Study On Current Collapse Effect And Structural Optimization Of GaN HEMT

GaN is a wide-bandgap semiconductor material.It has the charaterstics of large critical electric field,wide operating temperature range and high electronic mobility.It is suitable for the production of high-voltage,high-temperature,high-frequency and high-power devices.GaN HEMT devices made from it have a good application prospect in the fields of military,communications and new energy.Nevertheless,the device has many problems in process production,cost and reliability,among which reliability is worthy of attention.Current collapse is one of the reliability problems,which seriously affects the performance of the device.Based on the current collapse theory under DC bias,this thesis improved structure from two different perspectives to improve the current collapse effect of GaN HEMT devices.Firstly,this thesis introduced the characteristics and advantages of GaN materials and the application and development of GaN-based devices.Then,the current collapse phenomenon,current collapse mechanism and suppression measures of GaN HEMT devices were described in detail,including the manifestations of current collapse under direct current and high frequency,five current collapse mechanisms and four suppression measures.Based on the theoretical descriptions of current collapse,a GaN HEMT device structure with double micropits was designed using Sentaurus TCAD simulation software and related model settings,including trapezoidal micropits at the GaN/Si interface and AlGaN rectangular micropits with low Alcomposition.According to different sizes and quantities of micropits,we simulated and analyzed their temperature distribution,channel electron distribution,output characteristics and current collapse.It concluded that there were hot spots,interface thermal resistance and thermal diffusion phenomena inside the device.Compared the structure with conventional structure,the proposed double micro-well structure can reduce the device temperature by increasing the thermal diffusion areas of the interface and modulating channel temperature,and the temperature peak value is reduced by21.77K.In addition,the saturation region of the output characteristic curve was linearly approximated.It is obtained that the improvement ratio of the current collapse effect of the structure with double micropits at V_GS=0V was about 22.01%.Finally,a GaN HEMT composite structure incorporating a field plate and an AlN layer was proposed,including a discontinuous field plate between the gate and the drain with the same potential as the gate and the AlN layer between the gate and source.We used the simulation software to optimize devices with different structural parameters and analyze their electric field distribution,channel electron distribution,output characteristics and current collapse.We concluded that the field plate can introduce a new electric field to modulate the electric field distribution and ameliorate the current collapse effect.But field plate had a slight influence on the channel electrons below it,and the AlN layer can reduce the influence of the field plate on the channel electrons below it to improve the improvement degree of the current collapse effect.Using linear approximation method,it can be obtained that compared with the traditional structure,the improvement ratio of the current collapse effect of the introduced composite structure at V_GS=0V was about 25.05%,and the effect was better than that of the field plate structure.In summary,the two structures had been proposed can effectively improve the current collapse effect and provided some research value for the application of GaN HEMT devices in the high-voltage and high-power field.