The Transistor Laser: Theory and Experiment

The quantum-well (QW) heterojunction bipolar transistor (HBT) laser [the transistor laser (TL)], inherently a fast switching device, operates by transporting small minority base charge densities
$\sim\!\! {\hbox {10}}^{16}\ {\hbox {cm}}^{-3}$
over nanoscale base thickness (
$<$
900 A) in picoseconds. The base QW acts as an optical “collector,” in addition to the usual electrical collector, that selects out “fast” recombining carriers, resulting in a short lifetime (
$\sim$
29 ps) and higher differential laser gains. Charge and current continuity, together with the boundary conditions imposed by both collectors of the TL lead to a new charge control model, “unpinning” of population inversion beyond lasing threshold, quasi-Fermi level discontinuity across base QW, and a new equivalent circuit model requiring an extension to Kirchhoff's law. With the TL, the HBT becomes more than just a charge control device, but also a photon storage and switching device. The TL, owing to fast recombination speed, its unique three-terminal configuration, and the complementary nature of its optical and electrical collector output signals, enables resonance-free base current and collector voltage modulations, and compact realization of electro–optical applications such as nonlinear signal mixing, frequency multiplication, negative feedback, and optoelectronic logic gates.