Research On Mechanism And Electrical Characteristics Of New High Voltage AlGaN/GaN HEMTs

Wide bandgap semiconductor material GaN has excellent properties such as high critical breakdown electric field,high electron mobility,high electron saturation velocity,and capability to operate at high temperature.Based on Al GaN/GaN heterojunction,GaN HEMT is superior to traditional silicon-based devices in terms of breakdown voltage,switching speed and conduction loss,and is widely used in the field of power electronics.At present,the development of AlGaN/GaN HEMT still faces some challenges:Firstly,under the OFF-state,due to the peak electric field distribution at the drain side of the gate region and large leakage current,i.e,the surface leakage current and the bulk punch-through current,the device is prone to premature breakdown,and its breakdown voltage is far from the theoretical limit of GaN material.Secondly,due to the spontaneous and piezoelectric polarization,there is high density of two-dimensional electron gas（2DEG）at the interface of Al GaN/GaN heterojunction,which makes it difficult for devices to achieve enhancement-mode（E-mode）with a high threshold voltage.Based on the scientific problems above,this thesis has developed two kinds of E-mode AlGaN/GaN HEMT with high breakdown voltage.The specific innovations are as follows:1.A polarization super-junction E-mode AlGaN/GaN HEMT is proposed and developed,which is characterized by introducing a P-GaN layer above the Al GaN barrier layer in the active region to form a polarization super-junction structure.When the device is turned on,the Pi N diode in the active region increases the electron density in the 2DEG channel and increases the saturation current of the device.When the device is under blocking state,the polarization super-junction modulates the electric field distribution in the active region and the breakdown voltage of the device is effectively improved.The fabrication process such as low-damage and high-precision etching process,low-resistance ohmic contact process,dielectric post-annealing process and polarization super-junction process are optimized through experiments.The device is developed by integrating single-step process,and its electrical properties are tested and analyzed.The threshold voltage of PSJ HEMT is 0.8 V.The measured BV and on-resistance(R_on)are 635 V and 12.4Ω·mm at L_gd=7.5μm.2.A dual-channel E-mode AlGaN/GaN HEMT with P-GaN buried layer is designed and studied by simulation.In the off state of the device,the P-GaN buried layer shorted to the source assists in depleting 2DEG under the gate,and the device has a higher threshold voltage.When the device is turned on,under a higher gate voltage,the density of 2DEG in the lower channel recovers,and the electron accumulation layer formed on the GaN channel side of the upper layer provides a new conduction path for the device,improving the saturation current and on-resistance of the device.When the device is under blocking state,the P-GaN buried layer assists in depleting the drift region,and suppresses the leakage current between the source and the drain through the buffer layer,and improves the breakdown voltage of the device.By optimizing structure parameters,the device has a threshold voltage of 1.2 V.At L_gd=6μm,the BV,R_on,spand figure of merit（FOM）are 702 V,0.79 mΩ·cm²and 624 MW/mm²,respectively.