Q.What is a power diode?

A power diode is the simplest power switch and the building block of every rectifier. It conducts a large current in the forward direction with very little power loss, while blocking a large voltage in the reverse direction.

To block a large reverse voltage, a diode needs a wide depletion layer, and a wide depletion layer needs a region of low doping. So a lightly doped n region — written n- and called the drift layer (or epitaxial layer) — is sandwiched between a heavily doped p+ anode and a heavily doped n+ cathode. The end layers are doped to about $10^{19}\ \text{cm}^{-3}$, while the drift layer is much lower, about $10^{14}\ \text{cm}^{-3}$.

Under forward bias (p+ to the positive terminal), holes from the p+ region and electrons from the n+ region flood the drift layer. This double injection produces conductivity modulation, which greatly increases the drift layer's conductivity and keeps the on-state voltage drop small.

Under reverse bias the diode blocks. Two cases arise: in a non-punch-through diode the depletion layer stops before the n+ cathode and the electric field falls to zero within the drift region; in a punch-through diode the depletion layer spans the whole drift region and reaches the n+ cathode, so the field does not fall to zero. Understanding this structure explains why the diode both conducts efficiently and blocks a large reverse voltage.