= Doping and the equilibrium junction sketch
<Semiconductor doping> introduces impurities with levels close to the band edges. A <donor impurity> can release an <Electron> to the <conduction band>, leaving a fixed positive ion; an <n-type semiconductor> has <Electrons> as majority carriers. An <acceptor impurity> can take an <Electron> from the <valence band>, leaving a mobile hole and fixed negative ion; a <p-type semiconductor> has holes as majority carriers. The opposite species remains as a <minority carrier> population, and a neutral bulk sample balances the ionised dopants with mobile carriers.
At a <p-n junction>, concentration gradients initially drive <Electrons> and holes across the join. Their removal near the join exposes negative acceptor ions on the p side and positive donor ions on the n side, forming a <depletion region>. Its electric field opposes further majority-carrier diffusion. Equilibrium has a flat <Fermi level>, no net current and spatially bent band edges. With p on the left and n on the right, the <Electron> band edges fall towards the n side; the difference is the equilibrium barrier in <Electron> <energy>. A flat <Fermi level> does not mean flat conduction and valence edges.
\Image[/past-exam-of-the-physics-a-course-of-the-university-of-cambridge/2017/paper-2-c11-semiconductor-bands.png]
{title=Qualitative direct and indirect band gaps, and band bending across an equilibrium p-n junction}
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