacent to both sides of the second gate electrode  143 B. The impurity regions  147  may be aligned with sidewalls of the second gate electrode  143 B. The impurity regions  147 A may have different widths due to alignment errors of the second gate electrode  143 B present during its formation. A channel region  128  may be defined between the impurity regions  147 A. 
     Embodiment 7 
       FIGS. 13 through 24  are cross-sectional views taken along line I-I′ of  FIG. 2 , illustrating a method of forming a semiconductor device according to a seventh embodiment of the inventive concept. 
     Referring to  FIGS. 2 and 13 , a p-well  24 , an n-well  25 , and an isolation layer  23  may be formed in a semiconductor substrate  21 . In some embodiments, the semiconductor substrate  21  may comprise a semiconductor wafer formed of single crystal material. For example, the semiconductor substrate  21  may be a silicon wafer having p-type impurities. The p-well  24  may include single crystalline silicon having p-type impurities, while the n-well  25  may include single crystalline silicon having n-type impurities. The isolation layer  23  may be an insulating layer formed of silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof using a shallow trench isolation (STI) technique. The p-well  24  and the n-well  25  may be electrically isolated from one another by the isolation layer  23 . A top surface of the isolation layer  23 , the p-well  24 , and the n-well  25  may lie on substantially the same planar surface. 
     Referring to  FIGS. 2 and 14A , a first mask pattern  26 M may be formed to cover the n-well  25  and partially expose the p-well  24 . N-type impurities may be implanted into the p-well  24  using the first mask pattern  26 M as an ion implantation mask, thereby forming an n-drain region  26 . A channel region  28  may be defined adjacent to the n-drain region  26 . The channel region  28  may include single crystalline silicon material having p-type impurities. The first mask pattern  26 M may be removed. 
     Referring to  FIG. 14B , in applied embodiments, a first mask pattern  26 M may be formed to cover the n-well  25  and partially expose the p-well  24 . N-type impurities may be implanted into the p-well  24  using the first mask pattern  26 M as an ion implantation mask, thereby forming an n-drain region  26 , a first source/drain region  27 A, and a second source/drain region  29 A. A channel region  28  may be defined between the first and second source/drain regions  27 A and  29 A. The channel region  28  may include single crystalline silicon having p-type impurities. The first mask pattern  26 M may be removed. 
     Referring to  FIGS. 2 and 15 , a second mask pattern  36 M may be formed to cover the p-well  24  and expose the n-well  25 . P-type impurities may be implanted into the n-well  25  using the second mask pattern  36 M as an ion implantation mask, thereby forming a p-drain region  36 . The second mask pattern  36 M may be removed, thereby exposing top surfaces of the n-drain region  26  and the p-drain region  36 . 
     Referring to  FIGS. 2 and 16 , a first semiconductor layer  31  may be formed on the semiconductor substrate  21 . The first semiconductor layer  31  may be in contact with top surfaces of the n-drain region  26  and the p-drain region  36 . In some embodiments, the first semiconductor layer  31  may be formed using an epitaxial growth technique. The first semiconductor layer  31  may include an n-type semiconductor, a p-type semiconductor, or an intrinsic semiconductor. 
     Hereinafter, it is assumed that the first semiconductor layer  31  is a first p-semiconductor layer. For example, the first p-semiconductor layer  31  may include single crystalline silicon having p-type impurities. 
     Referring to  FIGS. 2 and 17 , a third mask pattern  32 M may be formed on the first p-semiconductor layer  31 . A first n-semiconductor layer  32  and a second p-semiconductor layer  34  may be formed in the first p-semiconductor layer  31  by performing an ion implantation process using the third mask pattern  32 M as an ion implantation mask. The third mask pattern  32 M may be removed. The first n-semiconductor layer  32  may be in contact with the p-drain region  36 . The second p-semiconductor layer  34  may be formed on the first n-semiconductor layer  32 . As a result, the first p-semiconductor layer  31  may be defined on the p-well  24 . 
