Scientists at Lund University in Sweden and the University of New South Wales in Australia have made a significant advance in nanofabrication techniques, developing the first wrap-gate nanowire transistor that has a concentric metal ‘wrap-gate’, metal wrapped around the entire nanowire, and that sits horizontally on a silicon substrate. The breakthrough came through a simplification of the fabrication process in tuning the length of the wrap-gate through a single wet-etch step.
With the microelectronics industry looking for ways to pack as many transistors as possible onto silicon chips, the study focused on developing a greater area for gate overlap without taking up more area on the chip. The usual solution is to build upwards, with a Fin Field-Effect Transistor (FinFET), a transistor made as a rectangular 'fin' of silicon that sticks out of the surface. This can then be used as the channel, and with metal on both the top and the sides, the gate overlap area can be increased without taking up more overall chip area.
However, reducing the overlap between the semiconductor channel through which the current flows and the metal gate makes it more difficult to switch the current on and off, so the team produced nanowire transistors with an all-around metal ‘wrap-gate’ that can lie flat on a semiconductor substrate, like conventional silicon transistors. This was managed by precisely setting the wrap-gate’s length using a single wet-etch step, with no need for any further lithography. These devices exhibit good electrical performance and can be produced reliably with high yield.
The innovation in the nanofabrication, which also offered a greater understanding of one-dimensional electron systems, was published online in Nano Letters [Storm et al. Nano Letters (2011) doi: 10.1021/nl104403g] and showed that wrapping the gate all the way around the channel can provide effective control, although managing to get the metal below the channel without compromising the device can be difficult.
The gate being wrapped all the way around the nanowire also allows the use of a highly doped insulated substrate which can be placed on a positive voltage to ensure the contacts conduct well, and also put a negative voltage on the wrap-gate to turn the channel off. As researcher Adam Micolich points out, “This independent control of contacts and channel is very useful, and isn't something you can do with previous designs.”
With the gate metal being gold, this research could offer the potential for leaving the wrap-gate unconnected to be used as a 'chemical' gate and the possible development of devices such as biosensors, as the exposed metal can be used to bind 'sensing' molecules to. The wrap-gated nanowires could also be used in one-dimensional quantum transport in semiconductors.
Laurie Donaldson