Low power AlGaAs/GaAs complementary FETs incorporating InGaAs n-channel gates
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present application is a continuation-in-part of the Abrokwah et al, application Ser. No. 747,501, filed June 21, 1985 and now abandoned. 
    
    
     This invention is directed to the fabrication of low power n-channel and p-channel (Al,Ga)As Heterostructure Insulated Gate Field-Effect Transistors (HIGFETs) on a common planar wafer surface to form high performance GaAs based complementary integrated circuits. In the operation of heterostructure insulated gate FETs with structures where the gate is directly deposited on AlGaAs, the threshold voltage (V t ) of the n-channel FET is near 1 V (a difference of the WSi metal barrier height and the AlGaAS/GaAs conduction band discontinuity). This high V t  requires correspondingly high power supply voltages (2-3 V) for device operation and therefore increased level of gate leakage with reduced voltage swing and noise margin. This invention reduces the HIGFET V t  using a pseudomorphic InGaAs semiconductor gate, to allow low power circuit operation. 
     The normal heterostructure used for HIGFETs consists of an undoped GaAs buffer layer grown on a semi-insulating GaAs substrate followed by an undoped (Al,Ga)As gate layer. Both of these layers are epitaxially grown under conditions to make them as close to intrinsic and insulating as possible. Both the n- and p-channel HIGFETs utilize the high mobility, two-dimensional (2D) electron (hole) gas which is induced at the (Al,Ga)As-GaAs hetero-interface by the application of a suitable gate bias voltage. A self-aligned gate (SAG) process is used to form the source drain regions of the transistors with p +  implanted regions for p-channel and n +  implanted regions for n-channel HIGFETs. The electrons (holes) in the channel come from the source drain contacts when a suitable gate bias is applied. The use of undoped heterostructures allows both n- and p-channel transistors to be fabricated in the same epitaxial layers on the same wafer surface by a planar process. The absence of impurities also leads to high electron (hole) channel mobility resulting in FET operation in the saturation velocity regime for high transconductance transistors. Complementary GaAs based circuits which utilize both n- and p-channel transistors offer significant advantages over circuits utilizing only n-channel transisitors in terms of noise margin and power dissipation and consequently circuit integration level. 
     The prior art has not taught this type of complementary planar structure in which both the n- and p-channel transistors utilize a 2D electron (hole) gas in an undoped high mobility channel. An article by Katayama et al, &#34;A New Two-Dimensional Electron Gas Field-Effect Transistor Fabricated on Undoped AlGaAs-GaAs Heterostructure&#34;, Jpn. J. Appl. Phys., Vol. 23, L150-2, March 84, shows an n-channel device which utilizes an undoped AlGaAs gate and is a 2D electron gas field-effect transistor. It is not a complementary device. Solomon et al, &#34;A GaAs Gate Heterojunction FET&#34;, IEEE Electron Device Letters, Vol. 5, No. 9, Sept. 84, p 379-80, describe an n-channel heterostructure FET with an insulating gate but this also is not a complementary device. A different kind of complementary device is shown in an article by Kiehl et al, &#34;Complementary p-MODFET and n-HB MESFET (Al,Ga)As Transistors&#34;, IEEE Electron Device Letters, Vol. EDL-5, No. 12, Dec. &#39;84, p 521-23. A similar disclosure is shown in Kiehl et al, &#34;Complementary p-MODFET and n-HB MESFET (Al,Ga)As FET&#39;s&#34;, IEDM &#39;84, p 854-55. Both of these publications describe a complementary device structure consisting of a conventional p-channel MODFET, made with a doped (Al,Ga)As gate and an n-channel MESFET. This is not a planar device and does not utilize a 2-dimensional electron and hole gas in an undoped high mobility channel for both the n- and p-channel devices. In the substantially different JFET technology, the fabrication of n-channel and p-channel GaAs-based devices on the same wafer has been disclosed, as for example by Zuleeg et al, &#34;Double-Implanted GaAs Complementary JFET&#39;s&#34;, IEEE Electron Device Letters, Vol. EDL-5, No. 1, Jan. 1984, p 21-23. J-FET devices do not utilize 2-dimensional electron (hole) gases in undoped, high mobility channels. 
     This subject matter herein is related to a copending application of Cirillo et al, Ser. No. 668,586, filed Nov. 5, 1984 (now U.S. Pat. No. 4,662,058), titled &#34;Self-aligned Gate Process for ICs based on Modulation Doped (Al,Ga)As/GaAs FETs&#34;, and assigned to the same assignee as the present invention. The self-aligned gate process of that application is also used in the present invention. 
     The HIGFET approach takes maximum advantages of the heterostructure technology to create two-dimensional (2D) gases of both electrons and holes in a planar undoped multilayer (Al,Ga)As/GaAs structure. The 2D, electron (hole) gas is created at the (Al,Ga)As/GaAs heterointerface by applying a positive (negative) gate bias. The 2D electron (hole) gas has a high electron (hole) mobility because of the reduction of the impurity scattering. This allows FET operation in the velocity saturation regime at low voltages for high transconductance transistors. The SAG approach is essential for HIGFET devices since electrical contact must be made directly to the 2D high mobility electron and hole gases, which are present only under the gates. The planar structure of this invention is important to achieve the high processing yield required for VLSI circuits. The insulating gate approach for both the n- and p-channel devices allows a larger voltage swing on the input gates without leakage or shorting. The insulating gates also improve the threshold voltage uniformity and control since the threshold voltage is much less critically dependent on the thickness and doping of the gate layer than normal MODFET device structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b are schematics of the device cross section for a n-channel HIGFET and a p-channel HIGFET. 
     FIG. 2a shows the band structure at the point of establishing the 2D electron gas for the n-channel device of FIG. 1a; FIG. 2b shows the band structure at the point of establishing the 2D hole gas for the p-channel device of FIG. 1b; and FIG. 2c shows the band structure at zero bias of the normally off (non-conductive) enhancement type complementary devices. 
     FIG. 3 shows the cross section of the complementary HIGFET inverter. 
     FIG. 4 shows the processing steps for the complementary HIGFET fabrication. 
     FIG. 5 shows band structure of an alternate HIGFET device structure. 
     FIG. 6 is an electrical line schematic of the CMOS-like GaAS structure shown in FIG. 3. 
     FIG. 7 is a graphical presentation of the transfer characteristics and transconductance of n-HIGFET at 77k. 
     FIG. 8 is a graphical presentation of the transfer characteristics and transconductances of p-HIGFET at 77k. 
     FIG. 9 shows a preferred embodiment and is a modification of FIG. 3 for low power use as taught in this application. 
     FIG. 10 is a schematic band diagram of the n-channel and p-channel devices shown in FIG. 9. 
    