     Referring to  FIGS. 2 and 18 , a fourth mask pattern  33 M may be formed to cover the second p-semiconductor layer  34  and expose the first p-semiconductor layer  31 . A second n-semiconductor layer  33  may be formed by performing an ion implantation process using the fourth mask pattern  33 M as an ion implantation mask. The fourth mask pattern  33 M may be removed. The first p-semiconductor layer  31  may therefore be defined between the second n-semiconductor layer  33  and the n-drain region  26 . 
     Referring to  FIGS. 2, 19, and 20 , a fifth mask pattern  37 M may be formed on the second n-semiconductor layer  33  and the second p-semiconductor layer  34 . The second n-semiconductor layer  33 , the first p-semiconductor layer  31 , the n-drain region  26 , the channel region  28 , the second p-semiconductor layer  34 , the first n-semiconductor layer  32 , and the p-drain region  36  may be anisotropically etched using the fifth mask pattern  37 M as an etch mask, thereby forming an n-source region  33 S, a p-vertical channel region  31 P, a p-source region  34 S, and an n-vertical channel region  32 N. 
     The n-drain region  26 , the channel region  28 , and the p-drain region  36  may be partially recessed and retained at a lower level than the top surface of the isolation layer  23 . The n-drain region  26  may thereby include an n-protrusion  26 P, and the p-drain region  36  may thereby include a p-protrusion  36 P. The n-protrusion  36  may be disposed under and have sidewalls that are aligned with those of the p-vertical channel region  31 P, while the p-protrusion  36 P may be disposed under and have sidewalls that are aligned with those of the n-vertical channel region  32 N. 
     Referring to  FIGS. 2 and 21 , a gate dielectric layer  41 A,  41 B, and  41 C may be formed to cover the resulting surface of the semiconductor substrate  21 . A gate conductive layer  43  may be formed on the gate dielectric layer  41 A,  41 B, and  41 C. The gate dielectric layer  41 A,  41 B, and  41 C may include a first gate dielectric layer portion  41 A covering sidewalls of the p-vertical channel region  31 P, a second gate dielectric layer portion  41 B covering the channel region  28 , and a third gate dielectric layer  41 C portion covering sidewalls of the n-vertical channel region  32 N. 
     The gate dielectric layer  41 A,  41 B, and  41 C may comprise a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a high-k dielectric layer, or a combination thereof. The first gate dielectric layer  41 A, the second gate dielectric layer  41 B, and the third gate dielectric layer  41 C portions may be formed using the same material layer at the same time. The gate conductive layer  43  may include a metal layer, a metal nitride layer, a metal silicide layer, a polysilicon (poly-Si) layer, a conductive carbon layer, or a combination thereof. 
     Referring to  FIGS. 2 and 22 , a sixth mask pattern  45 M may be formed on the gate conductive layer  43 . The gate conductive layer  43  may be anisotropically etched using the sixth mask pattern  45 M as an etch mask, thereby forming a first gate electrode  43 A, a second gate electrode  43 B, and a third gate electrode  43 C. The sixth mask pattern  45 M may cover the second gate electrode  43 B. Also, the sixth mask pattern  45 M may cover a gate pad  43 P. 
     Referring to  FIGS. 2 and 23 , a seventh mask pattern  47 M may be formed to cover the n-well  25  and the n-drain region  26 . N-type impurities may be implanted into the channel region  28  adjacent to both sides of the second gate electrode  43 B using the seventh mask pattern  47 M as an ion implantation mask, thereby forming first and second source/drain regions  27  and  29 . Thereafter, the seventh mask pattern  47 M may be removed. As a result, the channel region  28  may be defined between the first and second source/drain regions  27  and  29 . 
     Subsequently, the sixth and fifth mask patterns  45 M and  37 M may be removed. The gate dielectric layer  41 A,  41 B, and  41 C portions may also be partially removed. 
