Semiconductor Electronics

Lesson

Part 1 — why silicon is neither a good conductor nor a good insulator, and how a trace of impurity changes it completely.

In this lesson you will

Distinguish conductors, semiconductors and insulators using energy bands
Describe intrinsic semiconductors, electrons and holes
Explain n-type and p-type doping

Gaps drawn to scale: Si 1.1 eV, diamond 5.4 eV.

Concept

From energy levels to energy bands

In an isolated atom electrons have sharp energy levels. In a crystal, the atoms are so close that each level spreads into a band of very closely spaced levels.

Valence band: the band holding the valence electrons (full at 0 K in a semiconductor).
Conduction band: the next band up; electrons here are free to move and carry current.
Band gap $E_g$ : the forbidden energy range between them.

Gaps drawn to scale: Si 1.1 eV, diamond 5.4 eV.

Diagram

Conductors, semiconductors and insulators

Table comparing conductors, semiconductors and insulators: band gap, typical resistivity and effect of temperature
	Conductor	Semiconductor	Insulator
Band gap	none (bands overlap)	small, ≈ 1 eV (Si 1.1, Ge 0.7)	large, > 3 eV (diamond 5.4)
Resistivity (Ω m)	10⁻⁸ to 10⁻⁶	10⁻⁵ to 10⁶	10¹¹ to 10¹⁹
Effect of heating	resistance rises	resistance falls	stays very high

Compare the band gaps:

Conductors: bands overlap, so many electrons are free.
Semiconductors: a small gap; at room temperature a few electrons gain enough thermal energy to cross it.
Insulators: a large gap; practically no electrons cross it.

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