Diamond Keeps Growing as a WBG Material for High-power and Frequency Electronics

March 09, 2022 by Abdulwaliy Oyekunle

In the search for better wide bandgap (WBG) materials, researchers are looking towards the exploitation of diamond for the fabrication of field-effect transistors (FETs) for high-power and frequency electronics.

Silicon (si) has typically dominated as a material for high power and frequency electronics fabrication. Despite its widespread use, researchers and companies are still investigating the "next best" wide bandgap material such as silicon carbide (SiC), gallium nitride (GaN), and even diamond.

More specifically, these wide bandgap materials hope to have the edge over Si for high power and frequency electronics applications. 


Close-up view of a diamond field-effect transistor (FET) with hexagonal boron nitride gate insulator and graphite gate.

Close-up view of a diamond FET with hexagonal boron nitride gate insulator and graphite gate. Image used courtesy of Sasama et al


Additionally, alternative wide bandgap materials could be better suited in grid applications and offer reliability and improved energy conversion efficiency to achieve an efficient green electronic system. 

Notably, recent research carried out by the researchers at the National Institute of Materials Science, Japan, on diamond promises to pave the way to fabricate diamond-based P-channel FETs.

With that in mind, let's dive into better understanding the trend of diamond as a possible semiconductor material for power applications by answering how diamond might be employed in power electronics? And, what significant advantage does it have over silicon carbide and gallium nitride?


Diamond as a WBG Material in Electronic Devices

As mentioned, diamond as a wide bandgap material has been growing in popularity as Si is potentially reaching its theoretical limits to serve the growing needs of state-of-the-art high-power electronics.


An example diagram of using a diamond PiN diode in an emission current for vacuum power switching.

An example diagram of using a diamond PiN diode in an emission current for vacuum power switching. Image [modified] used courtesy of Araujo et al


One area where diamond excels beyond other materials like SiC is its high dielectric breakdown strength and a high critical electric field. This advantage could be beneficial for reducing the on-resistance of the breakdown voltage capability of Schottky barrier diodes (SBDs) in high power engineering devices and applications.

What's more, engineers could also exploit diamond to fabricate diamond-based FETs with improved radio frequency (RF) performance with a high cut-off frequency of 55 GHz, as demonstrated in the past by researchers at the University of Glasgow, United Kingdom.

Despite some of these inherent benefits, a big question remains of how difficult is it to manufacture and fabricate with diamond?


How is Diamond Processed for Electronics Applications?

Most times, diamond substrates could be synthesized by high-pressure high temperature (HPHT) or chemical vapor deposition (CVD) techniques. 

Chemical vapor deposition involves chemical reactions inside a gas phase and deposition onto a substrate surface. The growth of diamond films by CVD is usually conducted under non-equilibrium conditions. 

In addition, atomic hydrogen is employed to eliminate non-diamond carbon, including graphite, formed on the diamond surface.

Over the years, chemical vapor deposition processes have produced a large substrate and high-quality single-crystal diamonds. Due to relentless research, the largest single-crystal diamond substrate has been reported to have a diameter up to 99 mm.

Though the fabrication of diamond substrates is possible, there is, of course, the need for further scaling of this technology.


Diamond Breaks Into High-power Applications

Thanks to some of its electrical characteristics, diamond has found its way into high-power devices and applications. 

After looking at some of the research on diamond-based power electronics, these devices could operate comfortably at high temperatures without the extensive cooling and circuit protection requirements of state-of-art high-power electronic devices. 

This type of application can be seen in an example of diamond Schottky rectifiers, where high-temperature operation and low on-resistance with a high breakdown voltage of up to 8 kV has been achieved.


Schematic diagram of  MESFET.

Schematic diagram of  MESFET. Image used courtesy of Araujo et al


Furthermore, researchers have demonstrated high-frequency operation in a diamond metal-semiconductor field-effect transistor (MESFET) and metal-oxide-semiconductor field-effect transistor (MOSFET). 

Generally, these components are aimed at applications in high-power radio-frequency (RF) power amplifiers.


More Research on Diamond-based FETs

In an attempt to tackle the high on-resistance and high conduction loss in P-channel WBG FETs caused by low hole mobility, the researchers at the National Institute of Materials Science, Japan, have fabricated a high-mobility P-channel wide-bandgap heterojunction FETs consisting of a hydrogen-terminated diamond channel and hexagonal boron nitride (h-BN) gate insulator.


Close-up view of heterostructure graphite/h-BN/hydrogen-terminated diamond.

Close-up view of heterostructure graphite/h-BN/hydrogen-terminated diamond. Image [modified] used courtesy of Sasama et al


The researcher's FETs were fabricated on IIa-type (111) single-crystalline diamonds synthesized in a high-temperature, high-pressure process. 

Using the CVD processes, at a temperature of 650°C, the diamond was first annealed in H2 gas for 35 minutes. After that, it is exposed to hydrogen plasma at 600°C for about 10 to 12 minutes in a microwave plasma-assisted CVD (MPCVD) chamber.

The researchers claim that diamond has phonon-limited intrinsic hole mobility higher than 2000 cm2V-1s-1 at room temperature, making it suitable for the fabrication of high-mobility p-channel transistors.

At room temperature, the researchers recorded hole mobility of 680 cm2V-1s-1, which increased up to 1000 cm2V-1s-1 with decreasing temperature.

However, the researchers remarked that the results obtained for the diamond FETs are the best values reported for p-channel FETs made of wide-bandgap semiconductors.

All in all, as more research keeps happening to look into the validity and usefulness of diamond as a WBG material, it will be interesting to see how and where this material develops in the semiconductor industry.