A high aspect ratio design approach to millimeter-wave HEMT structures

Abstract

In MESFET and HEMT structures as the gate length is reduced below 0.5 µm in an attempt to achieve amplification at highest possible frequencies, it is essential that the depletion depth under the gate be also reduced in order to preserve a high aspect ratio that ensures a high device voltage gain factor (g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> /g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> ) and a reasonable value of stable power gain at high frequencies. Results based on this design approach indicate that an n-A1GaAs/GaAs HEMT structure with 0.25-µm gate length could provide stable power gain in excess of 6 dB at the unity current gain frequency of 92.4 GHz, and for an aspect ratio of ten it is difficult to reduce the gate length below 0.25 µm.