A comparison of high-speed enhancement and depletion mode GaAs MESFET´s

Previously, most MESFET GaAs IC´s were either all enhancement or all depletion mode. In our work, both types of devices were constructed on the same substrate utilizing ion implantation directly into chrome-doped semi-insulating substrates. A normally-off MESFET was fabricated by adding only one light etch to the process steps required to fabricate a normally-on MESFET. A novel proximity cap anneal, a native oxide passivation, and a dual layer metalization were used in the planar process. Performance goals included propagation delays of less than 250 picoseconds and 200 picoseconds; and power dissipations of 5mw and 0.5 mw for the normally-on and normally-off inverters, respectively (fanout of two). This work is an extension upon earlier LMSC GaAs MESFET device research in which MESFETS with 2 micron gate lengths and 250 micron gate widths provided gm = 20 mmho, ft= 10 GHz and dual input NAND circuits provided 50 picosecond propagation delays.