Abstract:
The present invention provides a silicon-compatible germanium-based high-hole-mobility transistor with high-hole-mobility germanium channel comprising a semiconductor material having a valence band offset instead of the conventional gate insulating film, a germanium channel region, and a quantum well formed by heterojunctions of the upper and lower portions of the germanium channel on a silicon substrate. Thus, the present invention enables to gain maximum hole mobility of the germanium channel by using the two-dimensional hole gas gathered into the quantum well for high-speed and low-power operations and device reliability improvement.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. 119 of Korean Patent Application No. 10-2013-0147924 filed on Nov. 29, 2013, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a semiconductor transistor, and more particularly to a silicon-compatible germanium-based high-hole-mobility transistor (HHMT) with high-hole-mobility. 
     2. Description of the Related Art 
     Throughout the history of semiconductor transistor development, a large number of high-electron-mobility transistors (HEMTs) using high-electron-mobility have been studied actively, but only a few high-hole-mobility transistors (HHMTs) with high-hole-mobility are studied. 
     This is because a large number of semiconductor materials having high-electron-mobility are present, but the semiconductor materials having high-hole-mobility are extremely rare. 
     Germanium has recently been studied for implementing germanium-based p-type metal-oxide-semiconductor field effect transistors (MOSFETs) due to its intrinsically high-hole-mobility. However, there have been a complexity of the manufacturing processing and a limitation of techniques for getting enough high hole mobility of germanium because of a high channel doping concentration required inevitably in a highly scaled channel. 
     For example, Korean Patent Nos. 10-0585111 and 10-0644811 disclose techniques for getting carrier mobility of channel higher than that of silicon by forming a germanium channel region. However, there have been such problems that it is difficult to form an oxide film as a gate insulating film on a germanium channel region unlike silicon and to get enough high hole mobility of a germanium channel because of a carrier scattering and the like at the interface between an oxide film and a germanium channel though the gate insulating film being formed of any oxide on the germanium channel region. 
     SUMMARY OF THE INVENTION 
     The present invention is contrived to overcome the mentioned problems. Also, the objective of the present invention is to provide a silicon-compatible germanium-based high-hole-mobility transistor with a high-hole-mobility germanium channel and a semiconductor material having a predetermined or more valence band offset to insulate a gate from the germanium channel instead of the conventional gate insulating film, e.g., the oxide film, to be able to be formed on a silicon substrate and to prevent a leakage current from the silicon substrate. 
     To achieve the objectives, a transistor comprising: a silicon substrate; a germanium layer formed on the silicon substrate; a semiconductor material pattern formed on the germanium layer by a heterojunction, the semiconductor material pattern having a lattice constant 2% or less different from that of the germanium layer and a predetermined or more valence band offset ΔEv from below a valence band edge of the germanium layer; a gate formed on the semiconductor material pattern; and source and drain regions formed separately in the germanium layer at both sides of the semiconductor material pattern. 
     The semiconductor material pattern may be formed of Al x Ga 1-x As, and the aluminum (Al) fraction, x, may be x&lt;0.7. 
     The germanium layer may form a channel region between the source and drain regions, the channel region may have a quantum well formed by heterojunctions with the semiconductor material pattern and the silicon substrate, and a two-dimensional hole-gas flow may be formed between the source and drain regions through the quantum well in an ON state. 
     The two-dimensional hole-gas flow may be concentrated at the interface between the semiconductor material pattern and the channel region in the quantum well. 
     The channel region may be an intrinsic undoped region, and the semiconductor material pattern, the source region, the drain region and the silicon substrate may be doped with p-type impurities. 
     The channel region may be formed at the bottom of a trench formed in the germanium layer between the source and drain regions, and the semiconductor material pattern may have a shape filled in the trench. 
     The thickness of the channel region may be 10˜30 nm. 
     The present invention provides a quantum well formed by heterojunctions of the upper and lower portions of germanium channel region contacting with a gate insulating film formed of semiconductor material having a valence band offset and a silicon substrate, respectively. It is possible to gain the maximum hole mobility of germanium by using the two-dimensional hole gas gathered into the quantum well and to have the effects of high speed, low power operation and device reliability improvement. 
