A nonlinear gain model for multiple quantum well transistor lasers

The gain spectra of single quantum well (SQW) heterojunction bipolar transistor laser (HBTL) is calculated with the gain coefficients and transparency carrier density as a function of the device structure factors by taking into account intraband relaxation. Inter-well coupling effects in a multiple quantum well (MQW) structure are considered as a correction factor in our approach. We introduce a six-order polynomial expression for a nonlinear carrier-dependent gain of MQW-HBTL considering ground, first excited and second excited states. We show that our gain model is notably more accurate than usual logarithmic equations, particularly in a large signal regime. Using computationally efficient numerical methods with a comprehensive rate-equation-based model, we obtain good agreement with experimental results for small signal, large signal and switching analysis of the SQW and MQW structures. We discuss the influence of different structure factors on the switching behaviour as well as on the optical bandwidth of the HBTL. Specifically, turn-on times (τ_on) of 160 and 540 ps are obtained for SQW and 4 QW structures, while τ_on considerably decreases below 100 ps for 6 QWs due to competing effects between tunneling and thermionic emission followed by carrier diffusion over the barriers. Finally, τ_on can be minimized independently of the QW number when a barrier width of ≈7 nm is used in the HBTL structure.