Research On GaN HEMT Enhanced Devices With P-GaN Cap Layer

Compared with silicon(Si)-based devices,AlGaN / GaN heterostructure-based high electron mobility transistors(HEMT)can work at higher voltage,current,frequency and temperature due to the high electric field strength of the material and the high mobility and electron density of Two-dimensional electron gas(2DEG)formed at the AlGaN / GaN interface.However,the presence of 2DEG makes conventional GaN HEMT devices depleted.For power electronics applications,enhanced devices are essential for safety reasons and to simplify drive circuits.The realization of enhanced GaN HEMT devices has become a research focus in recent years.Owing to it's reliability and process repeatability,GaN-enhanced devices with p-GaN cap layers have become mainstream commercial devices.In this paper,the effects of structural parameters on the DC characteristics of p-GaN HEMT devices are studied through simulation.P-GaN enhanced devices with high threshold voltage and large saturation current are prepared through process optimization.The research results obtained are summarized as follows: 1.The Silvaco software was used to simulate the change of the DC characteristics of the conventional structured p-GaN HEMT as the relevant structural parameters were changed.As the p-GaN cap layer acceptor concentration and thickness increase,the depletion effect on the 2DEG at the heterojunction interface under the gate increases,causing the device threshold voltage to increase.With the increase of Al composition and thickness in the AlGaN barrier layer,the net polarization charge generated by the AlGaN / GaN heterojunction increases,and the threshold voltage of the device decreases accordingly.The introduction of an AlGaN back barrier structure can effectively increase the conduction band energy level at the interface of the heterojunction,and improve the threshold of the device while improving the 2DEG confinement.As the Al composition increases,the device threshold voltage also shifts forward.2.In order to further improve the DC characteristics,the p-GaN cap layer under the gate is etched,and p-GaN HEMT with recessed-gate was introduced.The effects of the proportion of recessed-gate and the depth of recessed-gate etching on the characteristics of the device were studied.Controlling the depth of the recessed-gate to only etch away the p-GaN layer and changing the proportion of the recessed-gate has a negligible effect on the threshold voltage of the device.As the proportion of recessed-gate increases,the gate control capability and saturation operating current of the device also improve.Fixed the proportion of recessed-gate as 75%,only the depth of recessed-gate etching was changed.During the gradual increase of the depth from-5nm(residual 5nm p-GaN layer)to 5nm(AlGaN barrier layer etching 5nm),the device threshold voltage did not change significantly.However,the etching depth further increased,and the threshold voltage shifted significantly.In terms of saturation current,the residual p-GaN layer or the etching of the AlGaN barrier layer are not conducive to performance improvement.3.Based on the AlGaN / GaN heterostructure with 110 nm p-GaN cap,Ni / Au gate p-GaN enhanced devices with threshold voltage of 2.3V,source drain saturation current of 210 m A/mm,transconductance peak of 45 m S/mm and breakdown voltage of 340 V at gate drain spacing of 6.5 ?m were successfully fabricated.In order to study the effect of different work function metals on the DC characteristics of the device,the low work function metal Ti / Au was used as the gate,so that the threshold voltage of the device was increased to 2.9V and the breakdown voltage was increased to 410 V.In order to further improve the DC characteristics of the Ti / Au gate device,the device was subjected to a post-gate annealing at 300 ° C for 10 minutes in a nitrogen atmosphere.The annealing treatment helps to repair the interface state of the barrier layer and shallow-level N vacancy defects,and promotes a solid-phase interface reaction between the gate metal and the p-GaN cap layer interface,which reduces the gate Schottky barrier height.Post-gate annealing caused the device's threshold voltage to be reduced from 2.9V to 1.9V,the peak value of transconductance increased from 85 m S / mm to 115 m S / mm,and the saturation operating current increased from 310 m A / mm to 505 m A / mm,an increase of nearly 66%.Considering the tradeoff between threshold voltage and saturation current,the DC characteristics of the device are close to the international leading level compared with the published device indexes in recent years.