     The present invention can omit an oxide film step, etc. for simplification and easiness of a fabricating process. Especially, the present invention can be fabricated together with optoelectronic elements in silicon photonics and optoelectronic integrated circuit systems in which germanium is used as a light emitting layer on a silicon substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a transistor structure, especially, a simulated structure according to an embodiment of the present invention showing a two-dimensional hole gas  22  formed in a channel region  21  when a voltage is applied in an ON state. 
         FIG. 2  is a perspective view of a transistor structure according to another embodiment of the present invention showing a two-dimensional hole gas  22  formed in a channel region  21  when a voltage is applied in an ON state. 
         FIGS. 3(A)-3(D)  are the energy-band diagrams [ FIG. 3(A)  and  FIG. 3(C) ] and [ FIG. 3(B)  and ( FIG. 3D )] obtained in the vertical direction from a semiconductor material pattern  30  to a silicon substrate  10  and in the horizontal direction from a source region  24  to a drain region  26  through a channel region  21 , respectively, when a voltage is applied in an OFF state [ FIG. 3(A)  and  FIG. 3(B) ] or an ON state [ FIG. 3(C)  and  FIG. 3(D) ]. 
         FIG. 4  is a simulation result showing a two-dimensional hole gas  22  formed in a channel region  21  when a voltage is applied in the ON state [(C) and (D)] of  FIG. 3 . 
         FIGS. 5(A)-5(D)  are simulation results showing electrical characteristics of  FIG. 1 .  FIG. 5(A)  and  FIG. 5(B)  show transfer and output characteristics, respectively, where a channel length is L G =200 nm, a germanium layer thickness of a channel region is T Ge =10 nm, and an aluminum Al fraction x of a semiconductor material pattern Al x Ga 1-x As is x=0.3.  FIG. 5(C)  and  FIG. 5(D)  show a driving current I on  and a subthreshold swing S depending on a channel length and a threshold voltage V th  and a drain-induced barrier lowering DIBL, respectively. 
         FIGS. 6(A)-6(C)  are simulation results showing electrical characteristics according to a germanium layer thickness T Ge  of a channel region, where a channel length is L G =200 nm and an aluminum Al fraction x of a semiconductor material pattern Al x Ga 1-x As is x=0.3.  FIG. 6(A)  shows an operation current and ON/OFF current ratio,  FIG. 6(B)  shows a subthreshold swing S, and  FIG. 6(C)  shows a leakage current I off . 
         FIGS. 7(A)-7(C)  is a simulation result showing electrical characteristics according to an aluminum Al fraction x of a semiconductor material pattern Al x Ga 1-x As, where a channel length is L G =200 nm, a germanium layer thickness of a channel region is T Ge =10 nm.  FIG. 7(A)  shows an operation current I on  and ON/OFF current ratio,  FIG. 7(B)  shows a subthreshold swing S, and  FIG. 7(C)  shows a threshold voltage V th . 
     
    
    
     In these drawings, the following reference numbers are used throughout: reference number  10  indicates a silicon substrate,  20  a germanium layer,  21  a channel region,  22  a two-dimensional hole gas,  24  a source region,  26  a drain region,  30  a semiconductor material pattern, and  40  a gate. 
     DETAILED DESCRIPTION 
     Detailed descriptions of preferred embodiments of the present invention are provided below with accompanying drawings. 
     A transistor according to an embodiment of the present invention comprises, as commonly shown in  FIGS. 1 and 2 , a silicon substrate  10 ; a germanium layer  20  formed on the silicon substrate  10 ; a semiconductor material pattern  30  formed on the germanium layer  20  by a heterojunction, a the semiconductor material pattern  30  having a lattice constant 2% or less different from that of the germanium layer  20  and a predetermined or more valence band offset ΔEv from below a valence band edge of the germanium layer  20 ; a gate  40  formed on the semiconductor material pattern  30 ; and source and drain regions  24  and  26  formed separately in the germanium layer  20  at both sides of the semiconductor material pattern  30 . 
     Here, the semiconductor material pattern  30  replaces an oxide film used as the conventional gate insulating film. Also, it is preferable that the semiconductor material pattern  30  is formed of semiconductor materials having the following physical characteristic. 
