Popular semiconductors currently being used for RF applications include GaAs and InP. The operating frequencies for HEMTs built with these semiconductors covers the range from 800 MHz to 100 GHz. Although high-speed operation is attainable using GaAs or InP, product performance is limited when considering high-power applications, where a high breakdown field and thermal conductivity is needed. Proposed about fifteen years ago as a solution to this problem, GaN has emerged as a viable candidate to challenge, and perhaps overtake, GaAs and InP as the dominant semiconductor in RF products, particularly power amplifiers. This is due to its high breakdown field, large band gap, and high thermal conductivity (relative to InP and GaAs). GaN possesses other material parameter values that are favorable to existing technologies such as carrier saturation velocity, dielectric constant, and piezoelectric coefficients. On a worldwide scale, researchers and industry experts continue to work on the modeling of GaN-based HEMT devices in hope that GaN can be used commercially in the near future. The proposed model is physics-based, making use of the Schrodinger and Poisson equations to establish relationships between the sheet carrier density, Fermi Level, and device terminal voltages. The quantum well formed at the heterointerface is approximated as a triangular well with two eigenstates, both determined from the Schrodinger equation. The charge control equations were carried over into the I/V derivations and channel charge derivations for capacitance calculations. Spontaneous and piezoelectric polarizations at the heterointerface are responsible for the high density of carriers in the channel and are accounted for in the model in the expression for the device threshold voltage. Device performance is dictated by the aluminum mole fraction of the barrier layer. This is because the mole fraction controls the amount of polarization at the heterointerface and consequently the 2DEG density. I/V equations were derived incorporating both drift and diffusion components. A two region model was adopted for the saturation region to account for channel length modulation. Device conductances were derived from the drain current expressions and results compared to experimental data gathered. To address high frequency device behavior, parasitic gate capacitance (Cgs and Cgd) expressions and cutoff frequency expressions are derived and presented. High voltage conditions were assumed for the drain bias to simulate a high power scenario. Relationships between the cutoff frequency of the device, the length of the gate, and drain bias are shown and compared with published data reported by other authors.
Library of Congress Subject Headings
Modulation-doped field-effect transistors--Mathematical models; Gallium nitride
Department, Program, or Center
Microelectronic Engineering (KGCOE)
Sippel, Jonathan, "A physics-based analytical model of an AlGaN/GaN high electron mobility transistor" (2004). Thesis. Rochester Institute of Technology. Accessed from
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