     Referring to  FIGS. 2 and 24 , an etch stop layer  48  may be formed to cover the resulting surface of the semiconductor substrate  21 . An interlayer insulating layer  49  may be formed on the etch stop layer  48 . A top surface of the interlayer insulating layer  49  may be planarized. 
     Referring back to  FIGS. 2 and 3A , a first plug  51 , a second plug  52 , a third plug  53 , a fourth plug  54 , a fifth plug  55 , and a sixth plug  56  may be formed through the interlayer insulating layer  49  and the etch stop layer  48 . First and second interconnection lines  57  and  59  may be formed on the interlayer insulating layer  49  to form the resulting semiconductor device. 
     Embodiment 8 
       FIGS. 25 through 31  are cross-sectional views illustrating a method of forming a semiconductor device according to an eighth embodiment of the inventive concept. 
     Referring to  FIG. 25 , a p-well  24 , an n-well  25 , an isolation layer  23 , an n-drain region  26 , a channel region  28 , a p-drain region  36 , a first p-semiconductor layer  31 , a first n-semiconductor layer  32 , an n-source region  33 S, a p-source region  34 S, and a fifth mask pattern  37 M may be formed on a semiconductor substrate  21 . 
     Referring to  FIG. 26 , first insulating spacers  63  may be formed on sidewalls of the fifth mask pattern  37 M and the n-source region  33 S, and second insulating spacers  64  may be formed on sidewalls of the fifth mask pattern  37 M and the p-source region  34 S. 
     Referring to  FIG. 27 , the first p-semiconductor layer  31  and the first n-semiconductor layer  32  may be anisotropically etched using the fifth mask pattern  37 M and the first and second insulating spacers  63  and  64  as an etch mask, thereby forming a p-vertical channel region  31 P and an n-vertical channel region  32 N. 
     Referring to  FIG. 28 , the thicknesses of the p-vertical channel region  31 P and the n-vertical channel region  32 N in the horizontal direction may be reduced using a pullback process. The p-vertical channel region  31 P may have a smaller width in the horizontal direction than that of the n-source region  33 S. The n-vertical channel region  32 N may have a width in the horizontal direction that is less than that of the p-source region  34 S. 
     The pullback process may include isotropically etching the p-vertical channel region  31 P and the n-vertical channel region  32 N. During the pullback process, the n-drain region  26 , the channel region  28 , and the p-drain region  36  may become partially recessed and retained at a lower level than a top surface of the isolation layer  23 . The n-drain region  26  may include an n-protrusion  26 P, while the p-drain region  36  may include a p-protrusion  36 P. The n-protrusion  26 P may be disposed under and have sidewalls that are aligned with those of the p-vertical channel region  31 P, while the p-protrusion  36 P may be disposed under and have sidewalls that are aligned with those of the n-vertical channel region  32 N. 
     Referring to  FIG. 29 , a gate dielectric layer  41 A,  41 B, and  41 C may be formed to cover the surface of the semiconductor substrate  21 . A gate conductive layer  43  may be formed on the gate dielectric layer  41 A,  41 B, and  41 C. The gate dielectric layer  41 A,  41 B, and  41 C may include a first gate dielectric layer  41 A portion covering sidewalls of the p-vertical channel region  31 P, a second gate dielectric layer  41 B portion covering the channel region  28 , and a third gate dielectric layer  41 C portion covering sidewalls of the n-vertical channel region  32 N. 
     Referring to  FIG. 30 , a sixth mask pattern  45 M may be formed on the gate conductive layer  43 . The gate conductive layer  43  may be anisotropically etched using the sixth mask pattern  45 M as an etch mask, thereby forming a first gate electrode  43 A, a second gate electrode  43 B, and a third gate electrode  43 C. The sixth mask pattern  45 M may cover the second gate electrode  43 B. The sixth and fifth mask patterns  45 M and  37 M may be removed. The gate dielectric layer  41 A,  41 B, and  41 C and the first and second insulating spacers  63  and  64  also may be partially removed. 
