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This 40-Year-Old Transistor Changed the Communications Industry

The high electron mobility transistor is now an IEEE Milestone

2 min read
Satoshi Hiyamizu (left) and Takashi Mimura of Fujitsu testing the first HEMT IC. On the right is the first commercial HEMT: a cryogenic low-noise amplifier for the radio telescope at Nobeyama Radio Observatory, Nagano, Japan.
Satoshi Hiyamizu (left) and IEEE Fellow Takashi Mimura of Fujitsu test the first high-electron-mobility transistor. On the right is the first commercial HEMT: a cryogenic low-noise amplifier for the radio telescope at Nobeyama Radio Observatory, in Nagano, Japan.
Photos: Fujitsu/IEEE

THE INSTITUTEWhile working as an electronics engineer in 1977 at Fujitsu Laboratories in Atsugi, Japan, IEEE Life Fellow Takashi Mimura began researching how to make the metal-oxide-semiconductor field-effect transistor quicker. The MOSFET, which had been invented in 1966, was the fastest transistor available at the time, but Mimura and other engineers wanted to make it even quicker by enhancing electron mobility—how speedily electrons could move through semiconducting material.

Mimura began to research an alternative semiconductor to the silicon used in the MOSFET, hoping it would be the solution. He came across an article in the Applied Physics Letters journal on heterojunction superlatticesstructures of two or more semiconductors of significantly different bandgaps—developed by Bell Labs in Holmdel, N.J. The superlattices, which used a modulation-doping technique to spatially separate conduction electrons and their parent donor impurity atoms, inspired Mimura to create a new transistor.

In 1979 he invented the high-electron-mobility transistor. His HEMT used a heterojunction superlattice to enhance electron mobility, improving on speed and performance.

The invention now powers cellphones, satellite television receivers, and radar equipment.

The HEMT was dedicated an IEEE Milestone on 18 December. The IEEE Tokyo Section sponsored the Milestone. Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technical developments around the world.

ENHANCING ELECTRON MOBILITY

The HEMT consists of thin layers of semiconductors—n-type gallium arsenide and aluminum gallium arsenide—as well as a heterojunction superlattice; a self-aligned, ion-implanted structure; and a recess gate structure. The superlattice, which acts as a diode, forms between the layers of n-type gallium arsenide (a highly doped narrow bandgap) and aluminum gallium arsenide (a nondoped narrow bandgap). Using different bandgap materials causes a quantum well to form in the superlattice. The well lets electrons move quickly without colliding with impurities.

 The self-aligned, ion-implanted structure consists of a drain, a gate, and a source, which sit on top of a second layer of n-type gallium arsenide—the recess-gate structure. Electrons originate from the source and flow through the semiconductors and heterojunction superlattice into the drain. The gate controls the current flow between the drain and the source.

According to a paper in IEEE Transactions on Electron Devices, the recess-gate structure decreases the chance of a current collapse—a reduction of current after high voltage is applied. A current collapse would decrease the transistor’s response at high frequencies.

The Milestone plaque, displayed in the exhibition room on the ground floor of Fujitsu Laboratories in Atsugi reads:

The HEMT was the first transistor to incorporate an interface between two semiconductor materials with different energy gaps. HEMTs proved superior to previous transistor technologies because of their high-mobility channel carriers, resulting in high-speed and high-frequency performance. They have been widely used in radio telescopes, satellite broadcasting receivers, and cellular base stations, becoming a fundamental technology supporting the information and communication society.

 This article was written with assistance from the IEEE History Center, which is funded by donations to the IEEE Foundation’s Realize the Full Potential of IEEE campaign.

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Letting Robocars See Around Corners

Using several bands of radar at once can give cars a kind of second sight

10 min read
Horizontal
Illustration of the modeling of a autonomous vehicle within a urban city intersection.

Seeing around the corner is simulated by modeling an autonomous vehicle approaching an urban intersection with four high-rise concrete buildings at the corners. A second vehicle is approaching the center via a crossing road, out of the AV’s line of sight, but it can be detected nonetheless through the processing of signals that return either by reflecting along multiple paths or by passing directly through the buildings.

Chris Philpot

An autonomous car needs to do many things to make the grade, but without a doubt, sensing and understanding its environment are the most critical. A self-driving vehicle must track and identify many objects and targets, whether they’re in clear view or hidden, whether the weather is fair or foul.

Today’s radar alone is nowhere near good enough to handle the entire job—cameras and lidars are also needed. But if we could make the most of radar’s particular strengths, we might dispense with at least some of those supplementary sensors.

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