Study On Strain And Temperature-dependent Characteristics Of The Barrier Layer In GaN-based HFETs

GaN-based heterojunction field effect transistors(HFETs)have broad application prospects in the fields of high temperature,high frequency and high power due to their superior characteristics such as forbidden bandwidth,high saturation electron drift speed,and two-dimensional electron gas generation(2DEG)without any doping.Although GaN-based HFETs have many of the above advantages,the reliability of device in operation has always been the bottleneck hindering its commercial development.In order to improve the reliability of GaN-based HFETs,the degradation mechanism of devices operating at high temperatures and high field conditions can be analyzed in depth.The polarization effect is the essential characteristic of the III-group nitride material system.Gan-based HFETs benefit from the polarization effect,which can generate 2DEG of high order without any doping.However,the polarization effect is affected by the strain of the barrier layer,thus affecting the carrier transport characteristics of the device.Therefore,it is of great significance to study the strain and polarization of the barrier layer in a high-temperature environment for improving the reliability of the device.In this thesis,a method to determine the strain of the barrier layer under the gate of GaN-based HFETs under variable temperatures environment is proposed.Based on this method,the dependence between the temperature and the strain of the barrier layer under the gate of GaN-based HFETs with different sizes is studied.Then,the effect of gate length on the strain of barrier layer is further studied.Finally,the influence of barrier layer strain on the saturation current and transconductance of the device is discussed.It includes the following contents:1.According to the existing method of solving the strain of the barrier layer under the gate of GaN-based HFETs at room temperature,the strain at different temperatures is deduced from the experimental point of view,combining with Debye model and the variable temperature theory of polarization effect.Through the above method,the strain of GaN-based HFETs with different sizes and different gate lengths under the condition of temperature change is calculated.It is found that the dependence of strain on temperature is different due to the different geometric size of the device.2.In order to further understand the effect of strain on the electrical properties of GaN-based HFETs,the effects of the strain of the barrier layer under the gate of the three sample devices on the electrical parameters such as saturation current,threshold voltage,and transconductance at different temperatures were calculated and analyzed.The results show that with the increase of temperature,the strain of the barrier layer under the gate increases,which makes the Polarization Coulomb Field(PCF)scattering have a certain impact on the mobility of channel 2DEG,and then affects the electrical performance of the device.In conclusion,GaN-based HFETs may cause strain relaxation at high temperature,thus leading to device degradation.By solving the strain of the barrier layer under the gate at variable temperatures and associating it with electrical characteristics,it is beneficial to improve the electrical performance of GaN-based HFETs by adjusting the strain of devices,thus solving the reliability problem of devices at high temperature.