Manufacturing Bits: Dec. 18

Gallium oxide breakthroughs

Crystalline beta gallium oxide is a promising wide bandgap semiconductor material. It has a large bandgap of 4.8–4.9 eV with a high breakdown field of 8 MV/cm.

The technology has a high voltage figure of merit, which is more than 3,000 times greater than silicon, more than 8 times greater than silicon carbide (SiC) and more than 4 times greater than that of gallium nitride (GaN).

Potential applications for this technology include power devices. However, the technology is still in its infancy. And the figure of merit of the current devices are still far from the projected material limit, according to researchers. 

At the recent IEDM conference, Cornell University, Kyoto University and Novel Crystal Technology presented a paper on a beta gallium oxide, vertical-trench Schottky barrier diode technology. The diodes have been demonstrated on bulk gallium oxide substrates using a halide vapor phase epitaxial layer process.

The technology enables a breakdown voltage of 2.44 kV. It also demonstrated a figure-of-merit of 0.39 GW/cm2 from DC measurements and 0.45 GW/cm2 from pulsed measurements.

The breakdown of the devices occurs at the trench bottom corner. For this, the maximum electric field of over 5 MV/cm could be sustained. This paves the way toward reaching the promise of a high figure-of-merit for this technology.

“These properties make β-Ga2O3 an excellent material candidate for next-generation power electronic devices, especially under harsh environment,” said Wenshen Li,a researcher from Cornell, and others in the paper. “The availability of melt-growth methods for single-crystal bulk substrate provides added advantage towards lower cost and a head start for fast development of epitaxial growth and device technologies.”

Super-junction SiC

At IEDM, National Institute of Advanced Industrial Science and Technology (AIST) presented a paper on a SiC-based super-junction MOSFET using a multi-epitaxial growth method.

SiC is a compound semiconductor material based on silicon and carbon. Compared to conventional silicon-based devices, SiC has 10 times the breakdown field strength and 3 times the thermal conductivity, making it ideal for high-voltage applications, such as power supplies, solar inverters, trains and wind turbines. In another application, SiC is used to make LEDs.

AIST demonstrated a 1.2 kV-class super-junction  SiC MOSFET. “In this study, a multi-epitaxial growth method with a keV order energy implantation was developed for a 1.2 kV-class SiC SJ-MOSFET, and the dynamic characteristics were evaluated for the first time,” said S. Harada, a researcher from AIST and others in the paper.

“The dynamic characteristics were characterized, and the potential of a product-level device was identified for the first time. The switching characteristics with Schottky barrier diode showed no degradation in spite of the large drain-source capacitance (CDS). The reverse recovery characteristics of the body diode exhibited a soft recovery which may originate from the large CDS and the short lifetime of minority carrier. A high short circuit capability comparable to anon-SJ device was demonstrated,” Harada said.

Groovy MOSFETs

At IEDM, AIST also presented a paper on a 4H-SiC super junction, V-groove trench MOSFETs (SJ-VMOSFET). The on-resistance is the lowest among all SiC MOSFETs with the blocking voltage over 600 V.

A 1,170 volt device was realized. “Silicon carbide MOSFETs have been anticipated as a candidate for power devices, owing to the superior material properties of wideband gap, high critical electric field and high electron saturation drift velocity,”according to T. Masuda from AIST in the paper. “The V-groove trench MOSFETs (VMOSFET) have reduced the channel resistance by utilized high quality SiO2/SiC interface with a high channel mobility.”