    
     DESCRIPTION 
     The HIGFET device approach is illustrated in FIGS. 1 and 2. FIG. 1 shows schematically a cross section view of n- and p-channel devices while FIG. 2 shows schematically the energy band diagrams for the HIGFET devices starting at the gate electrode 15 and going perpendicular to the wafer surface. The HIGFET utilizes two epitaxial layers 12 and 13 grown on a semi-insulating GaAs wafer 11. The first layer 12 is pure, undoped GaAs, which is grown under conditions to make it as close to intrinsic as possible. The second layer 13 is a pure and undoped layer of (Al,Ga)As, which is again grown under conditions to make it as close to intrinsic as possible. 
     The basic mechanisms which give rise to n- and p-channel HIGFETs are shown in FIG. 2. FIG. 2c shows the relative positions of the conduction and valence bands in the insulating GaAs layer, the insulating (Al,Ga)As layer and the position of the Fermi level of the metal gate electrode in a device without external electrical bias. The position of the Fermi level is shown to be midway between the conduction and valence bands of the GaAs and (Al,Ga)As layers for purposes of illustration and in practice the exact Fermi level position will depend on the amounts and types of residual impurities and defects in these undoped layers. Similarly, the gate metal Fermi level is shown midway between the conduction and valence bands of the (Al,Ga)As layer, but its actual position will depend on the work functions of the metal chosen and the position of the Fermi level at the surface of the (Al,Ga)As surface. In the unbiased state shown in FIG. 2c, a conducting channel does not exist under the gate, i.e. both the n- and p-channel complementary HIGFETs are normally-off enhancement devices. The 2D high mobility electron (n-channel) gas or hole (p-channel) gas is created at the (Al,Ga)As/GaAs heterointerface by applying a positive (or a negative) gate voltage between the gate and the source contact, thereby providing in effect a current channel or trough between source and drain implanted regions which act as electron (or hole) reservoirs. The source and drain regions are heavily doped by ion implantation and have Fermi levels near the conduction (valence) band edges. FIGS. 2a and 2b show bands with applied gate voltages. In FIG. 2a, a positive gate bias is applied and when E c  crosses the Fermi level, E F , a 2D electron gas is induced. Likewise in FIG. 2b a negative gate bias is applied and when E v  crosses the Fermi level, E F , a 2D hole gas is induced. The electrons (or holes) are provided by the n +  type (or p +  type) implanted regions, and the electron (or hole) density is controlled by the gate voltage. To first order, the threshold voltages of these devices are determined by the gate barrier height φ and the energy band discontinuities ΔE: 
     
