# Patent

## Title
Process for manufacturing an improved analog transistor

## Abstract

An analog transistor useful for low noise applications or for electrical circuits benefiting from tight control of threshold voltages and electrical characteristics is described. The analog transistor includes a substantially undoped channel positioned under a gate dielectric between a source and a drain with the undoped channel not being subjected to contaminating threshold voltage implants or halo implants. The channel is supported on a screen layer doped to have an average dopant density at least five times as great as the average dopant density of the substantially undoped channel which, in turn, is supported by a doped well having an average dopant density at least twice the average dopant density of the substantially undoped channel.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a divisional application of U.S. application Ser. No. 13/076,006 filed on Mar. 30, 2011.


FIELD OF THE INVENTION
Improved analog transistors and manufacturing processes that are suitable for providing low noise and/or nanometer scale analog transistors for mixed signal, system on a chip, analog only, or other electronic die are described.
BACKGROUND
Like digital transistors, analog transistors have been available in decreasing sizes over time with transistor channel lengths that formerly were tens of thousands of nanometers being reduced a thousand-fold to a hundred nanometers or less in length. However, maintaining transistor quality and electrical characteristics for such downwardly scaled analog transistors is difficult at nanometer scales and can even be difficult for larger analog transistors useful in ultra-low signal or low noise applications. This is particularly true for mixed signal die that support both analog and digital transistors. Digital transistors and circuits benefit from design and processes that encourage tightly controlled on/off transistor switching while analog circuits often require transistors with linear response over a wide range of input/output conditions, improved Rout, or other electrical characteristics. In addition, analog circuits greatly benefit from analog transistors that have low thermal, shot, flicker, and/or burst noise levels as compared to transistors suitable for digital circuits and by analog transistor pairs (i.e. differential pair transistors) that have closely matched electrical properties, including in particular current-voltage response curves and threshold voltage.
Accordingly, cost effective transistor structures and manufacturing processes are needed for CMOS analog transistors alone or in combination with digital CMOS transistors. Such analog transistors must be reliable at nanometer scales and should not require expensive or unavailable tools or process control conditions. While it is difficult to balance the many variables that control transistor electrical performance, particularly when both analog and digital transistors must be manufactured on the same die with compatible processes, finding suitable transistor dopant structures and manufacturing techniques that result in acceptable noise and electrical characteristics are commercially valuable and necessary.
Having low noise and low variation in transistor characteristics is particularly useful for amplifier circuits. In a multistage analog amplifier circuit, noise output from the final stage can be almost entirely determined by noise generated in the initial stage of the analog amplifying transistors. Such noise created by the initial amplifying stage is carried along and amplified in later stages and can require incorporation of expensive hardware or software to support intensive filtering or noise rejection/compensation techniques. In another example, sense amplifiers are commonly used in digital solid state integrated circuit (IC) applications which require low voltage sensing. Sense amplifiers can be used in memory read circuits for memory bit sensing, in bus signal receivers, and interfacing with low voltage data paths in a processor. Typically, sense amplifiers are formed in the same IC die as the memory storage array and the processor data path. Normally, the inputs are considered analog while the outputs of a sense amplifier being full swing voltages can be digital. Such a conventional sense amplifier at its input has a source coupled matched differential pair, an active load such as a regenerative circuit to provide a full swing, and a current sink or source. However, as gate length and overall dimensions of transistors forming the input pair decrease, variations in the threshold voltage Vt of the input FETs also increases. This leads to increased offset voltages in the input pair and the active load which reduces the sensitivity of the sense amplifier and again increases cost to create additional amplifying or noise rejection circuitry.
