Author

Wang Zhang

Date of Award

12-2010

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

Advisor

Nuri W. Emanetoglu

Second Committee Member

Rosemary Smith

Third Committee Member

David E. Kotecki

Abstract

Optoelectronic mixers (OEM) are photodetectors which detect an optical signal and internally mix it with an electrical signal to obtain an electrical base-band (low frequency) signal. OEM devices have applications in optical communications and sensors such as laser assisted detection and ranging (LADAR) systems. Optoelectronic mixers can simplify signal processing in a chirped-AM LADAR system. Gallium arsenide (GaAs) and indium gallium arsenide (InGaAs) symmetric metal-semiconductor-metal (MSM) Schottky photodetectors have been implemented as optoelectronic mixers in chirped-AM LADAR systems by researchers at the US Army Research Laboratory. Such devices do not have a gain mechanism. Adding gain to the optoelectronic mixer allows the following transimpedance amplifier's gain to be reduced, increasing bandwidth and improving the system's noise performance. Symmetric Gain OptoElectronic Mixers (SG-OEMs) for chirped-AM LADAR operating in the "eye-safe" 1.55 um wavelength were previously investigated by S. Drew et al. These SG-OEMs were based on a symmetric heteroj unction phototransistor using indium aluminum arsenide (In0.52Al0.48As)/ indium gallium arsenide (Ino.53Gao.47As) heterostructures. Cracking defects in the thin films were revealed during device fabrication, leading to an investigation into an alternative device structure with indium phosphide (InP) layers to improve the growth quality. This thesis investigates the feasibility of InP/In0.53Ga0.47As heterojunction based symmetric gain optoelectronic mixer structures for LADAR applications. The Ino.53Gao.47As layer with a bandgap of 0.74 eV is used in the base of the device, to ensure absorption of 1.55 um wavelength light. InP is used in the emitter/collector layers, as it has a wider bandgap of 1.35 eV. Two-dimensional simulations are used to model the behavior of these InP based devices. The dependence of the DC responsivity and small signal model components on structure parameters such as base thickness and doping profile are investigated to reach an optimized structure design. The InP based SG-OEM devices are predicted to have better performance than previously demonstrated InAlAs based devices in terms of dark current and device reliability. An optimized device structure was designed through theoretical calculations and 2D simulations. These SG-OEM devices can lead to miniaturized LADAR-on-chip systems, which would significantly expand the application of LADAR technology into areas such as range finding, terrain mapping and face recognition.

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