     First, the semiconductor material pattern  30  may be formed on the germanium layer  20  by a heterojunction, wherein it is preferable that a lattice constant of the semiconductor material pattern  30  is 2% or less different from that of the germanium layer  20 . By this configuration, the semiconductor material pattern  30  can be formed with a minimum lattice mismatch through a crystalline growth on a channel region  21  of the germanium layer  20 . Thus, this configuration enables to remove the factors which suppress the flow of carriers, namely the below mentioned flow of a two-dimensional hole gas, at the junction interface between the semiconductor material pattern  30  and the channel region  21  and to obtain the maximum hole mobility of the germanium channel region  21 . In other words, if the semiconductor material pattern  30  is formed of a semiconductor material having a lattice constant more than 2% different from that of germanium, it is difficult to gain the maximum hole mobility as one of the objectives of the present invention. 
     Second, because the semiconductor material pattern  30  can replace the conventional gate insulating films formed of various oxide films, it is preferable that the semiconductor material to form the semiconductor material pattern  30  is selected to have a predetermined or more valence band offset ΔEv from below a valence band edge of the germanium layer  20 . Here, the valence band offset ΔEv may have a size only to confine hole carriers, i.e., a two-dimensional hole gas, e.g., to form at least one side wall of a quantum well mentioned below for protecting the hole carriers in the channel region  21 . 
     An exemplary material to form the semiconductor material pattern  30  is Al x Ga 1-x As. If the above conditions are satisfied, it is not limited to Al x Ga 1-x As. 
     In case of Al x Ga 1-x As, the valence band offset ΔEv against germanium is calculated by Equation 1, and it surely meets the second condition.
 
Δ Ev= 0.834+0.147 x  [eV]  (1)
 
     Here, x is a mole fraction of aluminum (Al) and is preferably x&lt;0.7. This is because when x is 0.7, the lattice constant becomes similar to that of germanium allowing good compatibility on the germanium layer  20 , but when x is more than 0.7, the lattice constant becomes greater than that of germanium and it causes a process difficulty. 
     GaAs, as x=0, also guarantees the good compatibility on the germanium (Ge) layer because GaAs has a lattice constant equal to that of Ge, and it completely satisfies the first condition. 
     The germanium layer  20  can be formed on the silicon substrate  10  by a heterojunction. Because of about 4% lattice mismatch with silicon, it can be just grown to a predetermined thickness through a crystalline growth. However, it is possible to form SiGe firstly on the silicon substrate and then the germanium layer  20  can be formed on the interposed SiGe. Thus, the words of “the germanium layer  20  is formed on the silicon substrate  10 ” in this description should be interpreted to cover the previous two cases. 
     Also, the germanium layer  20  is used to form a channel region  21  in many different forms according to the concrete shapes of the transistor devices. As shown in  FIG. 2 , a germanium layer  20  formed on a silicon substrate  10  can be just used to form separately source  24  and drain  26  regions and then a channel region  21  is interposed between the source and drain regions. Also, as shown in  FIG. 1 , a trench is formed in the germanium layer  20 , a semiconductor material pattern  30  can be embodied as a shape filled in the trench, source  24  and drain  26  regions can be formed in the germanium layer  20  at both sides of the semiconductor material pattern  30 , and the germanium layer  20  as the bottom of the trench under the semiconductor material pattern  30  can be used as a channel region  21 . Thus, the invented device cannot be limited to the structures mentioned above and can be in various structures according to the fabricating processes. 
     The fabricating processes of the transistors according to the present invention can be a little bit different from each other depending on concrete device structures, but it can commonly form a germanium layer  20 , a semiconductor material pattern  30  and the like on a silicon substrate  10  through a crystalline growth. Thus, it is possible to omit many steps related to fabricating an oxide film essentially processed for forming the conventional gate insulating film or for removing the dangling bond states generated at the interface between the gate insulating film and the germanium channel region. Therefore, it has advantages to simplify the fabrication processes and make them easier. 
     On the other hand, the device structure of  FIG. 1  offers favorable advantages compared with the structure of  FIG. 2 , for maintaining high-speed operation capability, etc., because the gate  40  has an excellent channel control. 
