Design, fabrication, and performance evaluation of field-effect transistors.

Abstract

Metal semiconductor field effect transistors (MESFET's) with 0.1 $\mu$m and 50 nm gate-lengths have been designed, fabricated, and characterized in order to obtain an improved understanding of the device physics, equivalent circuit representations, and scaling limits of deep submicron GaAs MESFET's. The FET's were fabricated using a bi-layer electron beam lithography process. Process parameters such as gate-length, gate width and gate recess etch depth were systematically varied in the GaAs MESFET's and the resulting DC and RF device performance, and extrinsic and intrinsic equivalent circuit parameters were characterized. The dependence of the FET characteristics on gate-length, gate width, and gate recess etch depth was studied after the equivalent circuit parameters were determined. The circuit parameters were determined by direct extraction using cold FET measurements to calculate values for the extrinsic elements. Cold measurements are done with $V\sb{ds}$ = 0 V, so that the intrinsic part of the device is essentially turned off. The dependence of the FET characteristics on the process variations was explained. Higher frequency S-parameter measurements were also taken from 2.0 GHz to 40.0 GHz and 75.0 GHz to 110.0 GHz. The measured S-parameters were compared to the simulated S-parameters from a small-signal equivalent circuit. The goals of these measurements were to determine $f\sb{max}$ more accurately and investigate the validity of the small-signal equivalent circuit at these higher frequencies. The devices that were fabricated and tested for this thesis exhibit state-of-the-art performance. The best 0.1 $\mu$m gate-length MESFET has an extrinsic transconductance $(g\sb{m,ext})$ of 795 mS/mm, cut-off frequency ($f\sb{t})$ of 103 GHz and maximum frequency of oscillation $(f\sb{max})$ of 172 GHz. The best 50 nm gate-length MESFET has an extrinsic transconductance $(g\sb{m,ext})$ of 650 mS/mm, cut-off frequency $(f\sb{t})$ of 130 GHz and maximum frequency of oscillation $(f\sb{max})$ of 187 GHz.