Investigation On The Low Temperature Characteristics Of AlGaN/GaN HEMT Hemt Devices

Since GaN material owns so many outstanding properties such as large band gap,high breakdown voltage,high electron mobility and high thermal conductivity,that makes the material have great potention in microwave high power field.At present,high electron mobility transistors based on AlGaN/GaN heterostructures is an outstanding representative of GaN-based devices,is widely used in aviation,aerospace.However,the large change in the ambient temperature of the space requires that the device has a very high temperature reliability to meet the requirements for long-term stable operation at extremely high temperatures and very low temperatures.In recent years,researches on the high temperature reliability of GaN HEMT devices has gradually become more mature,but the analysis of the low temperature characteristics of the device is obviously insufficient.Therefore,this paper mainly analyzes the low temperature characteristics of GaN HEMT by theoretical and experimental methods.During the experiment,Lakeshore TTPX probe station is used as a temperature controller,Aglient B1500 and Keithley 4200 are connected as external equipment to test the DC and frequency characteristics of devices.The output and transfer curves of typical AlGa N/GaN under the condition of 77K~300K show that drain current first increase and then decrease with the temperature decrease,the maximum value is obtained at T=100K.The ohmic contact performance of 77K~300K is analyzed by TLM model,and we can find that the block resistance and specific contact resistance both decrease with the decrease of temperature.Based on the fat-FET theory,we can calculate the mobility of 2DEG through the temperature dependent transconductance and C-V curves of a GaN HEMT with a gate length of 20?m.It is found that 2DEG mobility is maximized at 100K,and when the temperature is greater or less than 100K,the mobility is reduced,which can well explain the variation in output with temperature.By CV ring structure analysis,the density of 2DEG is almost no change with the temperature under the condition of 77K~200K.In this paper,the thin barrier enhanced devices and depletion devices were compared.The results show that both types of devices have obvious kink effect under the low temperature and high electrical field condition.In addition,the threshold voltage of enhanced device exhibits a significant positive drift with temperature decreasing,which is contrary to the depletion mode.Finally,the dispersion of C-V curves with a variable frequency is more significant for enhanced device at T=77K.The C-V curves of enhanced device appear significant right shift with the decrease of temperature at the frequency of 100KHz,which coincides with the variation of the threshold voltage.In this paper,the gate leakage of 77K~350K is analyzed.It is found that for the forward bias,thermionic emission is the main leakage mechanism at higher temperature while tunneling is the leading factor at low temperature.However,for the reverse bias gate leakage,there is a linear relationship betweenln(J E_b~2)and 1_bE at 77K~200K,it means that F-N tunneling is the main leakage mechanism.In addition,ln(J E_b)changes linearly with E_b at 250K~350K meanwhile the slope m(T)and intercept b(T)of the fit line both change linearly with 1000/T,it means that FP emission is the dominant reason for leakage in this temperature range.At last,conductance method is applied to study the AlGaN/GaN interface states at77K~200K.It is concluded that the trap density at the interface is gradually increased when the gate bias is changed from the deep depletion region of GaN buffer layer to the 2DEG accumulation region,but it is insensitive to the variation of temperature.Moreover,time constant of traps will only increase sharply at the critical point of threshold voltage and this is mainly related to the band structure changes at the interface.