The demonstration of the first single-crystal Ga2O3 transistor by the Jpaanese National Institute of Information and Communications Technology (NICT) in 2011 started the development of transistors with a lateral geometry. However, lateral devices are not amenable to the high currents and high voltages required for many applications owing to large device areas and reliability issues arising from self-heating and surface instabilities.
Vertical devices carry higher current drives without having to enlarge the chip size, simplified thermal management, and far superior field termination. A group at NICT led by Masataka Higashiwaki has used silicon as an n-type dopant in Ga2O3 devices, but the community has long struggled to identify a suitable p-type dopant. Earlier this year, the same group published on the feasibility of nitrogen (N) as a p-type dopant. The latest work with the Tokyo University of Agriculture and Technology (TUAT) integrates Si and N doping using ion implantation for the first time to produce a Ga2O3 vertical MOSFET.
“Our success is a breakthrough development that promises a transformational impact on Ga2O3 power device technology,” said Higashiwaki, Director of the Green ICT Device Advanced Development Center at NICT. “Ion implantation is a versatile fabrication technique widely adopted in the mass production of commercial semiconductor devices such as Si and silicon carbide (SiC) MOSFETs. The demonstration of an all-ion-implanted vertical Ga2O3 transistor greatly enhances the prospects for Ga2O3-based power electronics.”
“We initially investigated magnesium for p-type doping, but this dopant failed to deliver its expected performance since it diffuses significantly at high process temperatures,” said Man Hoi Wong, a researcher of the Green ICT Device Advanced Development Centre. “Nitrogen, on the other hand, is much more thermally stable, thereby creating unique opportunities for designing and engineering a variety of high-voltage Ga2O3 devices.”
The Ga2O3 base material used for fabricating the vertical MOSFET was produced by halide vapour phase epitaxy (HVPE) that can grow single-crystal films at high speeds and with low impurity levels. Three ion implantation steps were performed to form the n-type contacts, n-type channel, and p-type current blocking layers (CBLs) in the MOSFET. The device showed reasonable electrical properties including an on-current density of 0.42 kA/cm2, a specific on-resistance of 31.5 mΩ·cm2, and a high drain current on/off ratio larger than eight orders of magnitude. Further improvements in its performance can be readily achieved with improved gate dielectric quality and optimized doping schemes.
“Vertical power devices are the strongest contenders to combine currents over 100 A with voltages over 1 kV – the requirements for many medium- and high-power industrial and automotive electric power systems,” say Higashiwaki and Wong. “The commercialization of vertical SiC and gallium nitride (GaN) power devices has, to a certain extent, been hindered by the high cost of substrates. For Ga2O3, the high quality and large size of native substrates offer this rapidly emerging technology a unique and significant cost advantage over the incumbent wide-bandgap SiC and GaN technologies.”