(Invited) Electric Field Control in AlGaN/GaN Hemts Operating in the Kilovolt Regime

Abstract

AlGaN/GaN High Electron Mobility Transistors (HEMTs) are very promising devices for the next generation of high-voltage power electronics. Their ability to achieve a low on-state resistance due to the high electron mobility in the 2DEG conduction channel formed by the GaN quantum well created at the AlGaN/GaN interface, as well as the fact that they have effective breakdown fields on the order of 106 V/cm, due to the wide bandgap of GaN (Eg = 3.4 eV), make them promising devices to operate in the kilovolt (kV) regime. Additionally, AlGaN/GaN HEMTs are majority-carrier devices which allow enhanced switching speeds at high voltages, conditions of high power dissipation, and high temperature conditions. Although the wide bandgaps of GaN and AlGaN in theory can enable the high-frequency switching capability of such unipolar field effect transistors to be extended to the kilovolt regime, the measured Figure of Merit (FOM, i.e., the breakdown voltage squared divided by the area-normalized on-resistance) for state-of-the-art commercial HEMTs continues to be well below values predicted by the FOM expected for an ideal device exhibiting a uniform electric field profile between the gate and drain. In fact, the difficulty of scaling the breakdown voltage of AlGaN/GaN HEMTs to kV levels is largely due to large electric field spikes in the active region that cause premature breakdown. In today’s devices, much of the channel voltage drop in the off-state occurs near the drain-side edge of the gate electrode. This voltage drop produces an electric field which can cause excessive gate leakage currents, and breakdown occurs at a much lower voltage than predicted by assuming an ideal, uniform electric field profile extending from the gate to the drain edge. While use of appropriate device passivation and encapsulation increases breakdown voltage by eliminating surface flashover, the fundamental problem of voltage scalability due to the non-uniform field distribution is evidenced by studies which show that breakdown voltage will not continuously increase as a function of increasing gate-to-drain spacing. This is true even if existing approaches such as source- and gate-connected field plates are employed; these mitigate the problem, but still result in a field distribution that is far from optimum. Consequently, other schemes for mitigating high electric fields need to be considered. Hence, in this talk we present results for AlGaN/GaN HEMT simulations that compare the effectiveness of various field plate implementation schemes in mitigating electric field spikes in the GaN channel so as to provide a more uniform electric field distribution. From these studies, the value of utilizing a distributed impedance “field-cage” structure is made apparent, and we also discuss the implementation of continuous “resistive field plate” (RFP) structures for electric field management. In the field cage scheme, a resistive voltage divider network fabricated on the device surface provides electric field control throughout the active region. The electric field in this region is determined to become progressively more uniform as the source-drain voltage is increased, indicating that the field cage scheme is promising for achieving breakdown voltage scalability. We also discuss the optimization of parasitic capacitances in the field cage in order to improve electric field control for AC ramps as high as dV/dt = 100V/ns. The RFP scheme, on the other hand, relies on making use of a continuous resistive material deposited between the gate and the drain in order to achieve a linear drop in electric potential and hence a constant field. From a processing standpoint, implementing RFP structures is perhaps less challenging that that of the field cages, as the fabrication of discrete components on the passivation layer would not be needed. However, finding a material with the appropriate resistivity that is easily integrated into the HEMT process may be a challenge. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.