Manufacturing Bits: Dec. 13

Phonon lasers; GaN-on-GaN devices; SiC inverters.


Phonon lasers
A European consortium recently kicked off a new project that will develop a practical phonon laser.

The European consortium, called PHENOMEN, will be coordinated by the Catalan Institute of Nanoscience and Nanotechnology (ICN2).

Most chips perform functions using an electrical charge (electrons) or by light (photons). In comparison, a phonon “is a quantum of energy or a quasiparticle associated with a compressional wave such as sound or a vibration of a crystal lattice,” according to Wikipedia. The lattice vibrations have frequencies up to the tens of terahertz, according to the European group.

Phonon lasers have been in the works for years. The idea is that sound waves move at speeds around five orders of magnitude less than the speed of light, according to a paper from Johns Hopkins University in 2010.

The wavelength of sound waves is shorter than light waves at the same frequency. This, in turn, could enable precise nondestructive measurements and imaging, according to the paper.

The European consortium hopes to develop a technology that combines phononics, photonics and radio-frequency (RF) electronic signals. The group will focus on the development of phonon-based signal processing. This will enable on-chip synchronization and transfer information carried between optical channels by phonons.

It plans to build optically-driven phonon sources and detectors. This includes phonon lasers, which will deliver coherent phonons to the rest of the chip pumped by a continuous wave optical source.

GaN-on-GaN devices
At the recent IEEE International Electron Devices Meeting (IEDM) in San Francisco, Panasonic presented a paper on a vertical gallium nitride (GaN) power transistor based on a GaN substrate.

GaN power transistors are promising for power semiconductor applications. The current devices are based on a lateral transistor configuration using a AlGaN/GaN heterojunction and a silicon substrate.

These devices have a blocking voltage of 600 V or lower. The products work, but breakdown voltage is limited up to 1 kV for the lateral GaN transistor. This is due to the mismatch between GaN and silicon as well as the increasing thickness of GaN on the silicon substrate, according to Panasonic.

Vertical GaN transistors on bulk GaN substrates overcome these limitations. Several companies have tried to make these devices, but they are limited. GaN-on-GaN devices are expensive and difficult to make.

“One critical issue is smaller threshold voltage with poor pinch-off characteristics even though the devices claim that these are normally-off transistors,” according to the paper from Panasonic. “Another issue is the stability of (the) gate structure.”

To advance the technology, Panasonic has developed a novel vertical GaN-based transistor on a bulk GaN substrate. It has a low on-state resistance of 1.0 mΩ·cm2 and a high off-state breakdown voltage of 1.7 kV. A new gate structure enables high threshold voltage of 2.5 Volts with good off-state characteristics. It has a switching speed of 400V/15A.

In the flow, an n-GaN drift layer is deposited on top of the GaN substrate. The design of the drift layer is to achieve the blocking voltage of 1.5 kV or over. Then, a p-GaN layer is deposited, followed by a carbon-doped GaN/Mg-doped layer. Following that step, a “ V-shaped” groove is formed by an inductively coupled plasma (ICP) etching, according to the paper.

“The p-GaN/AlGaN/GaN triple layers are epitaxally regrown over the V-grooves by MOCVD,” according to the paper. “Selective etchings of p-GaN to form the gate and of AlGaN/GaN/carbon-doped GaN to form the source electrode are conducted by ICP etching. Then, Ti/Al source and Pd/Au gate electrodes are formed on the surface side, and a Ti/Al/Ti/Au electrode is formed on (the) backside of the GaN substrate as drain.”

SiC inverters
Also at IEDM, the University of Tsukuba moved to advance another wide band-gap technology called silicon carbide (SiC).

Researchers devised what they claimed is the world’s first a p-channel vertical 4H-SiC MOSFET. This could be used for a power device applicable for high frequency complementary inverter.

The breakdown voltage is over -730 V and the short circuit capability is 15% higher than that of 4H-SiC n-channel MOSFET, according to researchers. “The fabricated device exhibits higher gate oxide reliability and avalanche immunity during the short circuit test,” according to the paper. “From experimental results, SOA area of SiC p-MOSFET could be larger than that of SiC n-MOSFET and the devices can be applied in the actual power applications. Therefore, the complementary inverter circuit based on pairs of SiC p-MOSFET and SiC n-MOSFET can be realized for possible applications.”

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