= Solution
Heating promotes electrons across the <band gap>, creating mobile <conduction electrons> and <electron holes>. Their contributions add:
$$
\boxed{\sigma=e(nu_e+pu_h)},
$$
where $u_e,u_h$ are the <electron mobility> and <hole mobility>, and $e$ is the positive elementary-charge magnitude. The increase in <carrier density> gives the rising <electrical conductivity> in the intrinsic activation regime. Mobility need not itself increase with temperature; the model attributes the dominant increase to carrier generation.
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