Short Channel Effects And Structure Optimization Of Millimeter-wave AlGaN/GaN HEMT

Millimeter-Wave AlGaN/GaN HEMT is widely used in national defense industryand daily life because of its excellent representation of high speed, high power, hightemperature, anti-irradiation and so on. Current gain cutoff frequency （fT） and maximumoscillation frequency (f_max) must be improved in order to enhance the operatingfrequency of Millimeter-Wave AlGaN/GaN HEMT. Shrinking gate length is an effectiveway to enhance fTand f_max, while the electrical properties of AlGaN/GaN HEMT isseriously affected by short channel effects when the gate length （Lg） is shrunk down tonano scale, and the device even cannot work normally.Based on the analysis of experimental results of Millimeter-Wave AlGaN/GaNHEMT, the influence of short channel effects on DC and RF characteristics was studiedby numerical simulation, and the causes of degradation of electrical properties were alsodiscussed. Then the influence of T gate, Γ gate and τ gate on fTand f_maxwas studied indetail. Finally, the influence of different back barrier on the improvement of DC and RFcharacteristics was analyzed. The major results are as follows:When Lgis reduced to40nm from500nm, the saturation characteristics andpinch-off behavior of the device degrades seriously. The threshold voltage has a42%shift, and the maximum transconductance has a reduction of16%. In addition, the fT·Lgproduct decreases from19.1GHz·μm to5.03GHz·μm. The aspect ratio should be largerthan6to suppress short channel effects effectively.For T gate devices, the smaller gate-foot height is, the better DC and RFcharacteristics are. Gate-head increases the gate capacitances, so fTand f_maxareimproved as the increase of gate-foot height, while fTand f_maxcannot be improvedeffectively when the gate-foot height is larger than300nm. Low κ passivation materialunder gate-head enhances the RF performance greatly. Γ gate and τ gate affect DCperformance little. fTdecreases as the increase of field-plate length of both τ-gate andΓ-gate, and fTof Γ gate device is lower than that of τ gate device when they have thesame field-plate length. f_maxof τ gate device increases continuously as the increase offield-plate length, while f_maxdecreases at first then increases and decreases at last for Γ gate device as the increase of field-plate length.Graded AlGaN back barrier has better inhibition about leakage current in bufferlayer than constant AlGaN back barrier. fTand f_maxincrease slightly as the increase ofmaximum Al fraction of Graded AlGaN back barrier, and fTand f_maxalmost keepconstant when the maximum Al Fraction is larger than0.2. High Al fraction constantAlGaN back barrier reduces2DEG density and increases on-resistance greatly, leadingto a reduction of fTand f_max. Graded AlGaN back barrier has better improvement of DCand RF performance of Millimeter-Wave AlGaN/GaN HEMT than constant AlGaNback barrier.