TOKYO — NEC Corp. has developed a nitride semiconductor power transistor capable of 2.3-watt power amplification in the 30-GHz sub-millimeter band. The Japanese company claims this is a major advance in performance over the highest output power of 0.72 W previously available from single chips using the same materials.
The work is part of a five-year project, partially funded by Japan's Ministry of Trade and Industry, aimed at developing low-power HF devices using nitrides. The work started earlier this year.
NEC hopes to commercialize the technology within three to five years, and is working to raise the output power and develop mass production techniques for the power transistors.
The initial transistor is targeted at creating small, high-output power transmitter amplifiers for use in next-generation broadband wireless networks. NEC said deploying the power transistor obviates the need for the power divider/combiner that leads to larger chip sizes and increased power loss.
Designers should be able to create a transmitter amplifier more than 80 percent smaller than previous implementations, according to NEC researchers, but with a twofold to threefold increase in transmission power (up to 2 W), leading to ultrasmall, higher-power devices.
By eliminating the power divider/combiner, a single model of the new power transistor can also cover the 22-GHz, 26-GHz and 38-GHz high-frequency bands allocated to subscriber wireless access, leading to significant cost savings.
To make the transistors, NEC employed 0.25-micron ultrafine gate electrodes using electron beam lithography and heterojunctions of gallium nitride and aluminum gallium nitride. The transistors had a power gain cut-off frequency of 120 GHz and a drain current density of close to 1 ampere per 1-mm-wide gate. The bias voltage achieved was 30 V, four to five times higher than conventional gallium arsenide (GaAs) transistors.
In addition, by adopting a silicon carbide substrate with superior thermal conductivity, the device allows parallel transistor elements to be operated at the same channel temperature.
NEC stressed that GaAs transistors operating at a smaller bias voltage of 5 V to 8 V require passive circuitry to combine the output power of multiple transistors, resulting in larger chip and package sizes, increased power loss, and narrow frequency bandwidths.
