8 February 2018
Gallium oxide as replacement for silicon in lower-cost, smaller microelectronic devices
© Semiconductor Today Magazine / Juno PublishiPicture: Disco’s DAL7440 KABRA laser saw.
Masataka Higashiwaki and Gregg Jessen at the US Air Force Research Laboratory (AFRL) have outlined a case for producing microelectronics using gallium oxide in a Guest Editorial ‘The dawn of gallium oxide microelectronics’ in Applied Physics Letters 112, 060401 (2018).
Despite being the long-time incumbent material for microelectronics, silicon faces limitations, particularly with scalability for power applications. Pushing semiconductor technology to its full potential requires smaller designs at higher energy density.
“One of the largest shortcomings in the world of microelectronics is always good use of power: Designers are always looking to reduce excess power consumption and unnecessary heat generation,” says Jessen, principal electronics engineer at AFRL. “Usually, you would do this by scaling the devices. But the technologies in use today are already scaled close to their limits for the operating voltage desired in many applications. They are limited by their critical electric field strength.”
Transparent conductive oxides (TCOs) are key emerging materials in semiconductor technology, offering the unusual combination of conductivity and transparency over the visual spectrum. One conductive oxide in particular has unique properties that allow it to function well in power switching: gallium oxide (Ga2O3), which has a very large bandgap.
Higashiwaki and Jessen focus on field-effect transistors (FETs), which could greatly benefit from gallium oxide’s large critical electric field strength - a quality that Jessen says could enable the design of FETs with smaller geometries and aggressive doping profiles that would destroy any other FET material.
The material’s flexibility for various applications is due to its broad range of possible conductivities - from highly conductive to very insulating - and high-breakdown-voltage capabilities due to its electric field strength. Consequently, gallium oxide can be scaled to an extreme degree. Large-area gallium oxide wafers can also be grown from the melt, lowering manufacturing costs.
Picture: A false-color, plan-view SEM image of a lateral Ga2O3 FET with an optically defined gate. From near (bottom) to far (top): the source, gate and drain electrodes. Metal is shown in yellow and orange, dark blue represents dielectric material, and lighter blue denotes the gallium oxide substrate.
“The next application for gallium oxide will be unipolar FETs for power supplies,” Jessen says. “Critical field strength is the key metric here, and it results in superior energy density capabilities. The critical field strength of gallium oxide is more than 20 times that of silicon and more than twice that of silicon carbide (SiC) and gallium nitride (GaN),” he adds.
The authors discuss manufacturing methods for Ga2O3 wafers, the ability to control electron density, and the challenges with hole transport. Their research suggests that unipolar Ga2O3 devices will dominate. They also detail Ga2O3 applications in different types of FETs and how the material can be of service in high-voltage, high-power and power-switching applications.
“We are just beginning to understand the full potential of these devices for several applications,” reckons Jessen.