Effect Of Substrate Thinning On The Electronic Characteristics Of AlGaN/GaN HEMT

AlGaN/GaN high-electron-mobility transistors?HEMTs?have shown excellent performance in high-frequency and high-power applications because of the high sheet carrier concentration and high electron mobility in the two-dimensional electron gas?2-DEG?channel.Chemical mechanical polishing is used to thin the substrate of device in the current integrated circuit industry.Although,HEMTs can be processed with substrate thinning to improve their thermal conductivity with global planarization.However,the substrate thinning process has the possibility to affect the electrical performance of devices as well as the internal residual stress.In this work,we investigate how substrate thinning affects the characteristics of AlGaN/GaN HEMTs.First of all,we estimated the change of residual stress in sample epilayers with respect to substrate thicknesses.Then,we analyzed the changes in output current and the kink effect,and we measured current transients and performed pulsed measurements to study how thinning changed the electronic characteristics and trap states.Moreover,we applied a uniform in-plane stress to further investigate the effect of stain on the output characteristics and kink effect of the AlGaN/GaN HEMT.The details are as follows:?1?We used electrical methods to analyze the trap states of AlGaN/GaN HEMTs,which performed typically output characteristics without kink effect.The current transient measurements shown that there were four detrapping behaviors?labeled DP1,DP2,DP3,and DP4?,where DP1 and DP3 only appeard after the on-state filling pulse.DP2 appeard when channel was pinched off.This behavior could come from traps in AlGaN barrier layer.And DP4 appeard both in on-state and V_DS=0 state filling pulse.Then the pulsed current-voltage characteristics and capacitance-voltage characteristics were used to further characterize the trap states in AlGaN layer.It was found that traps existed both under the gate and on the surface.?2?We thinning the sapphire substrate from 460?m to 80?m by chemical mechanical polishing.Using Raman spectroscopy and X-ray diffraction,we found that the compressive residual stress increased in GaN layer and tensile residual stress decreased in AlGaN layer after thinning process.Which were probably caused by the chemical mechanical polishing.?3?The electrical test results show that after substrate thinning,the sample showed lower drain–source current,and appeard kink effect,which indicated the increased of trap states in GaN layer.To study the change of trap states,we performed current transient and pulsed current-voltage measurements.The 80-?m sample showed a larger time constant for the detrapping behavior,a positive shift of the threshold voltage and a decrease in peak transconductance,an aggravation in current collapse,and a higher degree of capacitance dispersion.All this changes could be related to the increase of traps after substrate thinned.The plan-view study showed an increase of pit density on the 80-?m sample surface.Moreover,considering that piezoelectric materials will generate strain after power is applied.To further investigate the effect of strain on the electrical characteristics of AlGaN/GaN HEMTs,we applied external mechanical stress to the 80-?m sample,founding that the saturation current and magnitude of kink effect of the sample decreased and the V_DS-kink shifted to a larger value.Moreover,the time constant of detrapping behavior increased with the increase of tensile stress.Combined with literatures,it was proved that the external stress affects the band structure of sample.