Depletion layer of a PN junction under reverse bias
When p-type and n-type semiconductors are joined, electrons and holes diffuse across the boundary and recombine, leaving behind a depletion layer that contains only ionized donors and acceptors. The charge of these ions sets up a potential difference, the built-in potential, which stops any further diffusion of carriers.
Applying a reverse bias increases the potential difference across the junction, so the depletion layer widens. Its width is proportional to the square root of (built-in potential + reverse voltage), and because the ionic charge on both sides must balance, it extends mostly into the more lightly doped side. The depletion layer acts as a capacitor with charge on either side, so the wider it gets, the smaller the junction capacitance becomes.
Try it: reverse bias and depletion width
Change the reverse voltage with the slider and the depletion width follows the equation. The conditions are Si, NA = 10¹⁷ cm⁻³ on the p side, ND = 10¹⁶ cm⁻³ on the n side and 300 K (the defaults of the PN junction calculator), and horizontal lengths are drawn to scale.