| In microwave and millimeter wave applications,It is the most important task to increase the cutoff frequency?fT?and the highest oscillation frequency(fmax)of AlGaN/GaN HEMT devices in the research work.And the simplest and effective way to improve the frequency characteristics of the device is to reduce the gate length?Lg?,the performance of the device will be seriously degraded by short channel effects when the Lg decreases.In this paper,the microwave and millimeter wave AlGaN/GaN HEMT devices are analyzed by Silvaco TCAD-ATLAS simulation and experimental test verification.The main contents of the study include short channel effect and the optimization of gate structure and LGD.In this paper,the research of short channel effects on the DC and RF characteristics of the device is analyzed by ATLAS software.With the gate length Lg decreasing to the nanometer level,the output saturation characteristic of the device is deteriorated,the threshold voltage is negative drift,and the maximum transconductance is reduced,It is shown that the two-dimensional distribution of the channel potential under the gate and the drain induced barrier effect?DIBL?are the causes of the short channel effect.Although the fT and fmaxincrease as Lg decreases,the frequency gate length Tf?Lg of the device decreases.When Lg>250nm,fT?Lg decreases slowly,so reducing Lg can effectively increase the value of Tf.When Lg<250nm,Tf?Lg decreases faster,althoughTf is still increasing as Lg decreases,The improvement of fT due to the influence of the short channel effect is limited.In order to suppress the short channel effect,the reduction of the thickness barrier below the gate is simulated to increase the aspect ratio.When the tgbar=15nm,the device gate depletion region is widened in the longitudinal direction,and when the VDS is increased from 0V to 10V,the barrier reduction is reduced by 27%compared to tgbar=23nm,which effectively suppresses the short channel effect.Based on the analysis of the short channel effect,the T-gate and LGD of the device are optimized to improve the frequency characteristics of the device.As the value of Lcap decreases,the value of fT increases,and the value of fT of Lcap=300nm increases by 58%compared to Lcap=900nm.Since the decrease ofLcap increases the gate resistance,the value of fmaxincreases first and then decreases as Lcap decreases,and Lcap is the maximum value of 76GHz at 500nm.With the LGD value decreases,the device fT and fmaxshowed an increasing trend,compared with LGD=3?m,LGD=1?m fT is increased from 48GHz to75GHz,fmaxis increased from 60GHz to 94GHz.fT and fmaxare increased 56%and57%when LGD=1?m,respectively.However,when the LGD is reduced from 3?m to 1?m,the breakdown voltage of the device is reduced by 10%.Therefore,the value of LGD should consider the device's breakdown voltage,DC and RF characteristics.Based on the simulation of the device structure and theoretical analysis,through the experimental test to verify the correctness of the simulation results.The test data show that VT negative drift is 42.70%,transconductance is reduced by 16.47%and short channel effect is severe with Lg decreasing to 100nm.Compared to conventional devices,the gm of the recessed-gate device is increased by 18.41%,|VT|is reduced by 17.65%,the device'sIon Iof fis 1.5×108,and the pinch-off characteristic is good.The fT andfmaxof the device are increased by 35.48%and 36.11%,respectively,which improves the frequency characteristics of the device.The use of recessed-gate structure can effectively inhibit the short channel effect,Because DIBL=12.8mV/V,the use of recessed-gate structure to suppress DIBL effect is good.On the basis,the T-gate device is formed.Compared with the I-type gate,the variation of Tf and fmax in the T-gate structure is the same as that of the simulation.When Lcap is 500nm,the value of fmax is the largest,and its value is 141GHz.When Lcap is reduced from 500nm to 300nm,the value of fmaxis reduced by 5.67%due to the increase in gate resistance RG. |
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