     According to the structure of  FIG. 1 , the thickness of the channel region, namely the thickness of the germanium layer  20  of the channel region is preferably 10˜30 nm. This is based on the simulation result on the electrical characteristics according to the germanium layer thickness T Ge  when the channel length is L G =200 nm, and the aluminum Al fraction x of the semiconductor material pattern Al x Ga 1-x As is x=0.3 in the structure of  FIG. 1 . When the germanium layer thickness T Ge  of the channel region is larger than 30 nm, the gate  40  loses the channel control and the driving current I on  gain is slight, but the ON/OFF current ratio drops [refer to  FIG. 6(A) ], the subthreshold sing S increases for the switching characteristic loss [refer to  FIG. 6(B) ], and the leakage current I off  increases [refer to  FIG. 6(C) ]. When the germanium layer thickness T Ge  is less than 10 nm, the lattice mismatch between the germanium layer  21  and the silicon substrate  10  can induce defective effects on hole mobility. 
     As shown in  FIGS. 1 and 2 , when the germanium layer  20  or the germanium channel region  21  contacts to form a heterojunction with the silicon substrate  10 , because germanium can have a valence band offset ΔEv from below a valence band edge of silicon, as shown in  FIGS. 3(A) and 3(C) , a quantum well to trap holes can be formed in the germanium channel region  21  not only in the OFF state [ FIG. 3(A) ], but also in the ON state [ FIG. 3(C) ] as shown in the energy-band diagrams obtained in the vertical direction from a semiconductor material pattern  30  to a silicon substrate  10 . 
     By this configuration, according to a voltage applied to a device, a flow of two-dimensional hole gas between source and drain regions can occur through the quantum well and realize high-speed and low-power operations. 
     Especially, at the ON operation of the device, it is preferable that the two-dimensional hole gas  22 , as shown in  FIGS. 1 and 2 , is concentrated at the interface between the semiconductor material pattern  30  and the channel region  21 . This is because the above configuration can prevent the hole-mobility loss by the lattice mismatch at the opposite side of the interface of the quantum well, namely at the interface between the germanium channel layer  21  and the silicon substrate  10 , and realize the high-speed and low-power operations. 
       FIG. 4  is a simulation result diagram showing a two-dimensional hole gas  22  formed at the upper interface of a channel region  21  when a voltage is applied as the ON state [(C) and (D)] of  FIG. 3 . 
       FIG. 5  is a simulation result showing electrical characteristics of  FIG. 1 .  FIG. 5(A)  and  FIG. 5(B)  show transfer and output characteristics, respectively, where a channel length is L G =200 nm, a germanium layer thickness of a channel region is T Ge =10 nm, and an aluminum Al fraction x of a semiconductor material pattern Al x Ga 1-x As is x=0.3.  FIG. 5(C)  and  FIG. 5(D)  show a driving current I on  and a subthreshold swing S depending on a channel length and a threshold voltage V th  and a drain-induced barrier lowering DIBL, respectively. 
       FIG. 6  is a simulation result showing electrical characteristics according to a germanium layer thickness T Ge  of a channel region, where a channel length is L G =200 nm and an aluminum Al fraction x of a semiconductor material pattern Al x Ga 1-x As is x=0.3.  FIG. 6(A)  shows an operation current and ON/OFF current ratio,  FIG. 6(B)  shows a subthreshold swing S, and  FIG. 6(C)  shows a leakage current Ioff. 
       FIG. 7  is a simulation result showing electrical characteristics according to an aluminum Al fraction x of a semiconductor material pattern Al x Ga 1-x As, where a channel length is L G =200 nm, a germanium layer thickness of a channel region is T Ge =10 nm.  FIG. 7(A)  shows an operation current I on  and ON/OFF current ratio,  FIG. 7(B)  shows a subthreshold swing S, and  FIG. 7(C)  shows a threshold voltage V th . 
       FIGS. 3 to 7  are based on the p-type germanium high-hole-mobility transistor (p-GeHHMT), as the structure of  FIG. 1 . The channel region  21  is an intrinsic undoped region, and the semiconductor material pattern  30 , the source region  24 , the drain region  26  and the silicon substrate  10  are doped with p-type impurities. 
     Also, although the unmentioned gate  40  material can be formed of any conductive materials, the simulation results of  FIGS. 3 to 7  are obtained by the gate  40  made of A 1  in  FIG. 1 . 
     This work was supported by the Center for Integrated Smart Sensors funded by the Korean Ministry of Science, ICT &amp; Future Planning as Global Frontier Project (CISS-2012M3A6A6054186).