Manufacturing Bits: Oct. 25

GaN-on-GaN power semis; GaN biochips; silicon HBT.

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GaN-on-GaN power semis
Power semiconductors based on gallium nitride (GaN) are heating up in the market.

Typically, suppliers are shipping devices using a GaN-on-silicon process. These devices are available with blocking voltages of up to 650 volts. Going beyond 650 volts is problematic, however. GaN-on-silicon processes suffer from lattice mismatches, cost and other issues.

At the upcoming the 2016 IEEE International Electron Devices Meeting (IEDM) in San Francisco, Panasonic will describe a vertical GaN device based on a GaN-on-GaN process. The technology uses a bulk GaN substrate. On the substrate, the device makes use of the following layers–p-GaN/AlGaN/GaN.

With the technology, Panasonic will demonstrate a record-setting 1.7 kV threshold voltage, plus a low on-state resistance of 1.0mΩcm2.
The technology could eliminate the need for liquid cooling in high-power electronic systems, thereby reducing their size, weight, complexity and cost. “To achieve this performance, researchers created a ‘semipolar’ gate structure that propels charge carriers with great efficiency,” according to Panasonic’s abstract in the IEDM program.

“They plasma-etched V-shaped grooves into an n-GaN drift layer atop the substrate,” according to the abstract. “Because these grooves were cut at a slant, they exposed a second facet of the crystalline GaN material and thereby created the possibility of semipolar operation. The researchers then expitaxially grew p-GaN/AlGaN/GaN layers in these grooves and built a ‘slanted’ channel with the gate on top.”

GaN biochips
GaN materials are possible candidates for use in a range of bioelectronic devices, according North Carolina State University.

GaN materials don’t easily degrade in the body and they are nontoxic. But the question, according to researchers, is clear—Can the surface texture of GaN influence the health of nearby cells in the body?

For this, researchers tested three different materials. First, they tested GaN. And then, they tested two variations of aluminum GaN.

Researchers manipulated the surface of each material. Then, they modified the surface chemistry. They made them more attractive to water (hydrophilic) or less (hydrophobic).

Gallium nitride affects cell behavior (Source: NC State)

Gallium nitride affects cell behavior (Source: NC State)

Following that, researchers used the various GaN materials as substrates for growing PC12 cells. “The work also details differences in the release of metal ions from clean and functionalized nanostructured III-polar and N-polar semiconductors in cell culture media, and their relationship to changes in cell response through quantification of cell viability and the production of reactive oxygen species,” according to researchers from NC State.

“But while living cells will survive in the presence of GaN, we wanted to know if we could influence the behavior of the cells by changing the make-up of the GaN material,” said Patrick Snyder, a Ph.D. student at N.C. State, on the university’s Web site. “Basically, we wanted to know if engineering the GaN could influence the health and metabolism of the surrounding cells.

“We found that the roughly-textured AlGaN compositions released more gallium into the cellular environment,” Snyder said. “While this did not kill the cells, it did cause metabolic changes.”

Albena Ivanisevic, a professor of materials science and engineering at N.C. State, added: “This tells us that the topography of the material matters, and can influence cellular behavior. The work demonstrates that surface textures of bulk materials–like those used to create devices–can have similar effects to what we’ve previously seen in nanoscale materials.”

Silicon HBT
A number of wireless and wireline communications applications require fast transistors that can send and receive signals at the millimeter-wave (mmWave) range.

For example, take the RF market. The power amplifier, which amplifies RF signals in today’s smartphones, is generally based on gallium arsenide (GaAs) heterojunction bipolar transistor (HBT) technology.

HBTs are usually made from III-V materials like GaAs. They have higher electron mobilities than silicon. On the other hand, III-V materials are hard to integrate with silicon.

At IEDM, IHP will describe the world’s fastest silicon-based HBT technology. IHP’s devices will feature an fT/fmax of 505-GHz/720-GHz, respectively, at 1.6V, and a gate delay of just 1.34 ps in a ring oscillator test circuit.

Researchers, according to the abstract, will attribute this record performance to three factors: optimized vertical profiles of the emitter-base-collector regions; the use of “flash” annealing and low-temperature backend processing to lower base and emitter resistance; and lateral device scaling.

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