Transistor mismatch is of particular concern for systems on a chip (SoC) or other CMOS die having a bandwidth inversely proportional to the device capacitance. As more SoC require greater bandwidth for increasing communication requirements, more SoC analog circuitry is moving towards using logic type transistor devices for high-bandwidth applications since these devices are significantly smaller and hence have much smaller capacitance. However, while bandwidth increases as the size of the devices shrink, so does the Vt mismatch between identical devices. This Vt variation can result from process variations in line etch roughness, oxide thickness, or gate granularity but can also result from more fundamental limitations such as random dopant fluctuations in nanometer sized channels. Unfortunately, Vt mismatch in small devices effectively reduces the headroom which the devices have to operate and can render the circuit useless. In order to achieve high-bandwidth SoC devices, Vt mismatch and especially random dopant fluctuations need to be controlled and reduced.
SUMMARY
Suitable analog transistors, with improved electrical characteristics and having low noise and threshold voltage variation, include a gate having a gate dielectric and gate electrode positioned between a source and a drain. Typically, the analog transistors should have a substantially undoped channel having an average dopant density of less than 5×1017 atoms per cm3 positioned under the gate dielectric between the source and the drain. This can be created by epitaxial growth of intrinsic silicon, growth of another compatible channel material such as silicon germanium, atomic layer deposition, or other conventional process. Care should be taken during processing to prevent dopant migration, diffusion, or inadvertent dopant implant into the channel. Low temperature anneal and manufacture processes as well as haloless or significantly reduced halo dose processes that do not require a standard halo or pocket implant are preferred. Positioned below the substantially undoped channel is a screen layer doped to have an average dopant density at least twice as great as the average dopant density of the substantially undoped channel. Preferably, the screen layer has a dopant density ten to hundred times as great as the average dopant density of the substantially undoped channel and can be formed by epitaxial growth on, or implant into, a doped well. The doped well has an average dopant density less than the screen layer that is typically less than one-tenth to one-fiftieth the dopant density of the screen layer. In certain embodiments, threshold voltage of the analog transistor can be adjusted with a threshold voltage setting layer positioned between the substantially undoped channel and the screen layer and extending at least partially between the source and the drain. In other embodiments, precise dopant control and placement of the screen layer can be used to set threshold voltage while, in still other embodiments, selection of a gate metal helps to set a targeted transistor threshold voltage. Control of length and dopant density for lightly doped drain extensions is still another technique that can be used to modify transistor threshold voltage as is reduction or increase in gate/channel length. As will be appreciated, all of the forgoing threshold voltage adjustment techniques can be used alone or in combination with each other as required to meet analog transistor design specifications.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present disclosure, reference is made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which:
FIGS. 1 and 2 respectively illustrate perspective and side cross sectional views of an analog transistor having an undoped channel and screen layer to set depletion depth;
FIG. 3 illustrates a side cross sectional view of a portion of a die supporting multiple transistor types including an analog transistor such as illustrated in FIGS. 1 and 2 and a smaller analog and/or digital transistor positioned adjacent;
FIG. 4 illustrates an analog transistor with asymmetric threshold voltage set layers and lightly doped drains;
FIG. 5 illustrates an analog transistor with a counterdoped threshold voltage set layer and a reduced length drain side LDD extension to improve capacitance effects;
FIG. 6 illustrates the respective two-dimensional dopant profiles of a haloless undoped channel analog transistor and a similarly sized conventional transistor with a doped channel;
FIG. 7 illustrates a one-dimensional vertical dopant profile of a haloless undoped channel analog transistor and a similarly sized conventional transistor with a doped channel;
FIG. 8 illustrates a two-dimensional profile along the length of the channel for a haloless undoped channel analog transistor and a similarly sized conventional transistor with a doped channel;
FIG. 9 illustrates I-V curves (drain current versus drain voltage) for a haloless undoped channel analog transistor and a similarly sized conventional transistor with a doped channel;
FIG. 10 illustrates simulation runs comparing Rout to Id-on for undoped channel and screen layer supported deeply depleted (DDC) analog transistors and conventional channel implant and halo processed devices;
FIG. 11 illustrates simulation runs comparing Rout for a range of reverse and forward biased voltages of conventional channel dopant implanted and halo processed devices; and
FIG. 12 illustrates simulation runs comparing Rout for a range of reverse and forward biased voltages for haloless undoped channel and deeply depleted (DDC) analog transistors.



