Effects of Quantum-Well Base Geometry on Optoelectronic Characteristics of Transistor Laser

Abstract

Since the discovery of the transistor, the base current (minority carrier recombination) has been the key to the device operation. Among all developments through which Bipolar Junction Transistors (BJT) have progressed, the most innovating modification may be the replacement of the homojunction emitter material by a larger-energy-gap material, thus forming a Heterojunction Bipolar Transistor (HBT). In a silicon bipolar junction transistor, both homo and hetero structures, the base current is dissipated as heat, i.e. through nonradiative recombination, but can yield substantial radiative recombination for a directbandgap semiconductor like GaAs. Thus we can reinvent the base region and its mechanism (i.e. carrier recombination and transport function) to decrease current gain significantly and achieve stimulated recombination. Researches on III–V high-current-density high-speed HBT, revealed the ability of the transistor to operate electrically and optically in the same time. The modified transistor, called in literature as light emitting transistor (LET), works as a three port device with an electrical and an optical outputs (Feng et al. 2004a). Further improvements including wavelength tunability obtained by incorporating a place for better carrier confinement, called a Quantum-Well (QW) (Feng et al. 2004b). Carrier recombination in quantum well (QW) can be modified with a reflecting cavity changing the optoelectronic properties of QWLET (Walter et al. 2004). Room temperature continuous wave operation of such a device at GHz develops a novel three-terminal device, called a HBT Transistor Laser (HBTL) or briefly TL, in which laser emission produces by stimulated recombination (Feng et al. 2005). In the TL, the usual transistor electrical collector is accompanied with an optical collector, i.e. the above mentioned QW, inserted in the base region of the HBT. Stimulated recombination, unique in the TL, causes “compression” in the collector I-V characteristics and decrease in gain. Combined functionality of an HBT, i.e. amplification of a weak electrical signal, and that of a diode laser, i.e. generating laser emission, is observed in the TL. In other words, a modulating base current leads to modulated signals of the laser output power and collector current. It raises the possibility of replacing some metal wiring between components on a circuit board or wafer chip with optical interconnections, thus providing more flexibility and capability in optoelectronic integrated circuits (OEIC) (Feng et al. 2006a). It has been planned that TL is appropriate for telecommunication and other applications because of its capability of achieving a large optical bandwidth (BW) and a