         V.sub.tn =φ.sub.n -ΔE.sub.c,                     (1) 
    
     
         V.sub.tp =-φ.sub.p +ΔE.sub.v.                    (2) 
    
     The two-dimensional electron and hole gas densities, n s  and p s , respectively, are determined to first order by the relations: 
     
         n.sub.s =ε(V.sub.g -V.sub.tn)/q(d+Δd.sub.n)  (3) 
    
     
         p.sub.s =ε(-V.sub.g +V.sub.tp)/q(d+Δd.sub.p) (4) 
    
     where d is the thickness of the (Al,Ga)As insulator, q is the electronic charge, and Δd is the width of the 2D gases. The threshold voltage can be minimized and the two-dimensional electron and hole gas densities can be maximized by making ΔE c  and ΔE g  as large as possible. The magnitude of ΔE c  and ΔE v  both increase with the fraction x of Al in the Al x  Ga 1-x  As layer. Consequently, the device physics favors as large an Al composition as possible, commensurate with the technical problems associated with using AlAs or near AlAs composition materials in devices. Grading of the composition of the i-AlGaAs layer between the i-GaAs interface and the surface may be desirable to minimize the problems associated with high Al compositions. 
     The structure and fabrication of our complementary n- and p-channel HIGFET will now be described. Referring to FIG. 1a there is shown first an n-channel portion of a planar complementary HIGFET device. A semi-insulating GaAs substrate wafer 11 has grown thereon by molecular beam epitaxy (MBE) an insulating GaAs (i-GaAs) layer 12. Then an insulating aluminum gallium arsenide (i-(Al,Ga)As) layer 13 is grown by MBE. Alternately, MOCVD could be used to form these epitaxial layers. On the epitaxial surface is fabricated a gate such as a WSi gate 15. The regions 16 and 17 on either side of the gate extending to the ohmic contact regions, as delineated by the lines 18 and 19 are high dose ion implanted with a donor n +  species, such as Si, the gate 15 preventing any of the ion implant beneath itself and so providing the self-aligned gate structure. An implant anneal is made and then metallic contacts 21 and 22 are deposited on the surface. In a preferred embodiment the metallization is AuGeNi-based. A conventional alloy heat treatment forms these contacts to the heavily implanted region as shown by the dashed lines. 
     Referring now to FIG. 1b there is shown in the second place a p-channel portion 10&#39; of a planar complementary HIGFET device. The p-channel portion of FIG. 1b is the same substrate wafer 11 and is normally positioned adjacent the n-channel portion described above. The insulating layers 12 and 13 are the same layers identified as layers 12 and 13 of FIG. 1a. A WSi gate 15&#39; is the same as in FIG. 1a. An acceptor p +  implant is made at regions 16&#39; and 17&#39; on either side of the metal silicide gate 15&#39; . An implant anneal is made and then ohmic contacts 21&#39; and 22&#39; are deposited and alloyed to the high dose p-implanted region. In a preferred embodiment the metallization is AuZnAu-based. 
     In FIG. 3 a cross-section view of a planar complementary HIGFET inverter is shown using as far as possible the same identifying numerals as used in the individual sections FIGS. 1a and 1b. The n-channel device 10 and the p-channel device 10&#39; are both formed on the common semi-insulating GaAs substrate 11. Specifically, the GaAs substrate 11 has grown thereon by MBE an insulating (undoped) GaAs layer 12 and then an insulating (undoped) (Al,Ga)As layer 13. In FIG. 5 one example of an alternate fabrication which has been demonstrated shows a thin layer of i-AlAs between the i-GaAs and the i-(Al,Ga)As layers, where the i-AlAs layer is added to improve the insulating properties of the gate and to increase the valence and electron band energy discontinuities E c  and E v , which increases the electron (hole) concentrations in the channels. In other embodiments, the composition profile of the gate can be graded or include various composition sublayers to optimize the device performance. 
     Referring to FIG. 4, a flow chart of the planar self-aligned gate by ion implantation n-channel and p-channel HIGFET IC process for completing the complementary IC of FIG. 3 is illustrated. The first step listed is the MBE heterostructure growth of the i-GaAs layer 12 and the i-(Al,Ga)As layer 13. Growth conditions are adjusted to obtain as close to intrinsic semiconductor material as possible with very low total doping. Then on the epitaxial surface 13a a high temperature stability refractory metal or metal silicide such as WSi x  is deposited to form the gates 15 and 15&#39;. A double masking of the surface allows sequential n +  and p +  selective ion implants. The source and drain regions 16 and 17 on either side of the gate 15 extending to the ohmic contact regions, as delineated by lines 18 and 19, are ion implanted with an n-implant while the regions around gate 15&#39; are covered. Then the regions 16&#39; and 17&#39; on either side of the gate 15&#39; are ion implanted with a p +  implant while the regions of gate 15 are covered. In one embodiment Si was used to create the n +  implanted regions and Mg or Be have each been used to form the p +  implanted regions. These two ion implants provide the self-aligned gates, the gates 15 and 15&#39; preventing any of the ion implant beneath itself and so providing the self-aligned gate structure. A high temperature implant anneal process is the next step to activate the ion implanted n- and p-species. This implant anneal can be in a conventional semiconductor anneal furnace, or rapid thermal annealing using an optical lamp can also be used. Dielectric anneal encapsulants, such as Si 3  N 4  or an As-overpressure can be used to optimize the anneal activation. Alternate processes for forming self-aligned gate structures which utilize SiO 2  and photoresist to mask the gate region during ion implantation and deposit the actual metal gate after implant anneal can also be used. 
