Abstract
We have obtained the expressions for the terminal currents in a heterojunction bipolar transistor laser the base of which contains a quantum well (QW). The emitter-base junction is assumed to be abrupt, leading to abrupt discontinuity in quasi-Fermi level at the interface. The expressions for the terminal currents as a function of collector-emitter and base-emitter voltages are obtained from the solution of the continuity equation. The current density in the QW located at an arbitrary position in the base is related to the virtual state current density. The threshold current density in the QW is calculated by using the expression for gain obtained from Fermi golden rule. The plot of collector current (IC) versus collector-emitter voltage (VCE) for different values of base current shows the usual transistor characteristics, i.e., a rising portion after a cut-in VCE, and then a saturation behavior. The dc current gain remains constant. However, as the base current exceeds the threshold, a stimulated recombination rate is added to the spontaneous recombination rate and the plots of collector currents become closer for the same increase in base current. This current gain compression is in agreement with the experimental observation. Our calculated values qualitatively agree with other experimental findings; however some features like Early effect do not show up in the calculation.
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