Scientists develop new transistor, made from diamond

Scientists at the Tokyo Institute of Technology have developed a new type of transistor – made from diamond – which could prove valuable for high-power applications.

Transistors are electronic switches that control the flow of current from one part of a circuit to the next, and form the foundations of modern computing. They are usually made from semiconductors – materials that allow electric current to flow through them only under certain controllable circumstances.

Researchers are looking into the possibility of using doped diamond as semi-conductors in order to create hard-wearing transistors with a wide band gap, high thermal conductivity and the ability to withstand high electric fields without breaking.

Mutsuko Hatano and co-workers at the Tokyo Institute of Technology, together with colleagues across Japan, have succeeded in fabricating a new design of transistor using diamond doped with phosphorus and boron. The new transistor operates accurately at high temperatures and can prove useful in power devices.

“Naturally, diamond is an insulator,” Hatano said. “However, it becomes semi-conducting when doped with boron or phosphorus.

“We and our co-workers at the National Institute of Advanced Industrial Science and Technology discovered a unique way of selectively growing doped diamonds. We applied this technique to fabricate diamond junction field-effect transistors.”

Junction field-effect transistors (JFETs) work by altering the conductivity of the channel through which the current flows. Each transistor has a drain, source and gate. The state of the gate (either ‘on’ or ‘off’) determines the current flowing through the channel between the source and the drain. Rather like pinching or squeezing a hosepipe to prevent water flowing, JFETs allow the channel to remain open (‘on’ state) or closed (‘off’ state).

Hatano and her team built up JFETs by doping diamond with impure gases containing either boron or phosphorus, during the chemical vapour deposition process. Phosphorus has five free electrons as opposed to diamond’s four, so every atom effectively adds an extra electron (n-type doping). Boron, on the other hand, has only three electrons so every atom creates a ‘hole’ (p-type doping).

The team created the desired shape and structure of each transistor. The flow channel was made up of p-type diamond, with the n-type diamond making a unique structure of two gates placed on either side of the channel.

When open, the p-type channel is full of holes so there is plenty of space for the current to flow through the holes. But once a voltage is passed simultaneously through the n-type gates, the holes are filled in to create a depletion layer that closes off the channel to current.

The flow of current can therefore be carefully controlled according to the voltage passing through the gates.

This is the first transistor of its kind to be made from diamond and to function accurately even at higher temperatures. The ability of the lateral-gated diamond transistors to withstand high currents and high voltages when stacked vertically means the new devices could be very valuable in power applications.

“The present work is still just the first step,” Hatano said. “We are going to evaluate and improve the device characteristics. At present, diamond substrates are expensive to use as a power device on a large scale. This will be solved if diamond is grown on another, cheaper substrate in future.”

Results of the research were published in Applied Physics Express.