     Referring to  FIG. 31 , n-type impurities may be implanted into the channel region  28  adjacent to both sides of the second gate electrode  43 B, thereby forming a first source/drain region  27  and a second source/drain region  29 . The channel region  28  may be defined between the first and second source/drain regions  27  and  29 . An etch stop layer  48  may be formed to cover the surface of the semiconductor substrate  21 . An interlayer insulating layer  49  may be formed on the etch stop layer  48 . 
     Referring back to  FIG. 4 , a first plug  51 , a second plug  52 , and a third plug  53  may be formed through the interlayer insulating layer  49  and the etch stop layer  48 . First and second interconnection lines  57  and  59  may be formed on the interlayer insulating layer  49  to form the resulting semiconductor device. 
     Embodiment 9 
       FIGS. 32 through 39  are cross-sectional views illustrating a method of forming a semiconductor device according to a ninth embodiment of the inventive concept. 
     Referring to  FIG. 32 , a p-well  24 , an n-well  25 , and an isolation layer  23  may be formed in a semiconductor substrate  21 . A first mask pattern  26 M may be formed to cover the n-well  25  and expose the p-well  24 . N-type impurities may be implanted into the p-well  24  using the first mask pattern  26 M as an ion implantation mask, thereby forming an n-drain region  26 , a first source/drain region  27 , a second source/drain region  29 , and a lightly doped region  67 . The lightly doped region  67  may be formed under the n-drain region  26 , the first source/drain region  27 , and the second source/drain region  29 . The first mask pattern  26 M may then be removed. 
     Referring to  FIG. 33 , a second mask pattern  36 M may be formed to cover the p-well  24  and expose the n-well  25 . P-type impurities may be implanted into the n-well  25  using the second mask pattern  36 M as an ion implantation mask, thereby forming a p-drain region  36 . The second mask pattern  36 M may be removed to expose top surfaces of the n-drain region  26  and the p-drain region  36 . 
     Referring to  FIG. 34 , a first p-semiconductor layer  31 , a first n-semiconductor layer  32 , a second n-semiconductor layer  33 , a second p-semiconductor layer  34 , and a fifth mask pattern  37 M may be formed. The first p-semiconductor layer  31  and the second n-semiconductor layer  33  may be sequentially stacked on the n-drain region  26  and the first and second source/drain regions  27  and  29 . The first n-semiconductor layer  32  and the second p-semiconductor layer  34  may be sequentially stacked on the p-drain region  36 . 
     Referring to  FIG. 35 , the second n-semiconductor layer  33 , the first p-semiconductor layer  31 , the n-drain region  26 , the first source/drain region  27 , the second source/drain region  29 , the second p-semiconductor layer  34 , the first n-semiconductor layer  32 , and the p-drain region  36  may be anisotropically etched using the fifth mask pattern  37 M as an etch mask, thereby forming an n-source region  33 S, a p-vertical channel region  31 P, a p-source region  34 S, and an n-vertical channel region  32 N. The n-drain region  26 , the first source/drain region  27 , the second source/drain region  29 , and the p-drain region  36  may be partially recessed and retained at a lower level than a top surface of the isolation layer  23 . The n-drain region  26  may include an n-protrusion  26 P, while the p-drain region  36  may include a p-protrusion  36 P. 
     Referring to  FIG. 36 , a sixth mask pattern  66 M may be formed on the semiconductor substrate  21 . The first source/drain region  27 , the second source/drain region  29 , the lightly doped region  67 , and the p-well  24  may be anisotropically etched using the sixth mask pattern  66 M as an etch mask, thereby forming a gate trench  66 T. The gate trench  66 T may penetrate not only a region between the first and second source/drain regions  27  and  29  but also the lightly doped region  67 . The lightly doped region  67  may be divided into two regions by the gate trench  66 T. A channel region  68  may be defined by the gate trench  66 T in the p-well  24 . The sixth mask pattern  66 M may be removed. 
