07 Worked examples (Lesson 1)
Example 1.1. The concentration of free electrons in pure silicon at room temperature is 1.5 × 10¹⁰ cm⁻³. What is the concentration of holes and the density of charge carriers in total? Because electrons and holes are created in pairs, p = n = 1.5 × 10¹⁰ cm⁻³. The total number of carriers is 3.0 × 10¹⁰ cm⁻³. (Compare this with a copper conductor that has about 8 × 10²² electrons per cm³: a pure semiconductor is a very poor conductor.)
✍️ Worked examples
Example 1.2.
- Explain what happens to the conductivity of pure silicon when it is heated from 300 K to 350 K.
- More bonds are broken per second, so n and p increase until a new thermal equilibrium; the conductivity increases.
- For a metal, the resistance rises instead.