The Study Of Fluorine-based Plasma Treatment On AlGaN/GaN HEMTs

In recent years, Gallium nitride (GaN) material are promising candidates in hightemperature, high frequency and microwave devices fields, owing to its wide bandgap,high electron saturation velocity and superior characteristic of thermal conductivity.AlGaN/GaN high-electron-mobility transistors (HEMTs) have found wide applicationsin the fields of radar and wireless communications. Flourine plasma treatment is widelyapplied to recess gate process of GaN devices and fabrication of enhancement modedevices, so it’s of great importance to study AlGaN/GaN HEMTs treated by fluorineplasma. In this paper, the studies on AlGaN/GaN HEMTs by flourine plasma treatmentin the following three aspects:1. Based on the current fluorine plasma etching process used to make gate plate,the different time that fluorine plasma treatment under the gate in the device is used, theinfluence of different fluorine plasma treatment time on AlGaN/GaN HEMTs is studied.This paper mainly make comparison on three different devices by20s,100s and180sfluorine plasma treatment time under50W low etching power, respectively. It is foundthat the saturation currents and threshold voltages of three kinds of devices have littlechanges. The devices with50W,100s fluorine plasma treatment time have the highestbreakdown voltage,75%higher than the devices by20s fluorine plasma treatment timeand the Schottky reverse leakage current is decreased an order of magnitude; For thetreatment power of three kinds of devices is small, there is no significant difference inthe amount of current collapse of among the three kinds of devices and all currentcollapse are below10%.2. Using of fluorine plasma treatment process is now an important way to producethe enhancement mode devices, but fluorine plasma treatment will introduce too manytraps in devices, so the different power that fluorine plasma treatment under the gate inthe device are used, the influence of different fluorine plasma treatment power onAlGaN/GaN HEMTs is researched. From DC characteristics analysis, with the treatmentpower increases, the threshold voltage of devices move forward largely, the saturationcurrent and the peak transconductance are decreased obviously. After annealing, thethreshold voltage, the saturation current and peak transconductance of the devices arepartially restored, however, Schottky reverse leakage current increases and thatconfirming the characteristics of devices by fluorine plasma treatment are instability inhigh temperature environments. From the CV characteristics analysis, the defects and damage produced in devices with different fluorine plasma treatment power areresearched, it is found that the higher the fluorine plasma treatment power，the more thedefects and damages, and annealing can remove most of the defects.3. In order to improve the breakdown voltage of AlGaN/GaN high electronmobility transistors, a feasible method of low density drain (LDD) HEMTs is reported.The fluoride plasma treatment using CF4gas is performed on drain-side of gate edge.The electric field distributions of the LDD HEMTs are simulated by using Silvacosoftware, and the peak of electric field on gate edge is effectively reduced from5×106V/cm to3.5×106V/cm. Experimental results show that, compared with theconventional HEMTs, LDD HEMTs have lower reverse leakage current of gate, and thebreakdown voltage are increased by36%. The current collapses characteristics of theLDD HEMTs are confirmed by dual-pulse measurement, and an obvious pulse currentreduction is due to the surface states by implanting F ions between the gate and drain.