Modulation doped GaAs/AlGaAs field effect transistor
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of MODFETs (modulation doped heterojunction field effect transistors) on n-AlGaAs/GaAs structures for analog operation, particularly at millimeter wavelengths. 
     Since the discovery by Dingle et al (&#34;Electronic Mobilities in Modulation-doped Semiconductor Heterojunction Superlattices&#34;, Appl. Phys. Lett. Vol. 33, p.665, (1978)) of high mobility two-dimensional electron gas in N-AlGaAs/GaAs heterojunction structures, considerable progress has been made in utilizing these structures for practical realization of FETs based on Schottky-barrier type gates. See for example S. Hiyamizu et al. &#34;MBE-Grown GaAs/n-GaAs Heterostructures and Their Application to High Electron Mobility Transistors&#34;, Japan J.Appl. Phys. Vol 21, p.161, (1981); D. Delagebeaudeuf et al, Metal-(n)AlGaAs-GaAs Two Dimensional Electron Gas FET&#34;, IEEE Trans Electron Dev. ED-29, p.955, (1982); T. J. Drumond et al, &#34;Influence of Al x  Ga 1-x  As Buffer Layers on the Performance of Modulation Doped Field Effect Transistors&#34;, Appl. Phys. Lett. Vol. 40, p.879, (1982); and P. C. Chao et al &#34;Quarter-micron Gate Length Microwave High Electron Mobility Transistors&#34;, Presented at the 1983 IEEE/Cornell Conference on High Speed Semiconductor Devices and Circuits&#34;, Ithaca NY, Aug. 15-17, 1983. For a review paper on MODFETs, see P. M. Solomon and H. Morcoc, IEEE Trans. on Electron Devices, ED-31, p.1015 (1984). Also of interest is a paper by M. Ogura et al. &#34;Self-aligned Enhancement Mode FET with AlGaAs as Gate Insulator&#34;, J. Vac. Sci. Technol. B 3 (2), Mar/Apr 1985, page 581. 
     U.S. patents of interest relating to field effect transistors include U.S. Pat. No. 4,494,016 to Ransom et al disclosing a GaAs depletion mode MESFET useful as a linear power amplifier. It includes a semi-insulating host GaAs substrate which has a uniform depth N+ source, gate and drain implant. The N+ implant region has a very high dopant concentration. Delagebeudeuf et al in U.S. Pat. No. 4,455,564 are concerned with a high frequency field effect transistor which uses a heterojunction between the GaAs and AlGaAs layers to create a high mobility electron accumulation layer and a Schottky grid to control the load. Decker et al in U.S. Pat. No. 4,157,556 disclose a heterojunction in a confinement field effect transistor. In that patent a GaAs conducting channel is bounded by an AlGaAs layer. Cho et al in U.S. Pat. No. 4,186,410 discuss the formation of ohmic contacts in GaAs FETs. Ohmic contacts using silicon doped aluminum are taught in Morcon et al U.S. Pat. No. 4,333,100. Yoshida et al in U.S. Pat. No. 4,492,974 disclose a DMOS which comprises a MOSFET with a gate protection diode. 
     SUMMARY OF THE INVENTION 
     An object of the invention is realization of a depletion-mode modulation-doped field-effect transistor with high gate-length to depletion-depth ratio that is capable of providing high power gain at millimeter-wave frequencies. 
     According to the invention, an ohmic gate or a heterojunction gate is used on the n-AlGaAs/GaAs layered structure. 
     The depletion-mode operation is desirable for analog signal amplifying circuits as opposed to the enhancement-mode device commonly used for switching or digital circuits. In the case of Schottky-barrier gate, high aspect ratio structures naturally operate in the enhancement mode. 
