Normally-off type GaAs MESFET for low power, high speed logic circuits

wave amplifiers, but for high speed switching circuits’. Some of the logic using normally-on type GaAs MESFETs have large power dissipation and complicated circuit construction. The normally-off type GaAs MESFET logic has not as yet been reported, even though it is expected to have some attractive features such as low power dissipation and simple circuit configuration. Figure 1 is a microphotograph of the buffered output 13-stage ring oscillator consisting of normally-off type GaAs MESFETs and epitaxial resistors. A cutaway view of the inverter used in the ring oscillator is shown in Figure 2. The devices were fabricated on a sulfur-doped N-type epitaxial layer grown by VPE onto a semi-insulating Cr-doped substrate. The do ing density and thickness of the epitaxial layer was 1 x 101’cm-3 and 0.1 pm, respectively. The N-type layer outside the active area was etched down to the semiinsulating substrate to isolate inverters from each other. A dual-metal system was used. 0.04-pm thick Au-Ge eutectic alloy and 0.4-pm thick Au were continuously deposited as the first metal layer and were alloyed at 450’C for 120 seconds to make ohmic contact. Next, O.5pm-thick Si02 film which was used for the isolation of the dual metal layers was deposited by Chemical Vapor Deposition (CVD). This film was etched selectively to open the gate windows and the contact holes to the first metal layer. The second metal layer made with Cr-Pt-Au was used for the Schottky gate and crossing over or connecting to the GaAs MESFETs are substantially useful not only for micro-