We investigated collector current spreading in InGaAs single heterojunction bipolar transistors (SHBTs) having a collector thickness of 75 nm. SHBTs were fabricated with three different emitter widths -- 200, 400, and 600 nm -- and the highest cutoff frequency that was obtained was 468 GHz. 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 90 nm.

The authors report ultra-high-speed digital IC modules that use 0.1-µm InAlAs/InGaAs/InP HEMTs for broadband optical fiber communication systems. The multiplexer IC module operated at up to 70 Gbit/s, and error-free operation of the decision IC module was confirmed at 50 Gbit/s. The speed of each module is the fastest yet reported for its kind.