DETAILED DESCRIPTION
An improved analog transistor manufacturable on bulk CMOS substrates is seen in perspective view in FIG. 1 and side cross sectional view in FIG. 2. An analog Field Effect Transistor (FET) 100 is configured to have reduced noise, improved mobility, and decreased variation in threshold voltage due, in part, to minimization of channel dopants. The FET 100 has various optional and required structures, including a gate electrode 102, source 104, drain 106, and a gate dielectric 108 positioned over a substantially undoped channel 110. Lightly doped drain extensions (LDD) 113 and 115, positioned respectively adjacent to source 104 and drain 106, extend toward each other, reducing effective length of the undoped channel 110. These structures are supported by a substrate 116 which can include a lightly doped well 114, punch through suppression region 117, and a highly doped screen layer 112.
In this exemplary embodiment, the FET 100 is shown as an N-channel transistor having a source 104 and drain 106 made of N-type dopant material, formed upon a substrate as P-type doped silicon substrate providing a P-well 114 formed on a substrate 116. However, it will be understood that, with appropriate change to substrate or dopant material, a non-silicon P-type semiconductor transistor may be formed from other suitable substrates. The source 104 and drain 106 can be formed preferably using conventional dopant ion implantation processes and materials and may include, for example, modifications such as stress inducing source/drain structures, raised and/or recessed source/drains, asymmetrically doped, counter-doped or crystal structure modified source/drains, or implant doping of source/drain extension regions according to LDD (lightly doped drain) techniques.
The channel 110 contacts and extends between the source 104 and the drain 106 and supports movement of mobile charge carriers between the source and the drain. Preferably, the channel includes substantially undoped silicon having a dopant concentration less than 5×1017 dopant atoms per cm3 adjacent or near the gate dielectric 108. Channel thickness can typically range from 5 to 50 nanometers with exact thickness being dependent on desired transistor operating characteristics and transistor design node (i.e. a 20 nm gate length transistor will typically have a thinner channel thickness than a 45 nm gate length transistor). In certain embodiments the channel 110 is formed by epitaxial growth of pure or substantially pure silicon. Alternatively, silicon germanium or other suitable channel material can be used.
When gate electrode voltage is applied at a predetermined level, the entire volume of the undoped channel 110 is within a depletion zone, since channel depletion depth is a function of the integrated charge from dopants in the doped channel lattice, and the undoped channel 110 has very few dopants. The depletion region when voltage is applied to the gate normally extends from the gate dielectric through the undoped channel 110 and a short distance into the highly doped screen layer 112. While the channel 110 is substantially undoped, and positioned as illustrated above a highly doped screen layer 112, it may be surrounded vertically or laterally by simple or complex structures and layered with different dopant concentrations that can modify various transistor characteristics. These structures or doped layers can include a threshold voltage set layer 111 with a dopant concentration less than the screen layer 112, optionally positioned between the gate dielectric 108 and the screen layer 112 in the channel 110. The threshold voltage set layer 111 permits small adjustments in operational threshold voltage of the FET 100 while leaving the bulk of the channel 110 substantially undoped. In particular, that portion of the channel 110 adjacent to the gate dielectric 108 can remain undoped. Additionally, an optional punch through suppression region 117 is formed beneath the screen layer 112. Like the threshold voltage set layer 111, the punch through suppression region 117 has a dopant concentration less than the screen layer 112 while being higher than the overall dopant concentration of the lightly doped well 114 and substrate 116.
Overall improvement of noise and electrical characteristics for a transistor require careful trade-offs to be made in doping density, length, and depth of the foregoing transistor structures. Improvements made in one area, for example channel mobility, can be easily offset by adverse short channel effects or greater variability in capacitance or output resistance. One particularly critical parameter for analog and digital transistor design is the threshold voltage at which the transistor switches on or off. Threshold voltage in conventional polysilicon gate transistors is commonly set by directly implanting a “threshold voltage implant” into the channel, raising the threshold voltage to an acceptable level that reduces transistor off-state leakage while still allowing speedy transistor switching.
