Fabrication of 200-GHz f max Resonant- Tunneling Diodes for Integrated Circuit and Microwave Applications

Absrract-Room-temperature current densities of 1.3 X lOs A/cml and peak-to-valley ratios of 2.5 bave been acbieved for resonant tunneling diodes (Rm's) in tbe GaAs/AlAs material system. Tbe devices were fabricated in a microwave-compatible process employing topside contacts and a semi-insulating substrate to allow device integration. Proton implantation creates a nonconducting surface compatible with higb-frequency coplanar transmission lines and other passive microwave structures. F OR resonant-tunneling diodes (RTD's), the charging time constant of the device capacitance eventually limits high-frequency operation. For this reason, high-current and low-capacitance devices can be equated with high-speed devices. Fabricating discrete devices, Brown et al. have dem'onstrated RTD's with current densities in excess of4 x 104 Alcm2 and capacitances less than 1.6 fFI#Lm2 [I]. These devices have demonstrated high-frequency operation, with maximum oscil-lation frequencies above 200 GHz, however fabrication methods have been unsuitable for the integration of devices. For circuit applications such as parity-bit generation, frequency dividers, and binary and multilevel logic circuits it is necessary integrate several devices in ~imple circuit configu-rations. To date, this has been achieved in the growth process, with several double-barrier structures grown sequentially on top of each other. This has resulted in circuits which have demonstrated the desired functionality, however, the current densities achieved have been less than 3 X 103 A/cm2 and high-frequency operation has not been attained [2], [3]. If devices are properly isolated then this integration can be easily accomplished on chip by a series connections of devices with no corresponding complexities in growth. Microwave integrated circuit applications such as mixers, frequency multipliers, and pulse-forming circuits require on-chip integration of high-quality transmission lines as w~ll as possible integration of futers, inductors, and other passive microwave elements. This necessitates the use of a low-los