SiC- and GaN-based power semiconductor devices have in the past few years enabled great improvements in the efficiency and power density of switching power converters. A wide variety of SiC devices (e.g. MOSFETs, JFETs, BJTs, thyristors, and PiN/Schottky/JBS/MPS diodes) are now available from a number of manufacturers, and the same is true for GaN HEMTs. Thus, while vertical GaN devices are not yet mature, new research in the field is increasingly turning to the “ultra” wide-bandgap (UWBG) semiconductors, including diamond, gallium oxide, and aluminum gallium nitride (AlGaN), due to the expected scaling of the critical electric field as the bandgap to the 2.0-2.5 power. Notably, AlGaN is an alloy system, so that heterostructures are available, and it is also a polar material, which enables polarization doping. Both of these benefits significantly expand the range of device architectures that may be considered, compared to materials that do not have these properties; this is especially significant for UWBG materials, all of which have energetically deep impurity dopants that do not fully ionize at room temperature. This talk will report on both vertical PiN diodes and lateral HEMTs composed of Al-rich Al_xGa_1-xN (x ≥ 0.7, E_G ≥ 5.2 eV) designed as prototype power switching devices.

The talk will begin with an overview of the properties of AlGaN that motivate its application to power switching, as well as an analysis of conduction and switching loss mechanisms as a function of AlGaN bandgap for various device types (PiN diodes, Schottky diodes, etc.) These results will be compared to similar metrics for SiC, GaN, and UWBG materials other than AlGaN. The talk will then present results on several PiN diode structures. All of the structures to be presented have thick (5-8 mm) Al_0.7Ga_0.3N drift regions doped in the 1-3×10¹⁶ cm^-3 range, and were grown on thick (1.3 mm) sapphire substrates; due to the insulating nature of the substrate, the cathode contact is to a heavily-doped n-layer on the front side of the wafer in a so-called “quasi-vertical” configuration. The diodes differ in the design of the p-type anode. Homojunction diodes suffer from low free carrier density in the p-Al_0.7Ga_0.3N due to the low fraction of Mg than is ionized in material of such a wide bandgap (E_A > 400 meV). Thus, two alternate approaches to achieve appreciable hole density were examined: heterojunction diodes in with the anode is composed of Al_0.3Ga_0.7N, in which reasonable Mg activation can be achieved; and diodes utilizing anodes in which the composition is graded from Al_0.7Ga_0.3N down to GaN. In order to avoid premature edge and/or surface breakdown, all diodes studied utilized a junction termination extension formed by nitrogen implantation into the p-region.

The talk will conclude with a presentation of a second class of device, which is a power switching HEMT utilizing an Al_0.85Ga_0.15N/Al_0.70Ga_0.30N heterostructure. This heterostructure, which as a 25 nm thick barrier, is characterized by a pinch-off voltage of -4.5 V, a channel mobility of 250 cm²/Vs, and a channel sheet charge density of 6×10¹² cm^-2. The key challenge for Al-rich HEMTs has been the formation of Ohmic source and drain contacts; the current density in previous HEMTs reported by our group has been limited by the quasi-rectifying nature of these contacts. The HEMTs in the present work utilize planar source and drain contacts, as opposed to the etched-and-regrown contacts that we have reported previously; for those previous contacts, the regrown material was heavily-doped n-type GaN. The planar contacts in this study are composed of a Ti/Al/Ni/Au stack deposited directly on the Al_0.85Ga_0.15N barrier. While the current-voltage characteristics of these contacts are still quasi-rectifying, they have enabled a current density of 46 mA/mm, which is more than ten times greater than that achieved in our previous-generation AlGaN-channel HEMT, which had comparable channel sheet resistance. An effective breakdown field of ~170 V/um was achieved in a field-plated device with 4 um gate-to-drain spacing. This effective breakdown field exceeds that typically reported for GaN-channel HEMTs (~100 V/um), demonstrating the potential of AlGaN-channel HEMTs for high-performance power switching devices.

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-mission 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.