     The device achieved with the invention is suitable for millimeter-wave air-to-air communications systems involving frequencies up to 94 GHz. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cross-sectional schematic diagram of a conventional Schottky-Barrier gate MODFET structure; 
     FIG. 2 shows schematic band and field diagrams for the conventional Schottky-barrier gate MODFET structure of FIG. 1; 
     FIG. 3 is a graph of the n-AlGaAs Layer thickness vs. threshold voltgage in Shottky-barrier gate MODFETs; 
     FIG. 4 is a cross-sectional schematic diagram of the ohmic-(and heterojunction) gate MODFET structure according to the invention; 
     FIGS. 5a-5c show ohmic-gate MODFET schematic band and field diagrams under (5a) zero, (5b) reverse, and (5c) forward bias conditions; and 
     FIGS. 6a-6c show heterojunction gate MODFET band and field diagrams under (6a) zero, (6b) reverse, and (6c) forward bias conditions. 
    
    
     DETAILED DESCRIPTION 
     A cross sectional schematic diagram of a typical conventional Schottky barrier gate MODFET structure is shown in FIG. 1, with its band and field diagrams given in FIG. 2. The Schottky barrier height φ B  is responsible for the partial depletion of the n-AlGaAs layer of thickness d t . The two-dimensional electron gas (2-DEG) is supplied by the fractional thickness d m , and d s  is the spacer layer thickness intended to maintain donor-carrier separation for high electron mobility. The electric field diagram illustrates the situation of complete depletion of the n-AlGaAs layer with zero applied gate voltage. The maximum n-AlGaAs layer thickness should not exceed d tmax  corresponding to the situation that yields the maximum sheet carrier concentration (n so ) in the 2-DEG at zero applied voltage. As d t  is reduced below d tmax , the threshold voltage (V TH ), at which the device channel conductance vanishes, becomes less and less negative and even positive as graphically illustrated in FIG. 3. The maximum positive threshold voltage for a given doping concentration, N D , in the n-AlGaAs layer corresponds to its minimum value d m  and slightly increases with the space layer thickness. 
     In submicron gate-length millimeter-wave MODFETs if a high power gain is desired the aspect ratio determined by the gate-length-to-gate depletion depth must be very high (see M. D. Das &#34;A High Aspect Ratio design Approach for Millimeter-Wave HEMT Structures&#34; IEEE Trans. on Electron Devices, Vol. ED-32, No. 1, p. 11, January, 1985.) Thus it is apparent that use of the conventional Schottky-barrier gate in high aspect ratio MODFETs would result in enhancement-mode devices, whereas for analog applications it is preferable that these devices operate in the depletion-mode. 
     The design to achieve the depletion-mode operation for the MODFET&#39;s is illustrated in FIG. 4. A semi-insulating GaAs substrate 12 is used, on which there is GaAs buffer layer 14, an n-AlGaAs layer 16, a n-GaAs layer 18, a thin n-AlGaAs layer 20, and another n-GaAs layer 22. An additionl undoped AlGaAs or AlAs spacer layer (not shown) may be placed between layers 14 and 16 to reduce the electronic scattering by donors in layer 16. The use of AlAs will increase the heterostructure barrier potential. This could be desirable to reduce leakage current in the ohmic-gate and heterostructure gate FETs. 
     A mesa is first etched in the layers from 22 through 14 down to the substrate 12. The layer 22 is further etched to leave an area for the gate. A metal contact 24 is formed at the top of the gate. Metal contacts 26 and 28 are also formed for the source and drain. 
     The structure in FIG. 4 realizes electrical isolation between the gate 24 and drain 28, and the gate and source 26 terminals, through the use of free-surface potential depletion and the gate itself is formed by the ohmic contact process. The thick n-GaAs topmost layer 22 is retained in the gate region and removed from its source and the drain regions by etching using the ultra thin AlGaAs layer 20 (&lt;50 Å) as the etch-stop layer. Because of the additional thick n-GaAs top-layer 22 under the gate metal 24 the ohmic contact penetration after alloying can be expected to be much less under the gate compared to that under the drain and the source metallization. By a proper selection of alloying time and/or composition of Au/Ge/Ni or other metals the ohmic contact penetration under the gate can be controlled either to the depth where n-AlGaAs begins (shown as line B in FIG. 4) or where n-GaAs layer, below the ultra thin AlGaAs layer, begins (as shown by line A under the gate). The doping and the thickness of the n-GaAs layer 18 and that of the n-AlGaAs layer 16 should be selected to realize the formation of the depletion regions as indicated by the dotted lines. The region 1 on both sides of the gate are due to the free-surface potential and the region 2 is due to the transfer of electrons to the 2-DEG. Typical values of doping and thickness of the n-AlGaAs layer 16 are 2×10 18  cm -3  and 140 Å, respectively, whereas those for the n-GaAs layer 18 immediately above the n-AlGaAs are 4×10 17  cm -3  and 300 Å, respectively. 
     The total depletion depth under the gate, when the ohmic contact penetration is up to the depth indicated by the line B, would be equal to the depth of region 2 including any spacer layer thickness (˜30 Å) (if present) plus the effective thickness of the 2-DEG (˜80Å). Altogether this can be as low as 200 Å, providing an aspect ratio of 12.5 for a 0.25 μm length gate. This device will operate well at zero gate bias voltage with nearly full channel conduction and limited extent of reverse or forward bias can be applied either to deplete or enhance the channel conduction with excessive gate leakage current flow due to tunneling or carrier injection processes as illustrated schematically in FIGS. 5a, 5b, and 5c. At liquid nitrogen temperature, the leakage current components will be negligible and the device will operate under both forward and reverse gate bias conditions. An important advantage is realized in this structure due to the elimination of the Schottky gate contact, i.e. the transconductance should be high (by a factor of 1.75-2.55) due to the absence of the voltage drop across the Schottky depletion region with comparable increases in the power gain and stability margin. 
     The device behavior becomes quite different when the ohmic contact penetration under the gate electrode is up to the line A (see FIG. 4). In this case there is a second hetero-junction between the n-GaAs and n-AlGaAs with the transfer of 2-DEG into the n-GaAs as shown schematically in FIG. 6a, including two spacer layers. It is not necessary to have the spacer layer between the n-AlGaAs and the n-GaAs layer, however. The device has two back-to-back hetero-junctions as considered above. The total minimum depletion depth that can be realized in this structure, with the full 2-DEG for the channel region, would be greater than 250 Å. However, a device with depletion depth of 200 Å or less can be forward biased to enhance the electron concentration in the 2-DEG. In addition, there will be increased transconductance over the conventional Schottky gate devices, although not to the degree found in the single hetero-junction device described previously. However, this configuration would have the advantage of reduced gate leakage currents as a result of the additional heterojunction barrier. The operational details in terms of the band and the field diagrams are depicted in FIGS. 6b and 6c for the double hetero-junction structure for the reverse and forward biased conditions of operations, respectively. 
     The ohmic gate single and double hetero-junction MODFET structures proposed above would provide depletion-mode operation and they can be built with high gate-length to gate depletion depth aspect ratio as required for the realization of stable high power gain. These devices will operate primarily under zero gate bias conditions at 296 K with limited positive and negative gate bias voltages. At liquid nitrogen temperature, operation at the full range of bias voltage changes should be possible. It has been shown that a 0.25 μm length gate MODFET with 210 Å depletion depth and Schottky barrier gate would provide a power gain greater than 6 dB at 92 GHz (see M. B. Das &#34;A High Aspect Ratio Design Approach for Millimeter-Wave HEMT Structures&#34; IEEE Trans. on Electron Devices, Vol. Ed-32, No. 1, p. 11, January 1985). The ohmic-gate device structures according to the present invention should be better than this predicted performance under depletion-mode operation and hence would be attractive for millimeter-wave integrated circuit analog amplifying system applications, as there will be no need to have an additional power supply for the gate electrode. 
     It is understood that certain modifications to the invention as described may be made, as might occur to one with skill in the field of this invention, within the scope of the appended claims. Therefore, all embodiments contemplated hereunder which achieve the objects of the present invention have not been shown in complete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims.