Practice portal › Semiconductor Electronics › Intrinsic and Extrinsic Semiconductors
Asked in GSEB Board March 2019 · Conductivity and mobility
Idea: a semiconductor conducts only through carriers that something has set free, so anything that creates carriers raises its conductivity and anything that removes them lowers it.
Germanium's forbidden gap is 0.72 eV, and at room temperature thermal energy lifts a small number of electrons across it, each leaving a hole behind.
Cool the crystal and there is less thermal energy to do that, so fewer electron-hole pairs exist at any moment and the conductivity falls. Towards absolute zero pure germanium becomes an insulator.
The other three choices all add carriers instead: acceptors add holes, donors add electrons, and UV photons create pairs.
So the conductivity decreases on decreasing the temperature -- the opposite of a metal, whose resistance rises as it is heated.
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