     Referring to  FIG. 37 , a gate dielectric layer  41 A,  65 , and  41 C may be formed to cover the surface of the semiconductor substrate  21 . A gate conductive layer  43  may be formed on the gate dielectric layer  41 A,  65 , and  41 C. The gate conductive layer  43  may completely fill the gate trench  66 T. 
     Referring to  FIG. 38 , the gate conductive layer  43  may be anisotropically etched, thereby forming a first gate electrode  43 A, a second gate electrode  66 , and a third gate electrode  43 C. The second gate electrode  66  may be retained within the gate trench  66 T. A first gate dielectric layer  41 A may be retained between the first gate electrode  43 A and the p-vertical channel region  31 P, and a second gate dielectric layer  65  may be retained between the second gate electrode  66  and the channel region  68 . Also, a third gate dielectric layer  41 C may be retained between the third gate electrode  43 C and the n-vertical channel region  32 N. 
     A bottom of the second gate electrode  66  may be formed at a lower level than the first and second source/drain regions  27  and  29  and the lightly doped regions  67 . A top of the second gate electrode  66  may be formed at a lower level than top surfaces of the first and second source/drain regions  27  and  29 . The second gate electrode  66 , the second gate dielectric layer  65 , the channel region  68 , the first source/drain region  27 , the second source/drain region  29 , and the lightly doped regions  67  may constitute a recess channel transistor. The recess channel transistor may be categorized as a non-vertical, or horizontal, transistor. 
     Subsequently, the gate dielectric layer  41 A,  65 , and  41 C are partially etched and the fifth mask pattern  37 M may be removed. 
     Referring to  FIG. 39 , an etch stop layer  48  may be formed to cover the surface of the semiconductor substrate  21 . An interlayer insulating layer  49  may be formed on the etch stop layer  48 . The etch stop layer  48  may cover the second gate electrode  66 . 
     Referring back to  FIG. 5 , a first plug  51 , a second plug  52 , and a third plug  53  may be formed through the interlayer insulating layer  49  and the etch stop layer  48 . First and second interconnection lines  57  and  59  may be formed on the interlayer insulating layer  49  to form the resulting semiconductor device. 
     Embodiment 10 
       FIGS. 40A through 43C  are cross-sectional views taken along lines III-III′, IV-IV′, and V-V′ of  FIG. 8 , illustrating a method of forming a semiconductor device according to a tenth embodiment of the inventive concept. 
     Referring to  FIGS. 8, 40A, 40B, and 40C , a buried oxide layer  122  may be formed on a semiconductor substrate  121 . An active region  124  and an isolation layer  123  may be formed on the buried oxide layer  122 . Top surfaces of the active region  124  and the isolation layer  123  may be exposed on substantially the same plane surface. A first mask pattern  126 M may be formed on the active region  124  and the isolation layer  123 . An n-drain region  126  may be formed in the active region  124  by performing an ion implantation process using the first mask pattern  126 M as an ion implantation mask. 
     The buried oxide layer  122  may be an insulating layer, such as a silicon oxide layer. In this case, the semiconductor substrate  121  may be a silicon-on-insulator (SOI) wafer. The active region  124  may include a single crystalline semiconductor having p-type impurities. The isolation layer  123  may penetrate the active region  124  and contact the buried oxide layer  122 . 
     Referring to  FIGS. 8, 41A, 41B, and 41C , a second mask pattern  136 M may be formed on the n-drain region  126 , the active region  124 , and the isolation layer  123 . P-impurities may be implanted into the active region  124  using the second mask pattern  136 M as an ion implantation mask, thereby forming a p-drain region  136 . 
     Referring to  FIGS. 8, 42A, 42B, and 42C , a first p-semiconductor layer  131  may be formed on the n-drain region  126  and the active region  124 , and a first n-semiconductor layer  132  may be formed on the p-drain region  136 . A second n-semiconductor layer  133  may be formed on the first p-semiconductor layer  131 , and a second p-semiconductor layer  134  may be formed on the first n-semiconductor layer  132 . 
