ABSTRACT
In this chapter, the authors describe a heterojunction bipolar transistor (HBT) vertically integrated with a quantum well (QW) LED. Standard photolithography and wet etching techniques were used to fabricate devices with various emitter sizes for electrical characterization. By modulating both the external base current and the optical excitation, this device can function as a high sensitivity optical switch or as an erasable programmable optical memory element. The current gain of GaAs HBTs increases with increasing emitter current density, as the contribution of the surface and bulk recombination currents becomes less important compared with the useful diffusion current of minority carriers across the base region. A small optical feedback allows the switching characteristics of the device to be controlled by the emitter-base junction bias of the N-p-n HBT. The authors demonstrate an N-p-n-p device which is suitable for driving LEDs for digital optoelectronic applications.
