Analytical model for threshold-base current of a transistor laser with multiple quantum wells in the base

The authors investigate in this study the effect of incorporating multiple quantum wells (QWs) in the base of a heterojunction bipolar transistor laser on its performance. The expression for the terminal current is obtained by solving continuity equation in the base relating the bulk carrier density with the two-dimensional (2D) carrier density via virtual states. The gain in the QW is obtained by considering strain, 2D density-of-states, polarisation dependent momentum matrix element, Fermi statistics and Lorentzian broadening. The authors have taken as the basis for comparison the calculated as well as the experimental threshold-base current of 21.5 mA for a 16 nm wide InGaAs QW in GaAs base placed at 59 nm away from the emitter junction. Calculated value of 7.06 mA for three 16 nm wide QWs placed at 29, 59 and 79 nms in the base indicate substantial reduction in threshold-base current. Calculations are also made for three QWs of different widths having variable barrier widths. Reduction of threshold-base current by different amounts is observed in all cases.