     The metallic drain and source electrodes 21, 22, 21&#39; and 22&#39; are deposited and delineated using standard photolithographic liftoff techniques. In one preferred embodiment this ohmic contact is AuGeNi-based for the n-channel HIGFET 10 and AuZuAu-based for the p-channel HIGFET 10&#39;. Then a conventional alloy heat treatment is used so that ohmic contacts are formed to the heavily implanted regions as shown by the dashed lines of FIGS. 1a and 1b. 
     The individual complementary devices such as 10 and 10&#39; of FIG. 3 are electrically isolated one from the other by ion implantation of a neutral species, such as protons, which destroys any residual electrical conduction between the devices. A photoresist is used to protect the active devices during the isolation ion implant. The n-channel and p-channel HIGFET devices can then be connected to form standard complementary logic gates using a standard metal interconnect system as is shown in FIG. 3 to form gates such as the simple inverter shown schematically in FIG. 6. 
     Both n-channel and p-channel HIGFET devices have been fabricated and tested at both room temperature and 77K. As shown in FIG. 5, two different (Al/GA)As/GaAs heterostructures have been grown for device fabrication. The first employs a homogenous undoped Al 0 .3 Ga 0 .7 layer as the gate insulator. The second incorporates an undoped AlAs layer next to the GaAs in an attempt to increase the conduction and valence band discontinuities, E c  and E v  respectively. 
     For a nominal 1 m gate length n-channel HIGFET, the extrinsic transconductance g m  is 218 mS/mm at room temperature and 385 mS/mm at 77K. FIG. 7 shows a plot of both the square-root of I ds  and g m  versus the gate-to-source voltage V gs  for an n-channel device. For a nominal 1 m gate length p-channel HIGFET the values of the extrinsic transconductance g m  are 28.3 mS/mm at room temperature and 59.3 mS/mm at 77K, respectively. FIG. 8 shows a plot of both the square root of I ds  and g m  versus the gate-to-source voltage V gs  for a p-channel device. 
     The specification has shown a complementary device based on (Al,Ga)As/GaAs material system, however the invention can clearly be applied to other semiconductor material systems such as (Al,In)As/(Ga,In)As, in which the gate material is a larger gap insulating semiconductor and the active channel material is a high mobility semiconductor. 
     Referring now to the low power preferred embodiment of FIG. 9, the same numbering scheme will be used as in the complementary device of FIG. 3 wherever possible. In the operation of heterostructure insulated gate FETs where the WSi gate 15 (FIG. 3) is directly deposited on AlGaAs, the threshold voltage (V t ) of the n-channel FET 10 is near 1 V (a difference of the WSi metal barrier height and the AlGaAs/GaAs conduction band discontinuity). This relatively high V t  requires correspondingly high power supply voltages (2-3 V) for device operation and therefore increased level of gate leakage with reduced voltage swing and noise margin. In FIG. 9 the structure is changed to provide an InGaAs semiconductor gate, to reduce the threshold voltage to 0.2-0.4 V to allow low power circuit operation. 
     As has been pointed out, the threshold voltages of such devices with WSi gate metals are typically +1 and -0.5 volt for n- and p-channels respectively in close agreement with the formulations (1) and (2). The high threshold voltage of the n-channel device requires high power supply voltage (2-3 volts) during device operation which can result in significant gate leakage, reduced voltage swing, low noise margins and also overall increased power dissipation. 
     The invention embodied in FIG. 9 shows a modified epitaxial structure such that lower n-channel threshold voltages are achieved to allow lower supply voltages of approximately 1.5 V and also lower circuit power dissipation. Referring now to FIG. 9, the n-channel device 110 and the p-channel device 110&#39; are both fabricated on the common semi-insulating GaAs substrate 11. The GaAs substrate 11 has grown thereon by molecular beam epitaxy an insulating (undoped) GaAs layer 12 (i-GaAs) and then an insulating (undoped) (Al,Ga)As layer 13 i-(Al,Ga)As). Optionally, an insulating (undoped) GaAs layer 113 (50-100 Å) is also grown. The low threshold is achieved by depositing by MBE a thin layer (&lt;1000 Å) of heavily doped n-type In x  Ga 1-x  As (0&lt;x&lt;0.2). Silicon can be used as the n-type dopant. The dopant concentration may be about 5×10 18 . FIG. 9 shows the cross section of the device with the InGaAs n-channel gate layer 115. The layer 115 may preferably be in a thickness range of 200-500 Å. The gate region is preferably defined by conventional lithography and an etch. The etchant for InGaAs may be a wet etchant such as H 2  SO 4  :H 2  O 2  :nH 2  O or HCl:H 2  O 2  :nH 2  O or a dry etchant such as CCl 2  F 2 . On the epitaxial surface is fabricated a gate such as a WSi gate 15 in accordance with the earlier teaching for FIGS. 1 and 3. 
     The lowered threshold voltage for the In x  Ga 1-x  As gate n-channel HIGFET device of FIG. 9 is (as x changes the threshold voltage may vary from 0 to 0.4 volts): 
     
