Analytical modeling of channel potential and threshold voltage of triple material gate AlgaN/Gan hemt and study of its self-heating effect

Abstract

AlGaN/GaN based high electron mobility transistors (HEMTs) are rapidly progressing towards becoming prime candidate for high power, high frequency and high temperature electronics applications and the reason is having several outstanding and unique electrical properties like large energy band gap, high electron mobility, high saturation velocity, low thermal impedance and high breakdown field. Device dimensions of HEMTs are being continuously scaled down with the advancement of ultra large scale integration (ULSI) and very large scale integration (VLSI) technology for achieving high speed, high packing density and improved performance. This continual downscaling has invoked dominance of several short channel effects (SCEs) because of reduced gate controllability over the channel and resulted in poor carrier transport efficiency and degraded performance. Numerous new architectures have been developed to diminish these SCEs. Gate material engineering technique in which the gate electrode is composed of more than one material with different work function has been adopted by many devices to acquire better SCE handling capability. In the triple material gate HEMT, the gate electrode is formed by fabricating three materials with different work functions laterally. The work function of the material near the source is the highest and that near the drain end is the lowest. The high work function near the source results in enhanced carrier transport efficiency due to quick acceleration of carriers in the channel and the low work function near the drain leads to decrease in electric field peak at the drain end and reduction of hot carrier effect. Moreover, two screen gates screen the effect of drain potential on the source-channel barrier and suppress drain induced barrier lowering (DIBL) effect. In this work, a two dimensional analytical model of channel potential, electric field and threshold voltage of triple material gate (TMG) AlGaN/GaN HEMT has been developed. Two dimensional Poisson’s equation with suitable boundary conditions has been solved to derive the channel potential using parabolic approximation method. Spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN interface have been incorporated in the model. Effect of traps produced due to process defects during device growth has been analyzed. The device performance has been explored with the variation in various device parameters like total channel length, ratio of length of different gate metals, barrier layer thickness etc. and compared with single material gate and dual material gate HEMT structures. Superior performance of TMG HEMT over other device structures has been found in terms of reduction of SCEs and enhanced carrier velocity. Results from the proposed analytical model have been validated against the data obtained from a commercially available numerical device simulator. Moreover, proper investigation of self-heating effect is necessary in GaN-based HEMT devices as this significantly impacts device performance and reliability. Therefore, a simulation model has been developed in this work to explore the self-heating effect of the proposed device structure. Temperature distribution profile across the device has been obtained and effects of different device parameters and bias voltages on maximum operating channel temperature have been demonstrated.