Effect of Heavily P-Doped Base on Radiative Recombination of Transistor Laser

The p-type doping in base regions of transistor lasers (TLs) plays an essential role in both the electrical and optical functionalities. The holes in heavily p-doped bases lay the ground for not only the control of current signals but also stimulated emission of lasing modes. In this paper, however, we show that an exceedingly high doping level does not further enhance the radiative recombination in TLs. With heavy doping, the distorted band profile induced by the slight charge separation of holes and ionized acceptors results in no bound valence subbands in the quantum well (QW). This effect stops holes from entering the QW, and most radiative transitions take place inefficiently between bound electrons in the QW and quasi-free holes in the base. As a result, the radiative lifetime is only shortened to the (sub-) nanosecond range, suggesting that the previously reported picosecond carrier lifetime of TLs might originate from other recombination processes.