Shottky Contact

In a Schottky contact, the Fermi energy is pinned to the middle of the gap by interface states. The semiconductor is p-doped.

(a) Draw the band diagram indicating the Fermi energy, the valence band, and conduction band.

Figure 1: Band diagram of a Shottky contact between a metal and a p-type semiconductor, indicating the Fermi energy, the valence band and the conduction band.

In such a Shottky contact electrons will flow from the low work function material (high Fermi energy, EF) to the high work function material, so the high work function material (low Fermi energy, EF) will get negatively charged. Therefore the electrons will have to be pushed uphill into the high work function material. This determines the band bending. So the bending or curvature of the bands – orange curvature in figure 1 – indicates a potential barrier for the charge carriers, so a certain amount of energy will be necessary to push the electrons over this well. This energy is equivalent to the built-in voltage, Vbi.

(b) Explain the difference between a Schottky contact, an ohmic contact, and a tunnel contact.

A Shottky contact, as shown in figure 1, is very similar to a pn-junction where the semiconductor on one side of the junction is replaced by a metal or the semiconductor on one side is heavily doped and therefore will behave more like a metal.

 Shottky contact is a contact between a metal and a semiconductor An ohmic contact we would get, if we would somehow manage to bend the band of the semiconductor upwards. If the bands bend up the conductivity inside the p-type semiconductor will get better and better the closer one gets towards the metal. As the bands bend up the valance band edge will get closer or even cross the Fermi energy – as shown in figure 2.

Figure 2: Band diagram of an ohmic contact between a metal and a p-type semiconductor.

Nevertheless one has to keep in mind that we will never end up with a good ohmic contact between a metal an a semiconductor as there are far too many defect states at the semiconductor surface, known as interface states. Such interface states arise because the metal lattice does not perfectly match the lattice of semiconductor materials. Therefore one ends up with dangling bonds, defects and impurities, whereas the latter two pin the Fermi energy to the middle of the band gap.

For a tunnel contact one would have to dope the p-region near the interface more heavily so one gets a p+-region giving us a so-called degenerate doped region between the metal and the “normally” doped p-region of the semiconductor. So the depletion width W – indicated blue in figure 3 – of the Shottky contact will become narrow enough, so the electrons can easily tunnel through it.

Figure 3: Band diagram of a tunnel contact between a metal and a heavily doped p+ semiconductor

The abbreviation MESFET stands for MEtal Semiconductor Field Effect transistor. Such a MESFET – as shown in figure 4 – is very similar to a JFET (Junction Field Effect Transistor), where we have a tunnel contact between the n+-regions below the (metallic) source contact and drain contact, BUT as in a MESFET the heavily doped p+-region is missing, we have a Shottky contact between the gate metal and the n-channel that connects the source and the drain.

Figure 4: Image of a Metal Semiconductor Field Effect Transistor.

The operation principle of such a MESFET is similar to those of a JFET. If one applies a reverse bias to the gate the electrons of the n-channel will be depleted. At a certain voltage, where the depletion width xn is as large as the channel width h the n-channel will be pitched- off and no more current will flow through the transistor. The voltage at which this happens is called the pitch-off voltage VP being the sum of the built-in voltage Vbi and the applied reverse bias V. For determining the pitch-off voltage and/or the depletion width xn one can use the same formulas which also hold for the JFET.