Research On The Characteristic Degradation Of Normally-Off GaN HEMT

As a third-generation semiconductor,Ga N has excellent performance such as a large band width,high breakdown field,high temperature resistance,and anti-irradiation.Especially HEMTs can be formed with materials such as Al Ga N.Due to the spontaneous polarization and piezoelectric polarization,HEMTs have the two-dimensional electron gas with high concentration and high electron mobility at the interface of the heterojunction.Therefore,it has the advantages of large working current and fast switching speed.It is very suitable for the fields of high temperature,high voltage and high frequency.So,HEMTs have potentiality in satellite communication,space station and other systems.Due to the existence of twodimensional electron gas(2DEG)at the channel,conventional Ga N-based HEMTs are normally-on devices.In many applications,in order to simplify the driving circuit and reduce the static power,the device is required to be a normally-off device.Among several reported methods for implementing normally-off device,Ga N HENTs with p-Ga N gate have received widespread attention due to high threshold voltages(2V),high gate voltage swings.So far,mainstream Ga N-based enhanced HEMT devices have been implemented based on p-Ga N gates,and p-Ga N enhanced HEMT also expose some reliability problems.In recent years,some reliability issues of p-Ga N enhanced devices have attracted the attention of researchers all over the world.These studies include device performance stability under forward gate bias and current collapse.In order to improve the performance of p-Ga N devices,it is necessary to further study the reliability of different regions of the device.This paper conducts related research on the degradation of active and ohmic regions in p-Ga N enhanced devices.First,the degradation research of p-Ga N enhanced HEMT devices was carried out under offstate stress.The analysis shows that the degradation of the device is mainly due to the accumulation of a lot of negative space charge in the Al Ga N barrier layer.In the barrier layer,there are a large number of Mg acceptor traps entered by external diffusion,and these traps are ionized under the large electric field.The ionized holes quickly move to the gate electrode under the electric field,thereby leaving an amount of negative space charges in the barrier layer,which finally appears as a positive drift of the threshold voltage and an increase in the on-resistance.When the trap is completely ionized under the electric field,the threshold voltage and the on-resistance will enter a new stable state.After the stress is removed,the holes that have lost the effect of the electric field force will recombine with the ionized acceptor,showing that the threshold voltage drifts to the left,and the performance of the devices recover.However,the potential barrier at the p-Ga N / Al Ga N junction prevents the rapid combination of positive and negative charges,so the recovery of the threshold voltage is not instantaneous.With the increase of voltage,the accumulated negative charge quickly raises the barrier,and the ionized holes may be trapped again by the trap at the gate.The trap-assisted tunneling effect weakens,which eventually leads the gate current drops.In addition,the large stress voltage and long stress time will cause the on-resistance to rise continuously,which may be caused by the extension of degradation area from under the gate to the gate drain active area or the Ga N buffer layer.Second,the effect of the p-Ga N gate enhanced HEMT's structure on performance is studied.The p-Ga N layer increases the distance between the gate and the channel,resulting in a decrease in the peak transconductance of the device.Therefore,a structure to improve the gate control capability of the device is proposed.This structure combines the p-Ga N cap layer with the recess,reducing the distance between the gate and the channel and increasing the peak transconductance.Finally,the device was simulated to verify the correctness of the function.The paper then conducts research on the ohmic contact of Ga N HEMT with p-Ga N gate.Based on the previous problems,this paper carried out the characterization study of ohmic contact resistance,and explored the degradation rule of ohmic contact under different conditions.First,a new method for extracting the sheet resistance of the ohmic contact area is proposed and compared with the traditional method.Then,the effect of electrical stress and irradiation on the relevant parameters of the ohmic contact area is studied.It is shown that the sheet resistance of the ohmic contact area and the active area is degraded under electrical stress,while only the sheet resistance of the active area of the device is degraded under proton irradiation.the effect of proton radiation on the ohmic contact area is abnormal.The contact parameters of the ohmic contact began to decrease with the increase of irradiation dose,which indicates that a certain dose of proton irradiation will improve the ohmic contact performance.