Transistor threshold voltage (Vt) in conventional transistors can also be modified by a technique variously know as “halo” implants, high angle implants, or pocket implants. Such implants create a localized graded dopant distribution near a transistor source and drain that extends into the channel. Halo implants are often required by transistor designers who want to reduce unwanted source/drain leakage conduction or “punch through” current but have the added advantage of adjusting threshold voltage. Unfortunately halo implants tend to introduce additional dopants into the channel. These additional dopants increase the variability of threshold voltage between transistors and decrease mobility and channel transconductance due to the adverse effects of dopant scattering centers in the channel. In addition, halo implants generally require at least two separate processing steps with the die wafer being rotated between different positions (e.g. 0, 90, 180, or 270 degrees) and die with multiple transistor types can even require multiple separate halo implants. Since advanced die manufacturing processes currently require dozens of high angle implants, eliminating or greatly reducing the number of halo implants is desirable for reducing manufacture time and simplifying die processing. For transistors having poly gate structures, threshold voltage setting via halo implants also introduces additional variation in the threshold voltage, since at least a portion of the halo implant can travel through the corner of the poly gate. Since poly gate sidewall shape and crystal structure affect final location of halo dopants in the channel, unavoidable variation in poly gate edge shape and poly gate crystal structure will result in variations in threshold voltage. Such transistor variation can reduce performance of circuit and is of particular concern for paired analog transistors that rely on close matching of transistor characteristics for best performance.
Unfortunately, while useful for setting threshold voltage in conventional transistors, both halo implants and the threshold voltage implant into a previously undoped channel results in permanent contamination of the channel along with a consequent decrease in channel carrier mobility and increase in transistor variation (due to unavoidable variations in channel dopant density). To set threshold voltage to a desired range in FET 100, different threshold voltage modification techniques that do not rely on halo implants (i.e. haloless processing) can be used. This advantageously reduces cost of manufacture because halo implant process steps are not required, reduces the chance of failure due to misaligned halo implants, and eliminates unwanted contamination of the undoped channel. Haloless processing can be used alone, or in combination, with various other threshold voltage setting structures and techniques, including screen layer placement, positioning of intermediate threshold voltage set layers, gate metal selection, lightly doped drain geometry adjustments, and application of bias to the transistor well. Each of these modified structures and/or haloless processing techniques will be discussed in the following description.
As previously noted, the screen layer 112 is a highly doped layer that typically contains dopant atoms with a concentration of between 1×1018 atoms per cm3 and 1×1020 atoms per cm3, positioned under the channel 110 defined below the gate dielectric 108. P-type dopants such as boron are selected for screen layers of NMOS transistors while N-type dopants such as arsenic, antimony, or phosphorus can be selected for PMOS transistors. The presence of a screen layer below the undoped channel 110 is necessary to define a depletion zone beneath the gate. Generally, the greater the distance the screen layer 112 is positioned from the gate dielectric 108, the lower the threshold voltage and, conversely, the closer the screen layer 112 is to the gate dielectric 108, the higher the threshold voltage. As shown in FIGS. 1 and 2, the screen layer 112 can contact the source and drain or optionally can be positioned at a greater distance below the gate to avoid direct contact with the source and the drain (not shown). In certain embodiments, it may be formed as a blanket or sheet extending under multiple source/drain/channel regions (for example, as later discussed with respect to the embodiment of FIG. 4) while, in other embodiments, it may be a self-aligned implant or layer coextensive with the channel. The screen layer 112 thickness can typically range from 5 to 100 nanometers. The screen layer 112 is highly doped relative to the undoped channel 110, the threshold voltage set layer 111 (if provided), and the substrate supported well 114. The peak dopant concentration of the screen layer 112 can be five times or greater than the dopant concentration of the substantially undoped channel 110 with a relative concentration that can be between ten to a hundred times the dopant concentration of the undoped channel 110. In practice, the screen layer 112 is typically doped to have a near uniform concentration of between 5×1018 atoms/cm3 and 5×1019 atoms/cm3. However, embodiments in which the screen layer 112 has a complex dopant profile or reduces sharply in concentration from an initial spike are also contemplated. In certain embodiments, dopant migration resistant layers of carbon, germanium, or the like can be applied along with or above the screen layer 112 to prevent dopant migration into the optional threshold voltage set layer 111 and the undoped channel 110. Typically, the screen layer 112 is formed by a tightly controlled implant into the well 114, but it can be formed as a separate epitaxially grown layer subjected to implant, in-situ doped, or any other conventional or known doping techniques.
