= Depletion region
The region around a <p-n junction> with reduced mobile <charge carrier> density.
An illustrative abrupt-junction depletion approximation assumes uniform, fully ionized <donor impurities> and <acceptor impurities>, negligible mobile space charge in the depleted layer, and <permittivity> $\varepsilon_s$. For area $A$, doping <number densities> $N_A,N_D$ and built-in <electric potential difference> $V_{\mathrm{bi}}$, it gives
$$
W\simeq\sqrt{\frac{2\varepsilon_s}{q}\left(\frac1{N_A}+\frac1{N_D}\right)(V_{\mathrm{bi}}-V)},\qquad C_j\simeq\frac{\varepsilon_s A}{W}.
$$
Here $q>0$ is the <elementary charge> and $V$ is positive for <forward bias>. Increasing both doping densities reduces $W$ in this estimate. That explains the preparation strategy for a <tunnel diode>; it does not make this sharp-depletion approximation a precise microscopic model of a <degenerate semiconductor>.
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