Heterojunction vertical FETs revisited: potential for 225-GHz large-current operation

The high-speed operation of submicrometer Al/sub x/Ga/sub 1-x/As/GaAs unipolar heterojunction transistors is examined using two-dimensional time-dependent self-consistent ensemble Monte Carlo simulation. Careful device design can significantly increase ballistic injection over the heterojunction in steady state by eliminating retarding gate-induced space-charge reversal there. Design for optimal large-signal transient operation must also avoid gate-voltage-dependent ballistic injection. General design principles for optimizing high-speed operation are proposed. The resulting VFETs show cutoff frequencies of 225 GHz at large drain currents at 300 K, with frequency-independent two-port y parameters.<
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