What is an intrinsic semiconductor? Explain how electrons and holes are produced in it and how they carry current. Draw its energy band diagram at T > 0 K.

1. 1 · Define it. An intrinsic semiconductor is a pure semiconductor such as Si or Ge.
2. 2 · Describe the bonds. Each atom forms four covalent bonds with its neighbours; at 0 K there are no free carriers.
3. 3 · Explain thermal generation. At room temperature heat breaks some bonds, giving a free electron and a hole each time, so nₑ = nₕ = nᵢ.
4. 4 · Explain the current. Electrons move to the positive terminal, holes to the negative terminal; I = Iₑ + Iₕ.
5. 5 · Draw the band diagram. [Diagram: valence band with a few holes, gap Eg, conduction band with the same number of electrons, T > 0 K]
6. 6 · Close in one line. So, a pure semiconductor conducts by equal numbers of electrons and holes, and conducts better when hotter.

Scoring tip: Draw the same number of electrons in the conduction band as holes in the valence band. Examiners check this.