A Study Of Breakdown Mechanism Of GaN High Electron Mobility Transistor

In this thesis, the breakdown mechanism and related reliability of GaN high electron mobility transistors (HEMTs) for microwave power amplifiers applications are systematically studied. Two main leakage current mechanisms triggering the device breakdown are investigated:1) buffer layer leakage current. 2) gate leakage current. A high electric field will also increase the leakage current. leading to breakdown. The relationships of device structure parameter and process condition with breakdown characteristic are investigated in depth by experiments and simulation. Furthermore, guiding methods to improve breakdown voltage and reliability are suggested. The main achievements are:1. AlGaN/GaN HEMTs with two different GaN channel thickness are fabricated. Gate-lag and drain-lag measurements are carried out to assess the trap related current-collapse. The pulsed Ⅰ-Ⅴ test results suggest that current collapse in the two devices mainly caused by the buffer-traps rather than the surface-traps. Moreover, current collapse of device with thick channel is much smaller than that of the thinner channel device, indicating that increased thickness of the channel reduces the trap effect in buffer layer.2. Drain current injection method and the off-state stress tests are used to characterize the off-state breakdown characteristics of the device. Experimental results show that the off-state breakdown in these two kinds of devices are triggered by the current injection from source. Although the device current increase slightly as the GaN channel thickness increase from 50 nm to 150 nm, the off-state breakdown voltage is significantly enhanced, that is, from 90 V to 120V. A win-win of breakdown voltage and current collapse is achieved by derating thinking.3. For further investigation of the breakdown mechanism and explain the experimental phenomenon, GaN HEMTs with different channel thickness, electrical property simulation models are build by Atlas. The models are corrected based on actual test results, so that the device electrical characteristics by simulation are consistent with the test results. Then, energy band and electric field distribution in the devices are simulated.Simulation results suggest that the increase of breakdown voltage of in thick-channel device is due to the reduction of the lateral electric field in channel and buffer layers under the gate. The more severe DIBL effect in buffer layer results in more electrons injected from source to drain through the buffer, causing more pronounced pinch-off source current in thick-channel device. AlGaN/GaN HEMTs therefore requires an optimum thickness of GaN channel and proper epitaxial structure to suppress buffer leakage and improve off-state breakdown voltage.4. AlGaN/GaN HEMTs with different surface passivation by low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) are fabricated to study gate leakage mechanism, respectively. The dual-gate structure is designed to detect the lateral surface leakage current separated from the vertical current. First, pulsed IV measurements were carried out to analyze the density of surface states in the two kinds of devices. The fact that current collapse is lower in device passivated by LPCVD indicating that it has less surface states, compared to PECVD. Device treated by LPCVD shows dramatically mitigated surface leakage, but with enhanced vertical currents, therefore vertical current is the main gate leakage mechanism. Surface leakage in device treated by PECVD is worse, which increases as the negative gate voltage increases and as the dual-gate spacing decreases, and directly results in device breakdown. The lateral surface current mechanism is the two-dimensional variable-range hopping (2D-VRH) conduction due to the high density of surface states. Although reduction of the surface state density is critical to decrease the lateral surface current, it must be combined with other design to prevent the enhancement of vertical leakage, then higher breakdown voltage and high reliability can be achieved.