# Study of Self-Heating and High-Power Microwave Effects for Enhancement-Mode p-Gate GaN HEMT
## 3 Results and Discussion
### Electrical Characteristics  
Figure 2 shows the simulation results which fit the previously reported experimental data in appearance [33]. The transfer characteristics curve and output characteristics curve agree with the experimental measurement well. The value deviation between simulation and experiment in output current density, threshold voltage, and turning point of the I\({}_{\text{D}}\)-V\({}_{\text{D}}\) curve is caused by the structure and material properties. The proportion of aluminum in the AlGaN layer, the doping concentration in the barrier layer, and the geometric size of \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **GaN** & **Al0.23 GaN** \\ \hline Heat capacity \(cv\) & 3.0 (J/(K\(\cdot\)cm\({}^{3}\))) & 2.76 (J/(K\(\cdot\)cm\({}^{3}\))) \\ Thermal conductivity \(\kappa\) & 1.3 (W/(K\(\cdot\)cm)) & 1.66 (W/(K\(\cdot\)cm)) \\ Bandgap \(E_{\text{S}0}\) & 3.51 (eV) & 4.14 (eV) \\ Initial mobility \(\mu_{0}\) & 1500 (cm\({}^{2}\)/(Vs)) & 1224 (cm\({}^{2}\)/(Vs)) \\ Electron effective mass \(m_{e}\) & 0.22 & 0.24 \\ Relative permittivity \(\varepsilon_{r}\) & 9.4 & 9.26 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The initial value (T = 300 K) of the key parameters. the device that the simulation adopted all contribute to the slight deviation [33,34]. The above results demonstrate the reliability of the simulation model in this paper. For HEMT semiconductors, it is far from enough to only study its IV characteristics; its internal mechanism also needs to be revealed. Before being turned on, there is a certain concentration of carriers in the channel layer as shown in Figure 3a, which cannot provide the normal work mode. What cannot be ignored is a carrier distribution of an order of magnitude of a normal distribution pattern under the channel layer. The off-state studied here refers to the situation that the gate voltage is less than the threshold voltage, and the voltage bias of the source and drain electrodes is normal (Vds = 18 V). The carriers of the barrier layer, the channel layer, and the buffer layer are led to both ends under the action of the source-drain potential difference, forming a weak leakage current path in the buffer area, which is obvious from the current density distribution results as shown in Figure 3b. After the channel is turned on, most of the carriers are concentrated in the two-dimensional electron gas (2 DEG) layer. The carrier concentration under the channel is negligible compared with the carrier concentration in the 2 DEG layer. The difference in electron distribution between on-state and off-state can be explained from the conduction band diagram in Figure 4: before the HEMT is turned on, there is a potential barrier of up to 3.8 eV, and only a small part of the carriers can pass through the barrier and enter the channel layer. As the channel turns on, the conduction band moves down to 1.8 eV due to the external gate voltage, and most of the carriers in the AlGaN barrier layer can enter the channel layer and be trapped in the quantum well to form a two-dimensional electron gas. Most of the carriers are concentrated in the direction perpendicular to the AlGaN/GaN interface which is defined as 2 DEG. Figure 2: (a) Simulation and measurement comparison in output characteristic (simulation—solid line; measurement—dotted line); (b) transfer characteristic (simulation, blue line; measurement, red line). With the heterojunction structure, the HEMT device separates the ionization center and free electrons spatially which aims to gain higher mobility for carriers. At the initial off-state moment, the lower carrier concentration in the channel layer leads to a higher mobility due to the weak scattering effect. With the injection of the gate voltage, the channel is turned on, the concentration of the two-dimensional electron gas rises sharply, and the scattering between the carriers is enhanced.