ANALYSIS OF ULTRAFAST PHOTOCARRIER TRANSPORT IN ALINAS-GAINAS HETEROJUNCTION BIPOLAR-TRANSISTORS

We present a detailed physical analysis of photo-carrier transport in heterojunction bipolar transistors (HBT's) which describes their optic al impulse responses on picosecond time scales. Theoretical prediction s are experimentally verified with AlInAs-GaInAs HBT's using femtoseco nd visible-wavelength optical pulses for photocarrier injection and el ectro-optic sampling for subpicosecond-resolution electrical response measurements; additional sampling-oscilloscope observations allow accu rate responsivity measurements and provide time-response information t o nanosecond time scales. The HBT photocarrier transport description i s shown to be in good qualitative agreement with the significant featu res 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 impro ving an HBT's photoresponse bandwidth; operating be derived at which s lower photoresponse be dramatically reduced in amplitude. An experimen tally measured HBT optical response of 2.4 ps, corresponding to a phot ocurrent bandwidth of more than 200 GHz, is unsurpassed for an active device.