As will also be appreciated, position, concentration, and thickness of the screen layer 112 are an important factor in transistor design. In certain embodiments, peak concentration of the screen layer 112 is near the edge of the depletion layer under the gate 102 and the screen layer 112 is located above the bottom of the source 104 and drain 106 junctions. Multiple delta doping implants, broad dopant implants, or long duration in-situ substitutional doping is preferred, since the screen layer 112 should have a finite thickness with 10 nm or greater being preferred. When transistors are configured to have the screen layer 112, the transistor can simultaneously have good threshold voltage matching, high output resistance, low junction leakage, good short channel effects, and still have an independently controlled and strong body effect. Simultaneous provision of these features is difficult for conventional transistors of a similar size. For example, conventional transistors with threshold voltage implants can provide good threshold voltage matching, but cannot simultaneously provide low junction leakage, an independently controlled body effect, or an independently controlled threshold voltage setting. In contrast, transistors designed to have an undoped channel 110, optional threshold voltage set layer 111 (as discussed hereafter), and a thick and highly doped screen layer 112 can simultaneously provide all of the transistor device parameters required for implementation of complex multi-transistor SOC or multi-transistor analog integrated circuits.
Modifying threshold voltage by use of a threshold voltage set layer 111 positioned above the screen layer 112 and below the undoped channel 110 is an alternative technique to conventional threshold voltage implants for adjusting threshold voltage. Care must be taken to prevent dopant migration into the undoped channel 110 and thus use of low temperature anneals and transistor processing is recommended for many applications. The threshold voltage set layer 111 thickness can typically range from 2 to 20 nanometers. The threshold voltage set layer 111 is highly doped relative to the undoped channel 110 but is typically doped to a level one-half to one-tenth that of the screen layer 112. However, embodiments in which the screen layer 112 has complex dopant profile are also contemplated. Like the screen layer 112, in certain embodiments, dopant migration resistant layers of carbon, germanium, or the like can be applied along with or above the threshold voltage set layer 111 to prevent dopant migration into the undoped channel. The threshold voltage set layer 111 can be formed by out-diffusion from the screen layer 112 into an epitaxially grown layer, by implant or in-situ growth of an epitaxial layer on top of the screen layer 112, by delta doping to form an offset doped plane (as disclosed in pending U.S. patent application Ser. No. 12/895,785 filed Sep. 30, 2010, the entirety of which disclosure in herein incorporated by reference), or any other conventional or known doping techniques.
Yet another technique for modifying threshold voltage relies on selection of a gate material having a suitable work function. The gate electrode 102 can be formed from conventional materials, preferably including but not limited to metals, metal alloys, metal nitrides and metal silicides, as well as laminates thereof and composites thereof. In certain embodiments, the gate electrode 102 may also be formed from polysilicon, including for example highly doped polysilicon and polysilicon-germanium alloy. Metals or metal alloys may include those containing aluminum, titanium, tantalum, or nitrides thereof, including titanium containing compounds such as titanium nitride. Formation of the gate electrode 102 can include silicide methods, chemical vapor deposition methods, and physical vapor deposition methods, such as but not limited to evaporative methods and sputtering methods. Typically, the gate electrode 102 has an overall thickness from about 1 to about 500 nanometers. In certain embodiments, metals having work functions intermediate between band edge and mid-gap can be selected. As discussed in pending U.S. patent application Ser. No. 12/960,266 filed Dec. 3, 2010, the entirety of which disclosure is hereby incorporated by reference herein, such metal gates simplify swapping of PMOS and NMOS gate metals to allow a reduction in mask steps and different required metal types for systems on a chip or other die supporting multiple transistor types.
Threshold voltage can also be modified by adjustments to lightly doped drain extensions (LDD) 113 and 115, source/drain extensions that are typically formed by out-diffusion under gate spacers. Source/drain extensions 113 and 115 slightly reduce channel length by extending the source/drain toward each other using dopant implants of the same dopant type as the source 104 and drain 106. Care must be taken to control dopant migration to keep the channel 110 extending between the extensions substantially undoped with a dopant concentration of concentration less than 5×1017 dopant atoms per cm3. As will be appreciated, variations in extension dimensions affect channel electrical characteristics and also result in adjustments to threshold voltage. As shown in FIGS. 1 and 2, the LDD 113 and 115 are symmetrically spaced and extending toward each other with a predetermined length that can be optionally increased or decreased to change threshold voltage. In other embodiments, asymmetrical LDD's are possible with, for example, LDD 115 being configured to extend a greater or lesser extent into the channel 110 than LDD 113, having a greater or lesser dopant density than LDD 113, or extending deeper downward or shallower than LDD 113.
