This paper presents an analytical model for AlGaN/GaN High Electron Mobility Transistor (HEMT) to predict the device performance with variations of ambient temperature. The basic model considers the spontaneous and the piezoelectric polarization induced charges at the AlGaN/GaN heterointerface. Also, the effects of channel length modulation in the saturation region and the parasitic resistance due to the undoped GaN layer have been considered. For incorporation of temperature as model parameter, the temperature variations of low field electron mobility, effective width of 2 dimensional electron gas (2 DEG), bandgap energy, schottky barrier height and the polarization charges have been investigated. The developed model is validated with experimental data for the temperature range of 300K–573K. The small signal microwave parameters have been derived to determine the unity current gain cut-off frequency (f_T) as function of temperature.

The device structure considered in this work is shown in Fig. 1. Spontaneous and piezoelectric polarization of AlGaN and GaN materials contribute to the total electrons in the 2-DEG conduction channel with zero gate bias. The model is based on the charge control method developed by Chang et.al. [1]. The effective width of 2-DEG carrier density, energy band offset between AlGaN and GaN and low field mobility are found as key parameters for temperature response of the device. Although the spontaneous polarization is very strong in group III nitrides, the pyroelectric coefficients, describing the change of the spontaneous polarization with temperature, are measured to be very small [2, 3].  Since the temperature effects in piezoelectric polarization are expected to be very small as in spontaneous polarization, the temperature dependence of piezoelectric polarization is neglected in this model.

Figure 2 shows the drain current versus drain to source voltage characteristics at 300 K. Figures 3 and 4 show the analytical transconductance and unity current gain frequency variations with gate to source voltage as   function of temperature. The unity current gain frequency decreases with an increase in the temperature. The 24% reduction in unity current gain frequency leaves the device suitable for microwave frequency operation at higher temperature.

Figure 1: Device structure of the AlGaN/GaN HEMT

                                  Figure 2: Current-voltage characteristics at 300 K

                                Figure 3: Temperature effect on transconductance

                      Figure 4: Temperature effect on unity current gain frequency

Reference

[1] Chang CS, Fetterman HR. An analytic model for HEMT’s using new velocity-field dependence. IEEE Trans Electron Dev 1987; ED-34(7): 1456-62.

[2] Shur MS, Bykhovshki AD, Gaska R. Pyroelectric and Piezoelectric Properties of GaN-Based Materials. MRS Internet J Nitride Semicond Res 1999; 4S1, G1.6.

[3] Chang Y, Tong KY, Surya C. Numerical simulation of current–voltage characteristics of AlGaN/GaN HEMTs at high temperatures. Semi Sci Tech 2005; 20(2): 188-92.

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