Research On Switching Characteristics And New Structure Of GaN Power Devices

GaN power devices have the characteristics of high mobility,high withstand voltage,and high current density.However,there are many key scientific problems need to be solved.For example,when the conventional structure GaN HEMT device is in the off state,the electric field crowding will appear at the gate,which will lead to the early breakdown of the device.There has a big gap between the switching characteristics and the theoretical limit,and the advantages of high frequency and high efficiency cannot be fully exploited.In order to solve the above-mentioned key scientific problems,thesis mainly focuses on the exploration of new device structures and the improvement of switching characteristics.The main research results obtained are as follows.First,a new structure device(FH-HEMT)is proposed in which high-K/low-K dielectrics are respectively inserted into the AlGaN buffer layer between gate and drain to make it appear as a finger-like hybrid dielectric layer.The finger-like hybrid dielectric layer can modulate the channel electric field,thereby improving the breakdown voltage of the device.The research shows that when the gate-drain distance is 6?m and the buffer layer thickness is 2?m,the breakdown voltage of the conventional AlGaN buffer layer structure device(A-HEMT)is 817V,while its 1702V of the new structure FH-HEMT device proposed in thesis,the withstand voltage is increased by up to 108%.At the same time,the specific on-resistance R_on,sp of the FH-HEMT device is 0.44m?·cm~2,and the power figure of merit reaches 6.58GW/cm~2.Compared with the A-HEMT structure,not only the on-resistance is reduced,but also the power figure of merit of the device is greatly improved.Second,a new structure device(VDB-HEMT)with vertical diodes introduced into the AlGaN buffer layer under the drain is proposed.When this structure withstands voltage,the reverse-biased PN junction bears a part of the voltage,which can reduce the peak value of the electric field on the side of the drain and raise the average electric field of the channel.At the same time,it is also found that the leakage current of the buffer layer is also significantly reduced due to the reverse bias of the PN junction,which is more conducive to improving the withstand voltage of the device.The research shows that the breakdown voltage of the VDB-HEMT device is up to 1505V when the gate-drain pitch is 6?m and the buffer layer is 2?m,which is 164%higher than that of the A-HEMT structure.At the same time,the specific on-resistance R_on,sp of the VDB-HEMT structure is 0.51m?·cm~2,and the power figure of merit reaches 4.44 GW/cm~2.Compared with the A-HEMT structure,a significant improvement in the device power figure of merit is achieved while maintaining low on-resistance.Finally,based on the analysis of the test results of the switching characteristics and parameters of the GaN HEMT device,the relationship between the switching characteristics and the device parameters is deeply studied through the device simulation.The test results show that with the increase of temperature,the threshold voltage of the depletion-mode device prepared is shifted in a positive direction,and the switching characteristics are degraded.At the same time,it is found that the dynamic on-resistance R_on,D increases with the increase of voltage stress and stress time.Then,by building a device-circuit hybrid simulation circuit,the switching characteristics of VDB-HEMT device are simulated and optimized.It is found that when the Al composition of the buffer layer is 0.25,the switching speed of the VDB-HEMT device is the highest,the rise time is 0.7ns,and the fall time is 3.83ns(at 166KHz),which is far better than the Conv-HEMT device.