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Estimation of Collector Current Spreading in InGaAs SHBT Having 75-nm-Thick Collector

We investigated collector current spreading in InGaAs single heterojunction bipolar transistors (SHBTs) having a collector thickness of 75nm. SHBTs were fabricated with three different emitter widths — 200, 400, and 600nm — and the highest cutoff frequency that was obtained was 468GHz. The relationship between the current density at the highest cutoff frequency and the emitter width could not be used to estimate the current spreading because it was independent of the collector-base voltage. However, the relationship between the current density with the increase in the total collector-base capacitance and the emitter width indicates current spreading in the collector. The current spreading was estimated to be approximately 90nm.