In this study, we observed changes in the characteristics of the crystal defects in the crystal layers forming aluminum gallium nitride/gallium nitride high electron mobility transistors at low temperatures. A kink phenomenon was observed in the IV characteristics as the temperature decreased. Moreover, the current gain increased as the temperature decreased. Furthermore, the parasitic capacitance showed a similar temperature characteristic. The mode in which crystal defects capture carriers can be explained by a switch in the transport phenomenon from the induction of carriers to crystal defects triggered by carrier capture, rather than by two-dimensional electron gas depletion.

In this study, we irradiated light with energy greater than the bandgap of GaN from the top of the GaN-HEMT to generate photogenerated carriers in the GaN layer. By investigating the behavior of photogenerated carriers, we elucidated the carrier behavior in the GaN layer. Photogenerated carriers were generated under different temperatures. The temperature dependence of the high frequency characteristics of photogenerated carriers was also investigated. According to the results, the attenuation of high-frequency gain due to high temperature was canceled out by photogenerated carriers. Furthermore, analysis of the parasitic capacitance between the gate and source electrodes confirmed that the parasitic capacitance tended to increase as the drain voltage increased. The hole accumulation effect caused by photogenerated-carrier holes of near the source electrode was experimentally found to inhibit current gain amplification.

In this study, the current-voltage (IV) and Radio Frequency (RF) characteristics were measured in a low-temperature environment, and the transient response waveform was analyzed by the simplified Isothermal Capacitance Transient Spectroscopy (ICTS) method. According to the measurement results of the IV characteristics, the current value decreased rapidly as the temperature decreased, and the linear region could not be confirmed. From the RF characteristics, it was confirmed that the gain varies with temperature in the high-frequency band. Furthermore, the results suggested that substantial structural changes occurred in the device under low-temperature environments. In addition, it was confirmed by simple ICTS analysis that the capture cross-section and crystal defect concentration increased with decreasing temperature. At room temperature, the thermal energy imparted to the crystal layer induces phonon scattering at crystal lattice points. This implies that the process of trapping electrons in crystal defects is suppressed. In a high-frequency region, carriers trapped in crystal defects are released and the crystal defects become conspicuous. As a result, an increase in the capacitance component of crystal defects was confirmed, and it is thought that both the capture cross-section and the crystal defect concentration increased compared to those at room temperature.

In this study, we experimentally observed the behavior of carriers generated by the photoelectric effect in the GaN crystal layer (channel layer) from the viewpoint of high-frequency characteristics to clarify the carrier transport phenomenon. It was found that the accumulation of holes generated by the photoelectric effect near the source electrode causes a new parasitic capacitance component that depends on the gate voltage.

In this study, we analyzed the measurement results of the frequency dependence of the current collapse in AlGaN/GaN high-electron-mobility transistors and confirmed the gate voltage dependence and temperature dependence in the high frequency region. An oscillation phenomenon was confirmed in the frequency dependence of the drain current. It was suggested that the oscillation phenomenon is caused by electrons moving back and forth between the crystal defect and the two-dimensional electron gas (2DEG) following the frequency. In the high-frequency region (0.8 GHz to 5 GHz), we calculated the inflection point from the oscillation waveform of the drain current, and succeeded in separating the response performance owing to crystal defects that enable high-frequency response. As a method for evaluating 2DEG carrier behavior in high-frequency bands, area value evaluation of areas with frequency dependence was investigated. The area value increases as the temperature rises (22°C to 80°C); however, it is confirmed that the area tends to decrease at higher temperatures (100°C or higher). The calculation result of the area value is linked with 2DEG behavior in GaN crystal.