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Diamond boost for p-type power transistors

Technology News |
By Jean-Pierre Joosting

Researchers in Japan have developed a p-type diamond field-effect transistor (FET) with high hole mobility, which can lead to higher operational speeds and complementary operation.

Diamond offers a wider bandgap than gallium nitride or silicon carbide with more desirable properties that could boost device performance of p-type devices.

The team at the International Centre for Materials Nanoarchitectonics (WPI-MANA) used a new fabrication technique using hexagonal boron nitride as a gate insulator and avoiding exposing the diamond’s surface to air. 

The key advantage is that this technique reduces the density of negative charges on the diamond surface. If there are negative charges, they produce random Coulomb potential, which scatters the holes when they conduct near the diamond surface. This degrades the effectiveness of hole conduction and decreases the mobility of the holes.

Even if no gate voltage is applied, there are holes, and so the transistor is “normally on,” and this is not suitable for power electronics applications.

The team showed that p-channel wide-bandgap heterojunction field-effect transistors can be created, without surface transfer doping, using a hydrogen-terminated diamond channel and hexagonal boron nitride gate insulator. Despite having a reduced density of surface acceptors, the transistors have a low sheet resistance (1.4 kΩ) and large ON current (1,600 μm mA mm−1) compared with other p-channel wide-bandgap transistors, due to a high room-temperature Hall mobility (680 cm2 V−1 s−1). The transistors also exhibit normally OFF behaviour with an ON/OFF ratio of 108.

“In contrast, in our new technique, we can reduce the density of negative charges on the diamond surface. So the holes are less scattered, and therefore we can obtain higher mobility,” said team leader Dr. Takahide Yamaguchi. “This also results in ‘normally-off’ operation, which is desirable for power electronics.”

Dr. Yamaguchi pointed to some possible applications of this FET breakthrough. “Our new approach for fabricating diamond transistors could be used to make low-loss switches for power electronics and high-frequency high-output amplifiers for communications.”

www.nims.go.jp/mana/index.html

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