Current Gain of an AlGaN/GaN Heterojunction Bipolar Transistor
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Current Gain of an AlGaN/GaN Heterojunction Bipolar Transistor Yumin Zhang Department of Electrical and Software Engineering, University of Wisconsin-Platteville, Platteville, WI 53818, U.S.A. P. Paul Ruden Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, U.S.A. ABSTRACT A novel hybrid model and simulation results for an advanced, graded base AlGaN/GaN heterojunction bipolar transistor structure are presented. The base of the n-p-n HBT examined has two parts, a linearly graded AlGaN layer on the emitter side and a heavily p-doped GaN layer on the collector side. In the hybrid model developed here the potential profile is first calculated self-consistently in the biased state taking into account ionized impurity charges, polarization charges, and majority carrier charges. The minority carrier transport is examined subsequently. Injection of electrons from the emitter is modeled as a thermionic emission process. The minority electron transport process in the graded region is drift-dominated due to the large built-in effective field strength. In the low-field GaN layer of the base, electron transport is assumed to be diffusion-dominated. High-level injection effects are modeled in the framework of the Gummel-Poon model. Example structure design parameters are presented and it is found that the calculated current gain can be greater than 25, with a collector current density of 104A/cm2.
INTRODUCTION In recent years there has been considerable progress in the processing techniques for IIINitride material systems [1,2]. UV detectors, blue lasers and LEDs have been fabricated successfully with III-Nitride materials, and their application for electronic devices is also under active investigation. Due to their large bandgaps and high electron drift velocities, these materials appear to be well suited for high power and high temperature applications. N-channel high electron transistors (HEMTs) with very promising characteristics have been fabricated from AlGaN/GaN epitaxy layers on either sapphire or SiC substrates [3,4]. Another promising high speed electronic device is the heterojunction bipolar transistor (HBT). However, due to the large ionization energy of acceptors and low mobility of holes the development of HBTs in the IIINitride material system has so far lagged considerably behind the development of III-Nitride HEMTs. Wurtzite structure semiconductors, such as the III-Nitrides, typically have relatively ionic bonds leading to large stress-dependent electric polarization parallel to the hexagonal crystal axis. It has been suggested previously [5] that polarization charges may be employed to induce mobile charge carriers in specified regions of a device structure with modulated material composition. However, due to the dependence of the lattice constants on the composition, at
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least part of the structure will be strained. These strained layers have to be kept relatively thin if the formation of misfit dislocations is to be minimized.
HBT DESIGN FOR LOW BAS
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