In this study, we investigated the drop in the drain current value of an AlGaN/GaN HEMT immediately after the collapse phenomenon occurred. Analysis of the IV characteristics clarified that this occurrence was owing to electrons being trapped by the current collapse phenomenon, causing a drop in the electron concentration. By applying a frequency that suppresses the current collapse phenomenon from the gate electrode side to the AlGaN-GaN crystal layer, we measured the process in which electrons were released from crystal defects in the AlGaN-GaN crystal layer. The frequency dependence of the carrier emission process was measured at 1 to 4 GHz, and we confirmed that the higher the frequency, the slower the carrier emission and the higher the saturated drain current value. This is because the time taken for electrons to be trapped in crystal defects and the process time for electrons to be emitted differ depending on the crystal defect. In crystal defects with long emission times, electrons are repeatedly emitted and recaptured even when the current collapse phenomenon is suppressed. Consequently, electrons cannot be recaptured at high frequencies, and the saturated drain current value increases. By comparing the carrier emission process at different frequencies, the components of crystal defects that cannot emit or capture electrons can be extracted. Crystal defects can be evaluated by comparing the carrier emission process at different frequencies.

Voltage stress of 1–12 V was applied to an AlGaN/GaN high-electron-mobility transistor (HEMT) from the drain electrode, and the$I$-$V$characteristics were measured precisely. The experimental results revealed that the change in$I$-$V$characteristics caused by voltage stress resulted from changes in the crystal structure of the AlGaN layer. There was no significant change when the voltage stress value was approximately 1 or 2 V. When 3–5$V$was applied, the drain current value increased, and the initial strain resulting from crystal defects remaining in the AlGaN layer was canceled by the electric field. However, under excessive stress of 6 V or higher, distortions caused by crystal defects occurred again in the AlGaN crystal structure, electron scattering increased, and the current value decreased. The current value changed greatly in the region where the drain voltage was between 1.0 and 3.2 V, but the I-V characteristics matched in the region where the drain voltage was 3.2 V or more. Therefore, it is believed that two-dimensional electron gas (2DEG) formed stably at approximately 3.2 V, and the crystal defects were not affected by the voltage stress change. Furthermore, when the measurement temperature was changed to 80°C and 100°C, an overshoot effect was observed near a drain voltage of 3.2 V. This is considered to be caused by the improvement in carrier mobility resulting from stable 2DEG formation.

In this study, we confirmed the dependence of voltage stress and temperature on the I–V characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs) immediately after applying voltage stress. The high voltage stress and high-temperature environment intensified the kink occurrence, likely due to the expansion of the crystal strain. Results showed that, as the voltage stress increases, a crystal strain occurs in the GaN or AlGaN layer, which affects carrier transport. It has been reported that when the drain voltage is lower than 6 V, two dimensional electron gas (2DEG) is not stably formed; thus, carrier scattering occurs due to crystal distortion. Meanwhile, 2DEG stably forms above 6 V; hence, the influence of distortion is assumed to be minimal. The inflection point of the drain conductance is further calculated based on the I–V characteristics obtained from the kink phenomenon. This inflection point suggests a transition from 3D drift motion to 2DEG carrier transport. The impact of lattice scattering becomes more noticeable at drain voltages under 6 V in the I–V characteristics, where three-dimensional transport is dominant. The lattice scattering is confirmed to be relatively mitigated at drain voltages above 6 V in the I–V characteristics, where two-dimensional transport by the 2DEG is prevalent. It is reported that the lattice vibration of crystal strain due to temperature rise saturates above a certain temperature, and electron scattering also gets limited. When a certain temperature is exceeded under high electric field stress, the drain voltage, which is the inflection point, no longer changes. This is because the amount of crystal distortion due to temperature saturates and no longer affects the 2DEG.

With the growing demand for high-performance, high-frequency electronic devices, the development of devices based on gallium nitride (GaN) has garnered increasing interest. In this study, we examined the current-voltage characteristics of AlGaN/GaN high-electron-mobility transistors subjected to electrical stress. Stress application induced a kink effect, which became more pronounced with increasing gate reverse-bias stress and stress application duration. This observation indicated that local lattice strain, caused by the inverse piezoelectric effect, degraded the device performance. When combined with lattice vibrations owing to heat, this strain led to further degradation. However, the combined effect eventually reached saturation. Moreover, lattice vibrations likely suppressed electron trapping and strain during stress application, resulting in a nonlinear performance degradation trend. The inverse piezoelectric effect was sensitive to thermal fluctuations, resulting in instability. Furthermore, transient response measurements of the drain current revealed a long-term current recovery process, indicating a variation in the interaction between polarization and thermal fluctuations.

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.