Question 1

What makes silicon a semiconductor?

Accepted Answer

At 0 K, intrinsic silicon has a filled valence band and an empty conduction band. Thermal energy can then excite some electrons into the conduction band, leaving mobile holes behind. Its conductivity therefore lies between that of a conductor and an insulator.

Question 2

How are electrons and holes generated in intrinsic silicon?

Accepted Answer

At finite temperature, some valence-band electrons gain enough energy to enter the conduction band. Each excited electron leaves a vacancy in the valence band, called a hole. The model therefore shows equal numbers of conduction electrons and valence-band holes.

Question 3

How do phosphorus and boron produce n-type and p-type semiconductors?

Accepted Answer

Phosphorus has five valence electrons. Four participate in covalent bonds and the extra electron is relatively free, producing n-type material. Boron has three valence electrons, so one bond site has an electron vacancy called a hole, producing p-type material.

Question 4

What does the energy band gap represent?

Accepted Answer

The energy band gap is the energy difference between the top of the valence band and the bottom of the conduction band. It is an energy difference, not a physical gap between atoms in the silicon lattice. A small gap makes thermal excitation easier than in a large-gap insulator.

Silicon Lattice: Band Gaps, Heat and Doping

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