     Referring to  FIGS. 8, 43A, 43B, and 43C , a p-vertical channel region  131 P and an n-source region  133 S may be formed on the n-drain region  126  in about the same manners as in the previous embodiments. The n-drain region  126  may include an n-protrusion  126 P. A first gate electrode  143 A may be formed on sidewalls of the p-vertical channel region  131 P. A first gate dielectric layer  141 A may be formed between the first gate electrode  143 A and the p-vertical channel region  131 P. 
     A second gate electrode  143 B may be formed on the active region  124 . A first source/drain region  127  and a second source/drain region  129  may be formed in the active region  124  adjacent to both sides of the second gate electrode  143 B. A channel region  128  may be defined in the active region  124  between the first and second source/drain regions  127  and  129 . A second gate dielectric layer  141 B may be formed between the second gate electrode  143 B and the channel region  128 . 
     An n-vertical channel region  132 N and a p-source region  134 S may be formed on the p-drain region  136 . The p-drain region  136  may include a p-protrusion  136 P. A third gate electrode  143 C may be formed on sidewalls of the n-vertical channel region  132 N. A third gate dielectric layer  141 C may be formed between the third gate electrode  143 C and the n-vertical channel region  132 N. 
     A gate pad  143 P may be formed on the isolation layer  123 . An etch stop layer  148  may be formed to cover the entire surface of the semiconductor substrate  121 . An interlayer insulating layer  149  may be formed on the etch stop layer  148 . 
     Referring to  FIGS. 8, 9A, 9B, and 9C , a first plug  151 , a second plug  153 , a third plug  154 , a fourth plug  155 , and a fifth plug  156  may be formed through the interlayer insulating layer  149  and the etch stop layer  148 . First and second interconnection lines  157  and  159  may be formed on the interlayer insulating layer  149  to form the resulting semiconductor device. 
     Experimental Example 
       FIGS. 44A and 44B  are current-voltage (IV) graphs showing drain current characteristics of Experimental Examples according to the inventive concepts. In  FIGS. 44A and 44B , the horizontal axis denotes a gate bias voltage expressed in units of volts (V). A vertical axis of  FIG. 44A  denotes a drain current expressed in units of A/μm on a logarithmic scale, while a vertical axis of  FIG. 44B  denotes a drain current expressed in units of μA/μm on a linear scale. 
     Referring to  FIG. 44A , curve L 1  shows a drain current characteristic of a planar transistor having a similar construction to the second gate electrode  43 B of  FIG. 3A , and curves L 2  through L 5  show drain current characteristics of vertical transistors having similar constructions to the p-vertical channel region  31 P and the first gate electrode  43 A of  FIG. 3A . In this case, each of the vertical transistors may be interpreted as a double-gate transistor. In the curve L 1 , the second gate electrode  43 B has a horizontal width Lg of about 16 nm. In the curve L 2 , the p-vertical channel region  31 P has a horizontal width DGt of about 28 nm and a vertical height Lg of about 16 nm. In the curve L 3 , the p-vertical channel region  31 P has a horizontal width DGt of about 22 nm and a vertical height Lg of about 16 nm. In the curve L 4 , the p-vertical channel region  31 P has a horizontal width DGt of about 16 nm and a vertical height Lg of about 16 nm. In the curve L 5 , the p-vertical channel region  31 P has a horizontal width DGt of about 16 nm and a vertical height Lg of about 74 nm. 
     As shown in  FIG. 44A , it can be seen that each of the vertical transistors may exhibit a lower leakage current characteristic than the planar transistor. Also, it can be inferred that with a reduction in the horizontal width DGt of the p-vertical channel region  31 P, the subthreshold current may increase, and off-current may decrease. 
     Referring to  FIG. 44B , it can be seen from curves L 11  to L 51  that each vertical transistor may exhibit a higher on-current characteristic than the planar transistor. Also, it may be inferred that with a reduction in a horizontal width DGt of a p-vertical channel region  31 P, on-current may increase. 
