EXPLORING POLARIZATION DOPING IN GAN FOR POWER APPLICATIONS

Abstract

GaN has made tremendous progress in photonic and radio frequency applications, largely thanks to its wide band gap (3.4 eV), the high electron mobility (up to 2200 cm 2 /V · s) and saturation velocity (>10 7 cm/s). With the wide band gap and high elec- tron mobility, GaN also possesses great potential in power applications. Unlike the traditional wide-band-gap material (SiC) used in power applications, GaN has a strong polarization effect, which enables dopant-free doping techniques that can realize two dimensional electron gas (2DEG) as well as three-dimensional (3D) bulk doping. This work focuses on exploring polarization doping for power applications. GaN high electron mobility transistors (HEMTs), which are based on the polarization-induced 2DEG, have already been well researched for power applications. The highest reported breakdown voltage of GaN HEMTs is as high as ∼3000 V. How- ever, the performance of 2DEG based diodes are lagging behind. A GaN 2DEG based Schottky barrier diode with a record-breaking breakdown voltage of 1.9 kV is developed and described in this work. The fabrication process of this diode is fully compatible with GaN HEMT fabrication. Thus the diode can be readily integrated with GaN HEMTs. In contrast to the polarization-induced 2DEG, the polarization-induced 3D bulk dop- ing is rarely studied for power applications. Here we start by studying the electron mo- bility in polarization-doped Al x Ga 1-x N with a low doping concentration of ∼1 × 10 17 cm −3 . The electron mobility is first experimentally extracted. Then a theoretical model of the electron mobility is built based on the experimental data. The theoretical model reveals that as the doping concentration is further reduced, it is crucial to reduce dislo- cation density and thus its adverse effect on the electron mobility. Both polarization-induced 2DEG and bulk doping are then applied to a GaN metal- oxide-semiconductor HEMT (MOSHEMT) with a polarization-doped p-type back bar- rier. This device, referred to as PolarMOSH is an integral component for the power transistor, referred to as PolarMOS. The successful demonstration of PolarMOSH paves the way towards realizing PolarMOS and eventually taking full advantage of polariza- tion doping in power applications.