Due to the superior material properties such as high breakdown voltage, high saturation velocity, and high thermal conductivity, III-nitride devices are expected to offer better high frequency, high power, and high temperature performance compared to conventional Si and GaAs devices. The AlGaN/GaN high electron mobility transistor (HEMT) is one of the most promising devices because of its modulation-doped structure and a significantly larger polarization field induced at the hetero-interface. Recent progress in growth and process technology has already led to very impressive results for AlGaN/GaN HEMTs. A transconductance as high as 270 mS/mm has been achieved in devices with 0.7 μm gate length [1]. AlGaN/GaN HEMTs grown on a SiC substrate with a power density as high as 9.8 W/mm at 8 GHz have also been demonstrated, which is about ten times higher than GaAs-based FETs [2], and current gain cutoff frequencies of 101 GHz have been reported [3]. Recent studies show that there is about a 20% deviation for the sheet charge density in the AlGaN system between theoretical predictions and experimental measurements [4]. Nitride device modeling [5-10] has assumed that the polarization in nitride alloys is linearly interpolated from the values of the parent binary compounds [11-12]. However, recent research indicates that the macroscopic polarization in III-nitride alloys is a nonlinear function of the material composition [4]. Moreover, recent experimental measurements in AlGaN/GaN heterostructures illustrate that reasonable agreement is obtained using the nonlinear polarization formulation [4,13-14]. Also our research has shown that excellent agreement between the simulation and experimental data is obtained when the nonlinear polarization formulation is employed in the AlGaN/GaN HEMT model [15].
Since both the spontaneous and piezoelectric polarization fields can induce a significantly larger sheet charge and alter the band bending at the AlGaN/GaN hetero-interface, the device performance in the AlGaN/GaN HEMT is quite different from that in the AlGaAs/GaAs HEMT. Therefore, it is important to make a comparison between AlGaN/GaN and AlGaAs/GaAs HEMTs to illustrate how this natural advantage translates into improved device performance.
Several theoretical AlGaN/GaN HEMT simulation models have been presented and reported in the literature [5-10]. Rashmi’s group presented analytical models including a one-dimensional (1D) and two-dimensional (2D) model to examine DC and RF characteristics of HEMTs [5-7]. A full band Monte Carlo simulation also was presented by Ando et al [8]. Sacconi et al. investigated DC performance by using a quasi-2D model [9]. Within the framework of the gradual channel approximation, Albrecht et al. included the thermal effects in the I-V calculation of a GaN/AlGaN HEMT [10]. Though much progress has been made by these researchers, to the author’s knowledge this is the first theoretical study of the comparison between AlGaN/GaN and AlGaAs/GaAs HEMTs with the same basic geometric structure. The AlGaN/GaN HEMT model includes a nonlinear formulation of the polarization effects [15].
In order to affect a reasonable comparison of the AlGaAs/GaAs and AlGaN/GaN HEMTs comparable device structures must be examined. Unfortunately, experimental data are only available for the specific structure examined for the AlGaAs/GaAs HEMT. Thus the AlGaAs/GaAs HEMT device calculation is calibrated by using measured data. In an earlier work, we have compared our calculations of AlGaN/GaN HEMTs to other structures for which experimental data were available [15]. Excellent agreement was obtained thus ensuring that the present calculations for the AlGaN/GaN device have also been calibrated albeit to different device structures.
The sheet charge density, a function of gate voltage, is calculated from the Schroedinger and Poisson equations self-consistently. The carrier mobility in the AlGaN/GaN HEMT is extracted from Monte Carlo simulation [16]. Based on the self-consistent charge control model and the field-dependent mobility, a quasi-2D model is used to calculate the drain-current characteristics in the HEMT devices. The most important characteristics including the sheet charge density vs. gate voltage, gate to source capacitance, drain current-voltage, transconductance, and cut-off frequency are used to examine the performance of GaN and GaAs HEMTs.