MOSFET: Effects of Real Surfaces
When MOS devices are made using typical materials (e.g., n polysilicon-Si02-Si), departures from the ideal case can strongly affect VT and other properties. First, there is a work function difference between the doped polysilicon gate and substrate, which depends on the substrate doping. Here, the heavily doped polysilicon acts as a metal electrode. Second, there are inevitably charges at the Si-Si02 interface and within the oxide which must be taken into account.
Work Function Difference:
We expect Φs to vary depending on the doping of the semiconductor. Figure(1) illustrates the work function potential difference Φms = Φm- Φs for n polysilicon on Si as the doping is varied. We note that Φms is always negative for this case, and is most negative for heavily doped p-type Si (i.e., for EF close to the valence band).
- If we try to construct an equilibrium diagram with Φms negative (Fig (a) given below), we find that in aligning EF we must include a tilt in the oxide conduction band (implying an electric field).
- Thus the metal is positively charged and the semiconductor surface is negatively charged at equilibrium, to accommodate the work function difference.
- As a result, the bands bend down near the semiconductor surface.
- In fact, if Φms is sufficiently negative, an inversion region can exist with no external voltage applied. To obtain the flat band condition pictured in Fig.(b), we must apply a negative voltage to the metal (VFB = Φms).