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Tunnel FET architecture reduces IC power consumption

23 Aug 2013

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Tunnel FET new architecture

Figure 1: Transmission electron microscope image of a cross-section of the tunnel FET with the new architecture.

The operation of a synthetic electric field tunnel field-effect transistor (FET) with a new architecture has been demonstrated by a team of researchers from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. The team was led by senior researcher Yukinori Morita.

The tunnel FET is a transistor that is based on electron tunnelling, which can switch on and off at lower voltages than the operation voltage of the MOSFET. It is expected to reduce the power consumption of electronic devices. However, the fact that the current through the tunnel FET is smaller than the current through the MOSFET has been an issue in practical application of the tunnel FET. The tunnel FET with the new architecture uses a new channel and electrode structure to allow a higher electric field to be applied at a specific gate voltage, and it has achieved an operating current 10 to 100 times that of conventional tunnel FETs. The developed tunnel FET is expected to help to reduce the power consumption of large-scale integrated circuits (LSIs).

Research details
The tunnel FET has the disadvantage that the current passing through it is smaller than that through the MOSFET. To obtain a higher current efficiently, it is important to apply a stronger electric field to the tunnel junction. A high gate voltage is required to apply a strong electric field, while the tunnel FET needs to operate at a lower gate voltage to reduce power consumption. Therefore, the researchers used a new channel and electrode structure that can achieve a stronger electric field at the same gate voltage.

Figure 2 (c) and (d) are schematics of the tunnel FET with the new architecture. After a very thin non-doped channel layer is epitaxially grown on a source with a high concentration of impurities, a three dimensional transistor is formed by placing a gate electrode around the double-layered channel. As shown in Fig. 2 (a) and (b), conventional tunnel FETs are designed to switch the current on and off by using the effect of only the electric field perpendicular (a) or parallel (b) to the electric field from the gate electrode. In the new architecture, the vertical and horizontal electric fields are superimposed at the interface on the three dimensionally structured channel side-wall between the high-concentration source and the non-doped channel layer, making it possible to apply a stronger electric field than in conventional architectures. The researchers named the tunnel FET with this new architecture a synthetic electric field tunnel FET (SE-TFET).

 Comparison of tunnel FETs

Figure 2: Comparison of tunnel FETs with conventional (a and b) and new (c and d) architectures.

Figure 3 shows the relationships between the gate voltage and the drain current obtained in a conventional tunnel FET and in the developed SE-TFET. The drain current was 10 to 100 times higher in the SE-TFET than in the conventional FET. Because an electric field even stronger than this can be applied to the tunnel junction by reducing the thickness of the epitaxially grown layer and the channel width, the performance of the SE-TFET can be improved by device scaling. In addition, although silicon (Si) was used in the developed SE-TFET, the new architecture can be effectively applied to tunnel FETs that use germanium or compound semiconductors including indium gallium arsenide; the performance of devices made of such materials exceeds the performance of those made from Si. Performance can be further improved in future by device scaling and advances in materials.

Conventional TFET

Figure 3: Drain current in tunnel FETs with conventional and new architectures.

Future Plans
The researchers will continue to optimise the process aiming at the low-voltage operation of complementary metal-oxide-semiconductor (CMOS) circuits. Also, they intend to develop an SE-TFET with much higher performance than that of conventional tunnel FETs and to reduce the operating voltage through further miniaturisation. By combining the use of experiments, simulations, and circuit compact models, they will investigate the reduction effect on the power supply voltage when it is applied to CMOS circuits.

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