# A-level Physics (Advancing Physics)/Semiconductors/Worked Solutions

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1. What is the resistivity of silicon, at room temperature?

${\displaystyle \rho ={\frac {1}{\sigma }}={\frac {1}{435\times 10^{-6}}}\approx 2300\Omega {\mbox{m}}=2.3{\mbox{k}}\Omega {\mbox{m}}}$

2. What sort of variable resistor would a semiconductor be useful in?

A thermistor, as the resistance of a semiconductor decreases as heat increases (but, assuming use of a potential divider, the voltmeter would have to be on the other resistor).

3. If positive ions are added to silicon (doping it), how does its conductivity change?

When a few positive ions are present, they present "holes" into which some electrons can fall. This decreases the over-all number of valence electrons in the lattice, and these are more easily occupied by drifting electrons under the application of a potential difference across the semiconductor, so the effect is greater conductivity.