Applied bias to well 114 is yet another technique for modifying threshold voltage of FET 100. The screen layer 112 sets the body effect for the transistor and allows for a higher body effect than is found in state of the art CMOS technologies. For example, a body tap 119 to the well regions 114 of the transistor elements may be formed in order to provide further control of threshold voltage. The applied bias can be either reverse or forward biased and can result in significant changes to threshold voltage. Bias can be static or dynamic and can be applied to isolated transistors or to groups of transistors that share a common well. Biasing can be static to set threshold voltage at a fixed set point or dynamic to adjust to changes in transistor operating conditions or requirements. Various suitable biasing techniques are disclosed in pending U.S. patent application Ser. No. 12/708,497 filed Feb. 18, 2010, the entirety of which is hereby incorporated by reference herein.
The problem of channel dopant contamination can become even more acute when a die supporting multiple transistor types or requiring multiple implants are manufactured. Multiple implants increase the likelihood of dopant diffusion into the channel with each implant becoming a potential source of channel contamination. Eliminating halo implants, while still being able to effectively adjust threshold voltage, greatly improves the ease of manufacturing die supporting multiple transistor types. This is seen in FIG. 3 which illustrates a portion of a multi-transistor die 150 supporting multiple transistor types, including FET 100 of FIGS. 1 and 2, formed on substrate 116 adjacent to a differently sized transistor 120 having substantially different electrical characteristics. Transistor 120 can optionally be a digital or an analog transistor, can have a higher, lower, or identical threshold voltage to FET 100, and can be formed to have a conventional doped channel (i.e. “legacy” transistor) or a substantially undoped channel 110 similar to FET 100. In the illustrated FIG. 3, transistor 120 is shown to have an undoped channel 130, a threshold voltage set layer 131, and a screen layer 132 similar to the structure of FET 100. The screen layer 132 and threshold voltage set layer 131 can be formed by blanket wafer, die, or block implants and/or epitaxial growth that simultaneously form the corresponding screen layer 112, threshold voltage set layer 111, and undoped channel 110. Shallow trench isolation structures 118 can be used to isolate transistors and bias 119 can be applied as needed. However, as will be appreciated, simultaneous layer creation is not required and masking, selective epitaxial growth techniques, and heterogeneous materials and/or implant conditions can be used to create differing types of transistors on the same die. In certain embodiments, transistors having very different electrical characteristics can be formed with the same screen layer 112 and/or threshold voltage set layer 111 or, alternatively, transistors having very different sizes can be formed to have compatible electrical characteristics based on adjustments to screen layer 112 position, threshold voltage set layer 111 presence or absence, LDD geometry and dopant concentration, and/or bias conditions.
Exemplary asymmetric transistor structures are seen in FIG. 4 which illustrates an analog transistor 200 on substrate 216, having a source 204, a drain 206, and an asymmetric threshold voltage set layer 211 on a screen layer 212. The asymmetric threshold voltage set layer 211 only extends partially below a substantially undoped channel 210. FIG. 4 also illustrates other asymmetric structures, including lightly doped drain extensions 213 and 215 that extend differing distances into the channel 210. In addition to extending closer to a center of the gate/channel, LDD 215 is deeper and has a greater dopant density than the shallower LDD 213.
Another contemplated variant transistor structure is shown in FIG. 5 which illustrates an analog transistor 250 on substrate 266 with a substantially undoped channel 260 and a separate counterdoped threshold voltage set layer 261 which has a dopant type opposite to that of screen layer 262 (e.g. N-type threshold voltage set layer 261 versus P-type screen layer 262 or the opposite). This allows for reduction in threshold voltage of transistor 250 as compared to a similar transistor which has no threshold voltage set layer 111 or a transistor with a threshold voltage set layer 111 with the same dopant type as the screen layer 112. FIG. 5 also illustrates a reduced length drain side LDD 265 extension (as compared to LDD 263) which can help to improve gate/drain capacitance effects. As will be appreciated, length of LDD 265 can be slightly less than LDD 263, substantially less than LDD 263, or even absent altogether.
