Reduction of the intrinsic base collector capacitance due to differential space charge effects in InP-GaInAs heterojunction bipolar transistors

Modulation of electron velocity in the collector depletion region by the base-collector voltage is shown, theoretically and experimentally, to considerably decrease the intrinsic base collector capacitance of HBTs. This effect is important because reduction of the intrinsic base collector capacitance increases f/sub max/ of small size HBTs. Most previous descriptions of high current density effects in bipolar transistors assumed that changes in the base-collector capacitance were due to current induced changes in the width of the collector depletion region. In this work, it is clearly shown that changes of the depletion width do not account for the large measured decrease in the capacitance.