Abstract
The base transit times of an InP/GaInAs heterojunction bipolar transistor (HBT) with an ultrathin and heavily doped base are investigated by carrying out a Monte Carlo (MC) simulation. The acceleration of electrons due to a conduction-band discontinuity between the emitter and base is taken into consideration. Scattering caused by the spontaneous emission of hole plasmons considerably increases the base transit time even when the thickness of the base is 20 nm. Band-gap grading in the base effectively sweeps slow electrons into the collector, and the increase in the base transit time is partially suppressed. The MC simulation shows that when the band-gap grading is just twice the thermal energy, the transit time of the 20-nm-thick base decreases by 40%.
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