To better understand the differences between conventional doped channel transistors and examples of transistors with undoped channels and haloless processing, chart 300 of FIG. 6 illustrates the respective two-dimensional dopant profiles of a haloless undoped channel analog transistor 302 and a similarly sized doped channel conventional transistor 304. The substantial differences in dopant profile through the channel, screen layer, and into the well is also illustrated by chart 310 in FIG. 7 which shows a one-dimensional vertical dopant profile of a haloless undoped channel analog transistor 314 and a similarly sized conventional transistor 312 with a doped channel. In addition, FIG. 8 is a chart 320 illustrating a two-dimensional profile along the length of the channel for a haloless undoped channel analog transistor and a similarly sized conventional transistor with a doped channel.
Operational differences between undoped channel transistors and conventional doped channel transistors are illustrated by FIGS. 9 and 10. FIG. 9 is a chart 330 illustrating I-V curves (drain current versus drain voltage) for haloless undoped channel analog transistor 334 and a similarly sized conventional transistor 332 with a doped channel. The improved flatter response of the undoped channel transistor 334 is apparent. As another example, FIG. 10 is a chart 360 illustrating simulation runs comparing Rout to Id-on for undoped channel and screen layer supported haloless undoped channel analog transistors and conventional channel implant and halo processed devices. The increased Rout and decreased variation in Rout for a range of drain currents (Id-on) is shown.
The output resistance Rout for analog transistors is a key parameter for determining circuit performance and adjustments to Rout can significantly change circuit efficiency and power requirements. Unfortunately, changing Rout with the combination of biasing and process variations such as well dopant implant concentration changes can be difficult. This is seen in chart 370 of FIG. 11 which is a simulation illustrating minimal changes in Rout for a wide range of reverse and forward well substrate bias voltages even with increasing well dopant density for an NMOS analog transistor with halo implants and a doped channel. In contrast, FIG. 12 is a chart 380 illustrating a substantial increase in Rout range that is possible with reverse or forward bias voltages and shows dramatic changes with increasing or decreasing screen dopant density. This simulation uses an NMOS analog transistor similar to that simulated in FIG. 11 except for the additional screen layer and use of an undoped channel.
Transistors created according to the foregoing embodiments will have a reduced mismatch as compared to conventional MOS transistors. As will be understood, analog transistors are often larger than digital transistors, since analog transistors and circuits are more affected by Vt mismatch between identical devices. This Vt variation, due in part to process variations in line etch roughness, oxide thickness, or gate granularity as well as more fundamental limitations such as random dopant fluctuations in nanometer sized channels, is more controllable for larger transistors. Fortunately, Vt mismatch in small transistor devices built with a screen layer 112 and an undoped channel 110 as described herein can effectively increase headroom which the devices have to operate. This allows high-bandwidth SoC devices with improved sensitivity and performance through improved Vt matching and reduction in variation due to random dopant fluctuations that affect carrier mobility, capacitance, and other channel dependent transistor characteristics.
As will be understood, wafers and die supporting multiple transistor types, including those with and without the described dopant structures, those having different threshold voltages, and with and without static or dynamic biasing, are contemplated. Electronic devices that include the disclosed transistor structures can incorporate a die configured to operate as “systems on a chip” (SoC), advanced microprocessors, radio frequency, memory, and other die with one or more digital and analog transistor configurations and are capable of supporting a wide range of applications, including wireless telephones, communication devices, “smart phones”, embedded computers, portable computers, personal computers, servers, and other electronic devices. Electronic devices can optionally include both conventional transistors and transistors as disclosed, either on the same die or connected to other die via motherboard, electrical or optical interconnect, stacking, or through use of 3D wafer bonding or packaging. According to the methods and processes discussed herein, a system having a variety of combinations of analog and/or digital transistor devices, channel lengths, and strain or other structures can be produced on silicon using planar bulk CMOS processing techniques. In different embodiments, the die may be divided into one or more areas where dynamic bias structures, static bias structures, or no-bias structures exist separately or in some combination.
Although the present disclosure has been described in detail with reference to a particular embodiment, it should be understood that various other changes, substitutions, and alterations may be made hereto without departing from the spirit and scope of the appended claims. Numerous other changes, substitutions, variations, alterations, and modifications may be ascertained by those skilled in the art and it is intended that the present disclosure encompass all such changes, substitutions, variations, alterations, and modifications as falling within the spirit and scope of the appended claims. Moreover, the present disclosure is not intended to be limited in any way by any statement in the specification that is not otherwise reflected in the appended claims.

