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 superlattices—structures 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.