Numerical design and frequency response of MQW transistor lasers based entirely on group IV alloys

A theoretical model is developed for n–p–n mid-infrared transistor lasers (TLs) with a strain-balanced Ge0.85Sn0.15 multiple quantum well (MQW) structure in their base. The variation of the optical confinement factor, the modal gain, and the threshold current density is rigorously investigated for different numbers (N) of QWs in the MQW structure. The results show that overall the optical confinement factor and the modal gain increase with N. The frequency response of the MQWTL in the common-base (CB) configuration is estimated from the small-signal relationship between the photon density and the emitter current density by solving the laser rate equation and the continuity equation, considering the virtual states as a conversion mechanism. Increasing N causes the modulation bandwidth first to increase then to decrease with N. This reveals a shift in the nature of the device for higher values of N. The results also suggest that judicious selection of N will enable the proposed device to become a viable monolithic light source.