         V.sub.tn =ΔE.sub.c1 -ΔE.sub.c2 -ΔE.sub.c3 
    
     where 
     ΔE c1  =Band Discontinuity of InGaAs/GaAs interface 
     ΔE c2  =Band Discontinuity of GaAs/AlGaAs and 
     ΔE c3  =Band Discontinuity of AlGaAs/GaAs. 
     Since ΔE c2  =-E c3 , the V tn  is approximately the discontinuity at the InGaAs/GaAs interface. 
     The composition of the In x  Ga 1-x  As is chosen so the band discontinuity at the InGaAs/GaAs interface results in the desired threshold voltage. Thus for example, by using In 0 .15 Ga 0 .85 As as a gate layer, the channel V t  is close to +0.3 V. In these structures, since InGaAs is lattice-mismatched to GaAs, the InGaAs layer thickness is made small so that the mismatch strain is accommodated by a slight tetragonal distortion. The strained layer is then referred to as pseudomorphic since no dislocations are generated. The layer is deposited in the standard fashion of MBE using preferably low substrate temperatures (550° C.) for high quality surfaces, after the GaAs growth. 
     In the device fabrication, standard lithographic techniques are used to define the n-channel gates and the WSi deposited on the InGaAs in the n gate regions. The p regions are then defined and the InGaAs removed by a wet or dry etching process which stops at the GaAs. In one embodiment, the InGaAs is removed from everywhere except under the n channel gates. The gates and ohmic contacts are deposited subsequently. In another embodiment, the highly-doped InGaAs is also left in the n-ohmic metal regions to facilitate n ohmics. Thus the InGaAs gate has the following advantages. Since the indium composition can be changes in the MBE process, it allows one degree of freedom whereby the band discontinuity can be chosen to obtain a better match of V tn  with the p-channel HIGFET device in a complementary circuit. The heavily doped InGaAs also improves n ohmic contacts and the source resistance of the HIGFET.