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, f
t
= 10 GHz and dual input NAND circuits provided 50 picosecond propagation delays.