## Claims

1. A process for manufacturing an analog transistor comprising:
providing a doped well;
forming a screen layer that contacts and overlies at least a portion of the doped well;
forming an epitaxial undoped channel layer above the screen layer, and the undoped channel not being subjected to contaminating threshold voltage implants or halo implants;
forming a dopant migration resistant layer above the screen layer;
forming a gate dielectric and gate electrode above the undoped channel and positioned between a source and a drain, the source and drain configured to respond to an analog signal; and
maintaining process conditions so that a portion of the undoped channel adjacent to the gate dielectric remains undoped in the final analog transistor.

2. The process of claim 1, further comprising the step of forming a threshold voltage setting layer positioned between the undoped channel and the screen layer, the threshold voltage setting layer extending at least partially between the source and the drain.

3. The process of claim 2, further comprising the step of counter doping the threshold voltage setting layer with respect to the screen layer.

4. The process of claim 2, further comprising the step of forming an asymmetric threshold voltage setting layer that extends only partially between the source and the drain.

5. The process of claim 2, wherein the threshold voltage setting layer is formed by out diffusion from the screen layer into an epitaxially grown layer.

6. The process of claim 2, wherein the threshold voltage setting layer is formed by growing an epitaxial layer on the screen layer and doping the epitaxial layer through either implantation or in-situ doping.

7. The process of claim 2, wherein the threshold voltage setting layer is formed by delta doping to form an offset doped plane.

8. The process of claim 2, wherein the threshold voltage setting layer is formed as part of a blanket layer underlying a plurality of gate dielectrics and gate electrodes.

9. The process of claim 2, further comprising:
forming a dopant migration resistant layer above the threshold voltage setting layer.

10. The process of claim further comprising the steps of:
forming a first channel LDD extending from the source toward the drain for a first distance; and
forming a second channel LDD extending from the drain toward the source for a second distance selected to be less than the first distance.

11. The process of claim 1, wherein the gate electrode is formed by depositing a gate metal, wherein the gate metal is selected to have a work function intermediate between band edge and midgap.

12. The process of claim 1, further comprising the step of forming an electrical tap to the doped well to permit biasing that adjusts threshold voltage.

13. The process of claim 1, wherein the screen layer is formed as part of a blanket layer underlying a plurality of gate dielectrics and gate electrodes.

14. The process of claim 1, wherein the screen layer is formed by epitaxial growth subject to at least one of implantation and in situ doping.

15. The process of claim 1, wherein the screen layer is positioned above the bottom of the source and drain.

16. A process for manufacturing an analog transistor comprising: providing a doped well; forming a screen layer that contacts and overlies at least a portion of the doped well; forming an epitaxial undoped channel layer above the screen layer, and the undoped channel not being subjected to contaminating threshold voltage implants or halo implants; forming a threshold voltage setting layer positioned between the undoped channel and the screen layer, the threshold voltage setting layer extending at least partially between a source and a drain; forming a gate dielectric and gate electrode above the undoped channel and positioned between the source and the drain, the source and drain configured to respond to an analog signal; forming an electrical tap to the doped well to permit biasing that adjusts threshold voltage; maintaining process conditions so that a portion of the undoped channel adjacent to the gate dielectric remains undoped in the final analog transistor.

17. The process of claim 16, further comprising counter doping the threshold voltage setting layer with respect to the screen layer.

18. The process of claim 16, wherein the gate electrode is formed by depositing a gate metal, wherein the gate metal is selected to have a work function intermediate between band edge and midgap.

19. The process of claim 16, further comprising:
forming a first dopant migration resistant layer above the threshold voltage setting layer;
forming a second dopant migration resistant layer above the threshold voltage setting layer.