Study On Thermal Storage And Electro-thermal Coupling Reliability Of AlGaN/GaN HEMT

The human demand for high frequency and high-power microelectronic technology has led to the emergence and development of the third generation of semiconductor materials represented by GaN.The excellent performance of GaN makes it become one of the best new semiconductor materials.Since the first AlGaN/GaN HEMT came out in the early 1990 s,GaN-based HEMT has been widely used in many aspects of people's life and production after nearly 30 years of research and development,but the reliability issues such as the self-heating effects,high temperature and high field reliability are still outstanding.These issues restricted its further development and application seriously.Therefore,the self-heating effects,high temperature thermal storage and electro-thermal coupling reliability of AlGaN/GaN HEMT are studied by combining simulation and experiment in this article.First,on the basis of understanding the working principle of HEMT,the self-heating effects of the devices are simulated by using Silvaco and COMSOL,and the temperature distribution and parameter changes inside the device are obtained.The simulation results show that the self-heating effects of the device with sapphire substrate are very serious,resulting in the decline of DC and AC characteristics of the device.The self-heating effects of the device can be effectively suppressed by the use of high thermal conductivity substrate materials such as SiC or the structure of the flip chip.Then the thermal storage experiments of AlGaN/GaN HEMT were carried out at different time lengths.The experimental results show that the thermal storage improves the DC output and transfer characteristics of the device,resulting in the increase of the drain current and the transconductance peak.By comparing the changes of gate capacitance before and after the thermal storage,it is known that the increase of 2DEG concentration and the decrease of effective barrier layer thickness are the reasons for this change.The thermal storage also improves the current collapse characteristics of the device,indicating that the number of surface states of the barrier layer is reduced by the thermal storage.However,there is no significant change in gate leakage current characteristics and gate delay characteristics after thermal storage,indicating that the traps in barrier interface and surface between gate and drain is not affected by thermal storage obviously.The optimal storage time for the improvement of the device parameters can be found by calculating the change of the parameters of the device with different storage time length.At last,the on-state and the off-state stress were applied to the AlGaN/GaN HEMT at different ambient temperatures in order to study electro-thermal coupling reliability.The experimental results show that the increase of ambient temperature will lead to the decrease of the drain current and the transconductance peak of the device,and the decrease rate varies uniformly with the temperature.The main reason is the decrease of 2DEG mobility in the high temperature environment.The transfer characteristics and gate leakage current characteristics of the device will be further degraded when applied to electrical stress at high temperature,but the degradation caused by off-state stress is greater.Comparing the recovery characteristics of two kinds of stress,it is found that the recovery characteristics of the device which is applied on-state stress is fine,while the degradation caused by off-state stress is partially permanent and difficult to recover.By analyzing the degradation rate under different electric stresses,it can be found that the degradation rate decreases with the increase of ambient temperature under on-state stress,but there is no definite relationship between the degradation rate and the ambient temperature under the off-state stress.According to the above analysis,the degradation of the device under the on-state stress is caused by the electron capture mechanism driven by hot-electron,while the inverse piezoelectric polarization driven by high field leads to the degradation of the device under the off-state stress.In addition,the accelerated life test is carried out to evaluate the lifetime of samples.