Simulation And Process Study Of AlGaN/GaN Recessed-Gate MISFET Devices

As third-generation semiconductor,GaN itself has the advantages of large band gap,high electron saturation drift mobility,large critical breakdown electric field and high temperature stability.It's very suitable for making microwave,high temperature and high power devices.And due to the effects of piezoelectric polarization and spontaneous polarization,a two-dimensional electron gas(2DEG)of high-density is generated on the side of the AlGaN/GaN heterojunction at the GaN side,making the AlGaN/GaN heterojunction HEMT device has advantages of large current density and fast switching speed.However,the conventional structure HEMT device is a normally-on device,and additional negative-voltage driving circuit is required in practical applications,which increases the complexity of the circuit design,increases the additional power consumption,and greatly reduces the reliability of the system.Therefore,the GaN-based normally-off(enhanced)HEMT device has become the focus of research.The AlGaN/GaN MISFET(MOSFET)with recessed gate structure is one of the enhanced HEMT devices,with small gate leakage and Large gate voltage operating range,high switching ratio,no need for additional driver circuits and easy to make enhanced devices,making it a top priority.The MISFET device of the recess gate structure requires groove etching and introduction of a new gate dielectric layer,the interface state of the groove surface and the fixed charge in the gate dielectric deteriorate the performance of the device,such as output current reduction,current collapse,Threshold hysteresis and threshold voltage negative shift,etc.Based on this,this paper uses Atomic Layer Deposition System(ALD)to grow Al2O3 as the gate dielectric,and studies the groove etching and surface treatment process of the device.The main research contents are as follows:(1)Firstly,a depletion HEMT device was fabricated based on the conventional HEMT device process flow.The threshold voltage was-2.3V,the maximum output current density was greater than 200mA/mm,and there was no threshold hysteresis.In the working state of large gate voltage and large drain voltage,the output current of the device has a tendency to decrease(negative transconductance effect),by the Silvaco-TACD simulation,the thermal electron effect is one of the main causes of current drop.It is also found through the transfer characteristic test that the increase of the gate length and the source-drain distance has less influence on the threshold voltage,but causes the drain current to decrease.Through the gate leakage test,it was found that for a conventional HEMT device,the gate breakdown voltage of the ring-shaped gate device is larger and has a larger output current density(239 mA/mm).(2)Secondly,the influence of the conventional field plate on the breakdown characteristics of the HEMT device is simulated.The peak point of the electric field in the conductive channel of the device is simulated on the side of gate.The field plate can reduce the peak value of the electric field in the channel.And the distribution of the electric field in the channel is more uniform.The simulation also found that with the increase of the field plate,the breakdown voltage first increases and then decreases,and the length of the field plate has an optimal value.This phenomenon is caused by the breakdown point of the device to be transferred from the vicinity of the gate.to the drain with the increase of the field plate.Through the simulation of the Recessed gate MISFET device,it is difficult to achieve a threshold voltage more than OV when the AlGaN barrier layer is not completely etched.The simulation of the interface state shows that the interface state of the groove surface is another important factor causing the negative shift of the threshold voltage of the device.(3)In the preparation of the recessed gate MISFET device,the cycle-etch method is used to reduce the etching rate to 15 nm/min,and the surface damage of the groove is repaired by 550?/N2 rapid thermal annealing.Combined with the KOH/70?,the root mean square roughness(RMS)of the groove surface was reduced to 0.306 nm.Under this condition,the device prepared by using Al2O3 as the gate dielectric has a threshold voltage of 2V,gate breakdown voltage is 14V,and threshold hysteresis less than 0.3V.For recessed gate structure MISFET devices,the ring gate is more susceptible to breakdown.