     Embodiment 11 
       FIG. 45  is an equivalent circuit diagram of a CMOS SRAM cell according to an eleventh embodiment of the inventive concept. 
     Referring to  FIG. 45 , the CMOS SRAM cell may include a pair of pull-down transistors TD 1  and TD 2 , a pair of access transistors TA 1  and TA 2 , and a pair of pull-up transistors TU 1  and TU 2 . Both of the pull-down transistors TD 1  and TD 2  and both of the access transistors TA 1  and TA 2  may be NMOS transistors, and both of the pull-up transistors TU 1  and TU 2  may be PMOS transistors. 
     The first pull-down transistor TD 1  and the first access transistor TA 1  may be connected in series to each other. A source of the first pull-down transistor TD 1  may be electrically connected to a ground GND, while a drain of the first access transistor TA 1  may be electrically connected to a first bit line BL 1 . Similarly, the second pull-down transistor TD 2  and the second access transistor TA 2  may be connected in series to each other. A source of the second pull-down transistor TD 2  may be electrically connected to the ground GND, and a drain of the second access transistor TA 2  may be electrically connected to a second bit line BL 2 . 
     Meanwhile, a source and drain of the first pull-down transistor TU 1  may be electrically connected to a power source VDD and a drain of the first pull-down transistor TD 1 , respectively. Similarly, a source and drain of the second pull-up transistor TU 2  may be electrically connected to the power source VDD and a drain of the second pull-down transistor TD 2 , respectively. The drain of the first pull-up transistor TU 1 , the drain of the first pull-down transistor TD 1 , and a source of the first access transistor TA 1  may correspond to a first node N 1 . Also, the drain of the second pull-up transistor TU 2 , the drain of the second pull-down transistor TD 2 , and a source of the second access transistor TA 2  may correspond to a second node N 2 . A gate electrode of the first pull-down transistor TD 1  and a gate electrode of the first pull-up transistor TU 1  may be electrically connected to the second node N 2 , while a gate electrode of the second pull-down transistor TD 2  and a gate electrode of the second pull-up transistor TU 2  may be electrically connected to the first node N 1 . Also, gate electrodes of the first and second access transistors TA 1  and TA 2  may be electrically connected to a word line WL. 
     Each arrow (→) of  FIG. 45  denotes a direction in which current flows. As shown in  FIG. 45 , current may flow through the pull-up transistors TU 1  and TU 2  and the pull-down transistors TD 1  and TD 2  in one direction, while the access transistors TA 1  and TA 2  may operate to have current flow in opposite directions. 
     The semiconductor devices and methods of forming the same described with reference to  FIGS. 1 through 43C  may be variously applied to the CMOS SRAM cell. For example, as described with reference to  FIGS. 2 and 3A , the n-drain region  26 , the p-vertical channel region  31 P, the n-source region  33 S, the first gate dielectric layer  41 A, and the first gate electrode  43 A may correspond to the first pull-down transistor TD 1 . The p-drain region  36 , the n-vertical channel region  32 N, the p-source region  34 S, the third gate dielectric layer  41 C, and the third gate electrode  43 C may correspond to the first pull-up transistor TU 1 . Also, the first source/drain region  27 , the second source/drain region  29 , the channel region  28 , the second gate dielectric layer  41 B, and the second gate electrode  43 B may correspond to the first access transistor TA 1 . 
     The n-drain region  26 , the first plug  51 , the first interconnection line  57 , the second plug  52 , the p-drain region  36 , and the first source/drain region  27  may constitute the first node N 1 . As described above, the first source/drain region  27  may be contiguous with the n-drain region  26 . As a result, an electric resistance of the first node N 1  may be markedly reduced. Furthermore, the sizes of the first source/drain region  27  and the n-drain region  26  may be relatively minimized. That is, a structure in which the first source/drain region  27  and the n-drain region  26  are in continuity with each other and at the same level may be highly advantageous to an increase in the integration density of the CMOS SRAM cell. The first pull-down transistor TD 1  and the first pull-up transistor TU 1  may have a heightened subthreshold characteristics and low leakage current characteristics. In addition, a circuit configuration including a combination of the first pull-down transistor TD 1 , the first pull-up transistor TU 1 , and the first access transistor TA 1  may exhibit remarkably reduced power consumption in a CMOS SRAM cell. 
