InGaAs/AlGaAs Heterojunction Phototransistors for Optical Signal Processing Applications
Doctoral thesis, 1997
This thesis deals with InGaAs/AlGaAs heterojunction phototransistors (HPTs) operating in the wavelength region where the GaAs substrate is transparent. Vertical integration of such a current amplifying detector with emitters or modulators will facilitate the fabrication of two dimensional arrays of components for parallel optical signal processing and transmission applications. Operation in the transmission window of the GaAs substrate simplifies cascading of arrays.
To achieve optical response in the GaAs transmission window, strained InGaAs is normally used as an absorber. Two schemes to overcome the problem of the critical thickness are investigated: use of a resonant cavity to enhance (RCE) the absorption in a thin pseudomorphic absorber and use of a non-resonant structure with a thicker partially relaxed absorber. For the RCE-HPTs the spatial matching of the absorber and the standing wave of the optical field is made less critical by using an optical design where the absorption in the multiple quantum well absorber is tolerant to growth errors and layer nonuniformities. With this design very good uniformity in responsivity is achieved, even between devices with different resonance wavelengths and devices where the resonance wavelengths have been post growth tuned by etching. In addition, the RCE-HPTs show the highest responsivities ever reported for this type of devices in this wavelength region.
The partially relaxed non-resonant HPTs have similar performance to non-relaxed resonant devices with electrical gains of up to 3000. They also have flat (±15%) spectral response over more than 20 nm. Devices with different relaxation are compared and the conclusion reached is that a certain amount of relaxation yields optimum performance but as the relaxation is further increased the current-voltage characteristics and the gain of the devices degrade. This is attributed to both a slight loss in the internal quantum efficiency of the detector which also affects the collector current of the device, and thus the inherent transistor performance.