Numerical simulation of the I-V characteristics of heterojunctions including thermionic emission mechanism

Abstract

Absaact-The effects of temperature, Al mole fraction, and doping on the I-V characteristics of abrupt GaAdAIGaAs hetgrojunctiom are studied by numerically solving the basic semiconductor equations. Numerical models based on drift-diffusion with and without inclusion of thermionic emission current at the heterointerface are used and their results are compared. The simulation shows that neglecting the thermionic emission Mi overestimate the current, especially, at lower temperatures, at higher mole fraction, or for n-N structures. Under high injection, as temperabe increases, current becomes limited mainly by the series resistance. The asymmetry of doping level is fouud to affect significantly the transport process at lower temperatures only. I. I"IR0DUcnON&CENTLY, heterostructure based devices, such as high mobility tramistors (HEWS) and heterojunction Mpolar transistors (HBT's) have stimulated great interest because of their potential for high-speed and bigh-frequency devices. The most extensively studied materid system to date is the AIxGal,As/GaAs heterojunction where the aluminum mole fiaction (x) in AlxGal.,As is varied from zero to approximately 0.3. Excellent performance AIGaAs/GaAs have been reported [l]. To help understand tbe opmtionai mechanisms of this structure and hence optimize its design, various modeling methods have been developed. Theoretical discussions of current transport tbrough p-N heterojunction have been presented by [2,3]. Drift-dSfusion bansport model has been applied to numericalIy simulate the characteristics of abrupt and graded p-N hetemjunctions used in HBTs [4,5]. Horio and Yanai 1990, presented a more accaate numerical technique by taking into a m u n t the thermionic emission process at the abrupt heterojunction interface in conjunction with the drift-diffusion formulation for the bulk region. The thermionic emission current formulated at the heterointerface is used as a boundary conditim for the Curtent transport across the interface [61. In this paper, :. le present the results of numerical simulation of a p-N GaAslAIGaAs heterojunction, examining the effect of temperawe (T), Al mole fraction (x), and doping on the current-voltage characteristics . The simulation takes into account the temperature dependence of material parameters. U. STRUcTLREi MODELING Electron and hole transport across the heterointerface can be described by thermionic emission over the bandgap discontinuities in the conduction band (A&) and valence band (my). The basic equations based on the drift-diffusion model are employed for the region where the material composition is constant. The modificau'.ns made to handle heterojunctions use the band parameter approach introduced by Sutherland and Hauser [7]. For one-dimensional steadystate. the equations are then expressed as follows: (3)Where, y is the electrostatic potential, y, and yp are the composition dependent band parameters, ND and NA are the donors and acqtors concentration, respectively, N, and N, are the density of states in the conduction and valen...