     Embodiment 12 
       FIGS. 46 and 47  are a perspective view and block diagram, respectively, of an electronic system according to a twelfth embodiment of the inventive concept. 
     Referring to  FIG. 46 , the semiconductor devices and methods of forming the same described with reference to  FIGS. 1 through 45  may be effectively applied to electronic systems  1900 , such as portable telephones, netbooks, laptop computers, or tablet personal computers (PC). 
     Referring to  FIG. 47 , semiconductor devices configured in accordance with the embodiments in connection with  FIGS. 1 through 45  may be applied to an electronic system  2100 . The electronic system  2100  may include a body  2110 , a microprocessor unit (MPU)  2120 , a power unit  2130 , a function unit  2140 , and a display controller unit  2150 . The body  2110  may be a mother board including a printed circuit board (PCB). The MPU  2120 , the power unit  2130 , the function unit  2140 , and the display controller unit  2150  may be mounted on the body  2110 . A display unit  2160  may be disposed inside or outside the body  2110 . For example, the display unit  2160  may be disposed on the surface of the body  2110  and display an image processed by the display controller unit  2150 . 
     The power unit  2130  may receive a predetermined voltage from an external battery (not shown), divide the voltage into voltages having required voltage levels, and supply the divided voltages to the MPU  2120 , the function unit  2140 , and the display controller unit  2150 . The MPU  2120  may receive the voltage from the power unit  2130  and control the function unit  2140  and the display unit  2160 . The function unit  2140  may perform various functions of the electronic system  2100 . For instance, when the electronic system  2100  is a portable phone, the function unit  2140  may include several components capable of portable phone functions, such as the output of an image to the display unit  2160  or the output of a voice to a speaker, by dialing or communication with an external apparatus  2170 . Also, when the electronic system  2100  includes a camera, the electronic system  2100  may serve as a camera image processor. 
     In applied embodiments, when the electronic system  2100  is connected to a memory card to increase the capacity, thereof, the function unit  2140  may be a memory card controller. The function unit  2140  may transmit and receive signals to and from the external apparatus  2170  through a wired or wireless communication unit  2180 . Furthermore, when the electronic system  2100  requires a universal serial bus (USB) to expand functions thereof, the function unit  2140  may serve as an interface controller. 
     Semiconductor devices configured in accordance with the embodiments described above in connection with  FIGS. 1 through 45  may be applied to at least one of the MPU  2120  and the function unit  2140 . For example, the MPU  2120  or the function unit  2140  may include the pull-down transistor TD, the pull-up transistor TU, and the access transistor TA. In this case, the electronic system  2100  may be effectively made more lightweight, thinner, simpler, and smaller and exhibit low power consumption characteristics. 
     According to the embodiments of the inventive concepts, a semiconductor device including a first vertical transistor, a second vertical transistor, and a non-vertical transistor may be provided. A first drain region of the first vertical transistor, a second drain region of the second vertical transistor, a non-vertical drain region of the non-vertical transistor, and a non-vertical source region of the non-vertical transistor may be formed at the same level. One of the non-vertical drain region and the non-vertical source region may be contiguous with the first drain region. The second drain region may be connected to the first drain region. As a result, a semiconductor device that may increase integration density and reduce power consumption may be embodied. 
     The foregoing is illustrative of embodiments and is not to be construed as limiting thereof. Although a few embodiments have been described, those skilled in the art will readily appreciate that many modifications are possible in embodiments without materially departing from the novel teachings and advantages. Accordingly, all such modifications are intended to be included within the scope of the inventive concepts as defined in the claims. Therefore, it is to be understood that the foregoing is illustrative of various embodiments and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims.