Superlattices and Microstructures, Vol. 8, No. 3, 1990 341 Pseudomorphic InGaAs HEMTs on Ga& Substrates with Undoped and Doped Channels G. Papaioannourz, E. Rodlti’, C. Michelakisl, E. Martin’, Z. Hatzopoul~s~~~, M. Papastamatio$, G. Kiriakidls1v3, G. Halkias1T3and A. Christou1*4 (1) FORTH, Crete, GREECE (2) Solid St. Section, Physics Dpt. University of Athens, GREECE (3) University of Crete, GREECE (4) Rutgers University, USA (Received 30 July lW3) A thorough investigation of the pseudomorphic HEMTs was carried out by investigating the effect of the position of the donor layer and/or doping profile ln the channel on the device performance. The PM structure’s transport properties and presence of interface and bulk traps were investigated. The transistor microwave performance was measured and transconductance was compared for the various transistors. For relaxed geometry HEMTs a typical transconductance of 28OnWmm was measured. Introduction Investigations for optimizing gain and power output at frequencies greater than 6OGHz have been concentrated on pseudomoruhlc structures on GaAs and on multi-channel High‘ Electron Mobility TransistorslZ. Recent results4 with InGaAs HEMTs have indicated that at 94GHz, 3dB gain and 15 percent efficiency may be attained with 0.25l.r.m gate length and 75pm gate width. A maximum power gain of 6.OdH at 6OGHz has been reported with b.15pm x 5Opm gates5. In maximizing power output and efficiency above 6OGHz, most attempts at multi-channel HEMTs did not realize the full potential of such approach due to the presence of a double-peaked transconductance curve3. Other problems were related to a low break-down voltage (doped channel being doped above 3x101*cnr3) or to a narrow gin versus gate voltage curve and hence a poor power output. The present investigation reports on the results of an optimized doped channel heterojunction field effect transistor with a delta doped InGaAs channel of 1.5x101*cm-b. The transistors were fabricated with a T-gate process and with 0.2$mx5Opm, PdAu or TiPtAu gates. The pseudomorphic InGaAs channel FETs were fabricated, characterized compared. and their performance Experimental Description Standard deep ultra-violet photollthographlc techniques were used for gate-lengths of up to 0.5Oum while electron beam lithography was applied for 0.25mn gate length field effect transistors. The gate recess e&h con&ted of an H,O,:NH 0H:H20 etch diluted with deionized water. be consisted either of 1OOA Pd followed by 1000~a~ or 1OOA Ti. 400A Pd. followed bv 1OOOA Au. The ohmic contacts consisted of 1OOOA AuGe., loOA Ni and 1OOOA Au as the top layerannealed ln forming gas The following DC characteristics were measured: ID -VDs vo tage, f behavior, transconductance ( ), threshold saturated drain current % D & The transmission line technique was used fo obtain specific contact reslstivity, transfer resistance and sheet resistance. The traps present in each structure were measured and identified by standard DLTS methods. Experimental Results a) Doped Channel versus Single Heterostnrcture. Figure 1 shows the doped channel and single HFET cross-section indicating that the donor layer was planar doped, in or&r to obtain a broad gm versus gate voltage curve which would enhance the large signal performance. The advantage of the doned channel is observable in the- transport prop&ies of the two structures. The undoped channel HFET had a sheet charge density of 2.5x1012cnr2 and mobllit of 64C&m*N.s at 300K and 2.45x10’*cm- 2y 19,ooOrm*N.s at 77K. The doped channel HFET resulted in a charge densitv of 3.5x101*cm-* and mobility of 85,000&*N.s at 77K. The gate transfer characteristics of the DCHFET and standard HFET are shown ln Figure 2 0749-6036/90/070341 1-04 $02.00/O 0 1990 Academic Press Limited