Manufacturing Bits: April 10

Higher power GaN; space chips; diamond diodes.


Higher power GaN
Imec and Qromis have announced the development of a new gallium nitride (GaN) substrate technology that enables power devices at 650 volts and above.

GaN is an emerging technology for power semiconductor applications. Based on a GaN-on-silicon technology, GaN-based power semis operate at 650 volts and above. In simple terms, the buffer layers between the GaN device and the silicon substrate are based on aluminum gallium nitride (AlGaN).

In some cases, though, GaN-based power semis run into issues beyond 650 volts. It has become difficult to increase the buffer thickness to the levels required for higher breakdown voltages and low leakage levels. This is due to a mismatch in the coefficient of thermal expansion (CTE) between the GaN/AlGaN epitaxial layers and the silicon substrate.

One solution is to use thicker silicon substrates. That would prevent wafer warp and bow in high-voltage apps. But thicker wafers can cause compatibility issues in wafer handling in some processing tools, according to Imec and Qromis.

In response, Qromis has devised high-performance, enhancement-mode p-GaN power devices on 200mm substrates. Processed in Imec’s silicon pilot line, the substrates have a thermal expansion that closely matches the thermal expansion of the GaN/AlGaN epitaxial layers.

This paves the way for power devices with 900-1200 volt buffers and beyond on standard thickness 200mm substrates. The substrates also open the door for thick GaN buffers, including the realization of free-standing and low dislocation density GaN substrates.

Space chips
As part of a project with NASA, Structured Materials Industries (SMI) has finished the initial studies of the radiation hardness of power devices based on gallium oxide (Ga2O3).

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 GaN.

Under a NASA SBIR Phase I funded project, SMI has devised Ga2O3 films. These films were grown on bulk doped and undoped Ga2O3 and other substrates in SMI’s metal-organic chemical vapor deposition (MOCVD) systems.

Potential applications for this technology include power devices. But for NASA, they must pass certain radiation-hardened tests for space missions. There are three types of radiation in space missions that are of concern–particles trapped in the Earth’s magnetic field; solar particle events; and galactic cosmic rays.

For these devices, total ionization dose (TID) and single event effect (SEE) were used by SMI as radiation hardness testing metrics. These metrics determine whether Ga2O3-based power devices are candidates for NASA’s Power Management and Distribution (PMAD) systems among other applications.

The testing was deemed to be a milestone in the assessment of Ga2O3 radiation hardness. All Phase I objectives were completed.

Image shows Ga2O3-based power devices being setup for radiation hardness testing. (Source: Structured Materials Industries)

Earlier this year, SMI was able to grow silicon-doped Ga2O3 films on sapphire substrates using the MOCVD technique. The demonstration has proven that doped Ga2O3 films can be deposited uniformly on large-area substrates for device applications.

Ga2O3 has been grown in one of SMI’s two Ga2O3 MOCVD reactors, one of which features a rotating disc reactor with a 13-inch diameter susceptor. The other features single two-inch wafers processing through 1200°C.

Before and after growth of Ga2O3 film on wafers. (Source: SMI)

Diamond diodes
Advent Diamond, a spin-off from Arizona State University (ASU), has been awarded a National Science Foundation (NSF) Small Business Innovation Research (SBIR) grant of $225,000 to conduct R&D on advancing single-crystal diamond diodes capable of operating at high temperature and power.

Diamond’s electrical properties make it suited to operate in harsh environments. In the works for years, diamond FETs are capable of handling high voltages, currents and temperature for power processing.

Advent Diamond plans to develop diamond diodes. “Advent Diamond is committed to resolving challenges associated with high power and harsh environment devices with the development of single crystal diamond devices,” said CEO and co-founder Manpuneet Benipal, a former research specialist with ASU’s Department of Physics.


Darrell Hambley PE says:

I’m surprised to see the ancient TO3 package for GaN transistors for space. The benefit of GaN is speed. Modern SMT packages allow high speed due to both low internal inductance and low inductance of planar mounting techniques. The TO3 package is not easily mounted to achieve such low inductance.

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