Modeling Resonance-Free Modulation Response in Transistor Lasers With Single and Multiple Quantum Wells in the Base

We have developed the expressions for terminal currents in transistor lasers (TLs) having a single quantum well (SQW) as well as multiple quantum wells (MQWs) of different well and barrier widths in the base by solving a continuity equation relating the bulk carrier density with the 2-D carrier density via virtual states (VS). The gain in the quantum well (QW) is obtained by considering strain, 2-D density-of-states, polarization-dependent momentum matrix element, Fermi statistics, and Lorentzian broadening. A calculated value of 7.06 mA of threshold base current for three 16-nm-wide QWs in the base indicates a substantial reduction from the calculated and experimental value of 21.5 mA for a 16-nm-wide InGaAs QW in GaAs base. A similar reduction is also obtained for three QWs of different widths having variable barrier widths. The estimated modulation bandwidths (BWs) are higher in the case of MQW structures than in the SQW TL. Above threshold, the effective base recombination time, including spontaneous and stimulated processes, gives rise to a fast recombination process in the base, which leads to resonance-free modulation response. The estimated recombination time compares favorably with the value reported from the analysis of experimental data.