2.3.2 Depleted-Substrate Devices

As outlined above, punchthrough is one of the main problems in MOS devices. A straightforward countermeasure is to use devices where the substrate is partially or fully depleted [13]. Since there are no free carriers except in the channel, punchthrough cannot happen. Depleted-substrate devices can be realized using silicon on insulator substrates. The structure and conduction band edge of a fully-depleted single-gate SOI device is shown in the left part of Fig. 2.9. It consists of a standard MOSFET with a substrate that is insulated from the wafer by a layer of SiO$_2$. The gate can have an even better control over the inversion charge if a double-gate SOI transistor is considered, as shown in the right part of Fig. 2.9. Double- or even triple-gate MOSFETs can be achieved using a FinFET. This is a device where a small silicon channel -- the fin -- is surrounded at two or three sides by the gate electrode [25,26,27]. Fin thicknesses down to 6.5nm have been reported [28] which means that the channel between source and drain consists of only about 15 atomic layers of silicon.

Figure 2.9: Conduction band edge in a fully-depleted turned-on single-gate (left) and double-gate SOI transistor (right).

A. Gehring: Simulation of Tunneling in Semiconductor Devices