Analysis of ultrafast photocarrier transport in AlInAs-GaInAs heterojunction bipolar transistors

We present a detailed physical analysis of photocarrier transport in heterojunction bipolar transistors (HBT's) which describes their optical impulse responses on picosecond time scales. Theoretical predictions are experimentally verified with AlInAs-GaInAs HBT's using femtosecond visible-wavelength optical pulses for photocarrier injection and electro-optic sampling for subpicosecond-resolution electrical response measurements; additional sampling-oscilloscope observations allow accurate responsivity measurements and provide time-response information to nanosecond time scales. The HBT photocarrier transport description is shown to be in good qualitative agreement with the significant features of the measured transients, and the time scales of the transients can be predicted with good accuracy From simple expressions involving equivalent-circuit device parameters. The model suggests ways of improving an HBT's photoresponse bandwidth; operating parameters can be derived at which slower photoresponse components can be dramatically reduced in amplitude. An experimentally measured HBT optical response of 2.4 ps, corresponding to a photocurrent bandwidth of more than 200 GHz, is unsurpassed for an active device.< >