Research On Breakdown Mechanism And New Structures Of GaN HEMT

Gallium nitride(GaN)with wide energy band gap,high breakdown voltage,and high saturation electron mobility rate,etc.show outstanding performance in the fields of high voltage,high temperature,high frequency,and high power.However,the measured breakdown electric field of GaN high electron mobility transistor(GaN HEMT)devices are much smaller than the theoretical value of GaN materials.Aiming at the increase in breakdown voltage of GaN HEMT device,this paper proposes three new high-breakdown-voltage structures based on the device breakdown voltage mechanism and validate them by Sentaurus TCAD simulation software.(1)In order to solve the buffer current leakage of GaN HEMT device,this paper proposes a low Al component double heterojunction AlGaN/GaN HEMT device structure,using Aluminum Gallium Nitrogen(AlGaN)with wider energy band gap instead of GaN as buffer layer material to improve the buffer barrier height and reduce the buffer current leakage caused by electrons in the channel entering the buffer layer.The simulation results show that the AlGaN buffer layer can reduce the buffer leakage current and also reduce the concentration of electrons in the channel,especially the concentration of electrons in the channel decreases as the Al component increases.Combined with the simulation results,the AlGaN buffer layer with an Al component of 0.05 can maintain a low leakage current without excessive loss of electron concentration in the channel.(2)In order to reduce the electric field concentration at the gate edge,an AlGaN/GaN HEMT structure with polarization modulation layer is proposed on the basis of the double heterojunction AlGaN/GaN HEMT structure.According to the polarization modulation mechanism,holes are induced in the AlGaN polarization modulation layer whose Al component decreases linearly along the material growth direction.There are two main function of holes in the polarization modulation layer:when the device is reversed off,the holes help to deplete the electrons in the channel,thereby expanding the region of the depletion and increasing the breakdown voltage of the device;Another,the negatively charged polarization charge left after the depletion of holes in the polarization modulation layer will absorb the power lines emitted by the positive polarization charge in the depletion area of the channel,thus alleviating the gate electric field concentration effect.Simulation results show that the breakdown voltage of the proposed structure is increased to 645V from147V in the conventional structure,while the specific on-resistance in only increased by 0.38m?·cm².(3)This paper also studies the structure of the vertical AlGaN/GaN HEMT(GaN VHEMT)device.The experiments show that the main reasons for limiting the breakdown voltage of GaN VHEMT are device current leakage and uneven electric field distribution in the buffer layer.Aiming at the low breakdown voltage caused by device current leakage,this paper proposed a vertical AlGaN/GaN HEMT(CIL-VHEMT)structure with a SiO₂current isolation layer and changing the electron transfer path.The simulation results show that the breakdown voltage of the CIL-VHEMT structure is 2127V,and the specific on-resistance is 1.38m?·cm²,and has a threshold voltage of 2.2V.The Vertical AlGaN/GaN HEMT(BP-VHEMT)structure with P-type buried pillar is proposed based on the CIL-VHEMT structure,and the electric field in the buffer layer is enhanced by introducing the reverse p-n junction formed by the P-type GaN buried pillar and the N-type GaN in the buffer layer.According to the simulation results,when the number,thickness,length and doping concentration of the buried pillar are 3,0.8?m,8?m and 5×10¹⁶cm^-3 respectively,the electric field in the buffer layer is uniformly distributed and the theoretical maximum breakdown voltage of 2654V is obtained.