Simulation Of The Scaling-down Laws Of N-face GaN HEMT And Fin-HEMTs

The large band gap and high electron saturation velocity of the Ga N material system are well known,and GaN high electron mobility transistor?HEMT?has been one of the excellent candidates for high-speed,high-power applications.For a long time,the researchers have focused on the preparation of Ga-polar GaN heterojunction materials and relevant HEMTs.The recent breakthrough of the N-polar Ga N material growth brought the rapid development of N-polar Ga N HEMTs,and the device characteristics of maximum oscillation frequency(f_max)of 405 GHz and the current gain cutoff frequency?f_T?of275GHz were reported.In order to increase the operating frequency of the device,the size of the device must be down-scaled.However,the short channel effect become more and more important as the devices shrink to the nanometer scale.In this context,this work has researched the down-scaling law and short channel effect of N-polar GaN HEMT.The novel N-polar GaN Fin-HEMT structure was proposed,and the device performance was calculated and analyzed.The main research work and achievements are as follow:1.The device performances of N-polar and Ga-polar Ga N HEMTs with gate length of 100nm and 200 nm were simulated and compared.It was found that the N-polar GaN HEMT had lower sub-threshold swing and drain-induced barrier lowing?DIBL?than the Ga-polar counterpart,which indicated that the N-polar device provided better suppression of short-channel effect.2.The change of AC/DC performance of N-polar Ga N HEMT with lateral scaling down of device size was obtained.It was found that with the device down-scaling the saturation drain current,the transconductance and the drain induced barrier lowering?DIBL?were increased,but the saturation drain voltage,the on-resistance and the threshold voltage were decreased.The simulated AC performance showed that with the device down-scaling f _T and f_max were increased,but the f_T·L_g product was decreased.Some certain relationship among f_T,L_g and the channel thickness was revealed and formulated and the physical meaning of the formula was given by device physics analysis.3.It was found that serious short channel effect appeared when the gate length of the concerned N-polar Ga N HEMT device shrinked to 100 nm.The phenomena included that drain current could not be saturated,the DIBL decreased to 33 mV/V and the f_T·L_g product reduced to 9.2GHz·?m.Reducing the channel thickness helps to effectively suppress the short channel effect,so it is necessary to adjust the vertical device structure to improve the device performance in highly down-scaled cases.4.The effect of the scaled down electrode parameters of the T-gate N-polar GaN HEMT on the device performance was revealed.The length of gate top determined the tradeoff between gate resistance and parasitic capacitance.The operation frequency of the devices increased with increasing height of the gate stem,but the effect was weaker for higher gate stem.It was found that the spacing between the source and drain electrodes had a significant effect on the device frequency performance.For the devices with symmetric gate,it affected the frequency performance by affecting the transconductance.For the devices with offset gate,it affected the frequency performance by affecting the gate-source capacitance.The effect of source-drain spacing on the symmetric-gate device was greater than that on the offset-gate device.With the ohmic contact resistance of source and drain decreases,the on-resistance decreases and the maximum oscillation frequency increases.5.The exploration of the N-polar GaN Fin-HEMT device was realized by using the three-dimensional simulation,and it was found that the Fin-HEMTs structure could effectively suppress short channel effects.We found that the Fin-HEMT showed a positively shifted threshold voltage and 10 times smaller off-state current compared with standard N-polar GaN HEMT at the same gate length,and meanwhile sub-threshold swing was reduced by 31 mV/dec and DIBL reduced by 9 mV/V.