Transistor squeezing approach improves power efficiency

A different breed of transistor promises to bring the power consumption of microchips to a low. While this component is known to suffer from significant current leakage, applying a shell of piezoelectric material is claimed to reduce this leakage by a factor of five.

Researchers from the University of Twente MESA+ research institute and its spinoff company SolMateS have developed a prototype of this transistor, which they said could address either the current leakage while the transistor is inactive or the energy consumption while it is active. The consumption can be reduced by approximately 10 per cent.

The research was performed by Buket Kaleli, Ray Hueting and Rob Wolters from the University of Twente. Staff from SolMateS applied the piezoelectric layer to the transistor. The presentation was detailed in the June issue of Transactions on Electron Devices.

Intelligent squeezing

The trick lies in a piezoelectric material which is applied to the exterior of the transistor. The piezoelectric material expands when you apply a voltage to it and compresses the silicon in the transistor with a pressure of about 10,000 atmospheres. This high pressure ensures that electrons flow through the transistor faster. You can therefore make microchips more efficient by 'intelligently squeezing the transistor'.

Incidentally, existing transistors are already put under high pressure in order to improve their efficiency. In this case, however, the pressure is permanently built in, which actually increases the current leakage. In the prototype designed by the UT, the transistor is only put under pressure when required and this makes a big difference. The electric current needed to switch the transistor from on to off is thereby partly replaced by mechanical tension.

Figure 1: Sketch of the transistor prototype. Source: University of Twente

The figure above shows the transistor current flowing through tiny silicon bars, which are enveloped by a package of conductor layers and piezo-electric material. The conductors control the amount of mechanical tension, and also the quantity of electrons in the silicon. In this way, the power can be turned on and off.

This is an initial prototype that can already produce energy savings, said Semiconductor Components Chair Ray Hueting. "The design is still fairly crude where the material is concerned. With the further development of the transistor, it should therefore be possible to achieve a further significant increase in efficiency."

The operating principle of this transistor was theoretically predicted in 2013 by the same research group. But in advance it was by no means certain that the transistor would be a success since piezoelectric materials and silicon are difficult to combine. The researchers solved this by inserting a buffer layer between the two materials.

Figure 2: Top view of the upward movement of a piezo FET with five parallel silicon bars (also referred to as 'fins') as a result of the piezo-electric effect. When an electrical current is applied across the piezoelectric material, the green parts do not move, but the red ones do. Source: University of Twente