Transport Study of Heterojunction Bipolar Transistors

Autor: Her Yuan-Jan, 何源展
Rok vydání: 1998
Druh dokumentu: 學位論文 ; thesis
Popis: 86
In this thesis, the I/V characteristics of InGaP/GaAs heterojunction bipolar transistors (HBTs) are investigated. The potential profiles are obtained by solving the Poisson's equation. Different heterostructures including abrupt,abrupt with setback layer, graded, and combined of graded and setback layers are simulated. The transmission coefficients are calculated using the scattering matrix method. The thermionic field-diffusion model, including the reflections of carriers above the potential spike, and the self-consistent method are applied to solve the electron current density through the spike. The recombination currents within the space charge region (SCR) are also investigated. We use the flat-Fermi method to solve the recombination rate and recombination current. In addition, we use the scattering matrix method to solve the wave function, then calculate the carrier concentration and recombination rate. Two methods obtain comparable recombination currentsexcept the abrupt with setback layer junction. It is due to the potential well exists in the notch of the base edge. The abrupt with setback layer, graded, and combined of graded and setback layer junctions can decrease the barrier height at the interface and enhance the emitter injection current density, but they are accompanied by the increases in recombination within the SCR. The results show that the current gain of abrupt InGaP/GaAs HBT is the highest among these junctions. For abrupt with set back layer AlGaAs/GaAs HBTs, the higher curr ent gains occur at Wi = 50~75A. For graded AlGaAs/GaAs HBTs, Wg = 100A has the highest current gain and the gains decrease due to the increase of recombination current within SCR when the graded width exceeding 100A. Our calculations reveal a higher current gain when the emitter doping is 5E17 cm^-3 and the base doping is 5E18 cm^-3 for abrupt InGaP/GaAs HBTs. The current gain of InGaP emitter HBT shows relatively higher than that of the AlGaAs emitter. This is because the InGaP/GaAs heterojunction has lower conduction band discontinuity and higher valence band discontinuity than it in AlGaAs/GaAs junction.
Databáze: Networked Digital Library of Theses & Dissertations
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