# Study of Self-Heating and High-Power Microwave Effects for Enhancement-Mode p-Gate GaN HEMT
## 2 Device Structure and Simulation Model  
A two-dimensional device model is built in this paper. In the specific simulation process, Sentaurus TCAD will assume a \"thickness\" (effective width along the z-axis) of 1\(\upmu\)m. The cross-sectional schematic of the enhancement-mode AlGaN/GaN heterostructure HEMT is shown in Figure 1a. The physical models of AlGaN/GaN HEMT adopted in this simulation consist of a 10 nm thick AlN nucleation layer,2 \(\upmu\)m thick Al\({}_{0.05}\)Ga\({}_{0.95}\)N buffer layer, a 10 nm thick GaN channel layer, a 15 nm thick Al\({}_{0.25}\)Ga\({}_{0.77}\)N barrier layer, a 0.2 \(\upmu\)m thick SiN passivation layer, and a 110 nm thick p-GaN cap layer. The length of the gate Lg, the gate-source extension Lgs and the gate-drain extension Lgd are 1.4 \(\upmu\)m, 1 \(\upmu\)m, and 1 \(\upmu\)m, respectively. The device is 4.4 \(\upmu\)m wide. The doping concentrations of the p-GaN layer, AlGaN barrier layer, GaN channel layer, and AlGaN buffer layer are \(3\times 10^{17}\) cm\({}^{-3}\), \(1\times 10^{18}\) cm\({}^{-3}\), \(1\times 10^{15}\) cm\({}^{-3}\), and \(1\times 10^{14}\) cm\({}^{-3}\), respectively. A part of oxygen atoms or nitrogen atoms will be generated which makes AlGaN buffer show slight N-doping characteristics due to the AlGaN epitaxial growth process [25]. In this paper, a light N-type doping is applied to simulate this effect. The SiN passivation layer can moderate the current collapse effect to a certain extent [26]. P-GaN cap layer is used to elevate AlGaN/GaN heterojunction conduction band that makes an enhancement-mode device [27]. The channel layer is doped to increase the two-dimensional electron gas (2 DEG) electrons concentration. The 2 DEG electrons are mainly from Al\({}_{0.23}\)Ga\({}_{0.77}\)N barrier layer doping. The concentration of 2 DEG electrons will be slightly affected by the polarization effect and surface state as well [28]. The simulation work of this study was performed by Sentaurus TCAD software [29]. Some material parameters used in this work such as mobility \(\mu\), bandgap \(Eg\), and thermal parameters are related to temperature(\(T\)) or other factors. Take mobility as an example, it is constrained by factors such as electric field, doping concentration, and temperature. The carrier mobility at different moments and positions is different. The initial values of some parameters are shown in Table 1. However, using initial values during the entire simulation process will lead to an inaccurate simulation result, so it is important to consider necessary physical models to calibrate the simulation. Default values are used for other material parameters in Sentaurus TCAD. A brief description is given below:  
Figure 1: (a) Cross-sectional schematic of the enhancement-mode p-gate AlGaN/GaN HEMT; (b) partially enlarged view under the gate. The Fermi statistical distribution model is applied for heavily doped active regions to characterize that when the doping concentration exceeds \(1\times 10^{19}\) cm\({}^{-3}\), Boltzmann statistics can no longer be used to approximate Fermi statistics. The Fermi model should be chosen to improve simulation accuracy. With the effect of scattering, the constant mobility model that previously reported work used cannot match the actual movement of carriers well. Dynamic mobility model was applied which includes lattice scattering effect, ionized charge scattering effect, carrier scattering effect, doping-dependence effect, and high-field-saturation effect. Piezoelectric polarization model is involved for the polarization effect that is unique to GaAs HEMTs. The electric field induced by the spontaneous and piezoelectric polarization can result in an increase in the carrier concentration at the AlGaN/GaN heterointerface [28]. In our work, the interface charge induced by the polarization at AlGaN/GaN is set as \(6\times 10^{12}\) cm\({}^{-2}\). Sentaurus provides two models (strain and stress) to compute polarization effects in GaN devices.