NUMBER of impurities. Heavy doping results in a NARROW depletion region.When compared to a normal p-n junction diode, tunnel diode has a narrow depletion width. The depletion width (W) is given by\(W = \sqrt {\frac{{2\varepsilon }}{q}\left( {\frac{1}{{{N_A}}} + \frac{1}{{{N_D}}}} \right){\rm{\Delta }}V} \)NA is doping CONCENTRATION on the acceptor sideND is doping concentration on the donor sideAs the doping concentration on the acceptor side (NA) is large\(W \approx \sqrt {\frac{{2\varepsilon }}{q}\frac{1}{{{N_D}}}} \)\(W \propto \sqrt {\frac{1}{{{N_D}}}} \)Therefore, the width of the junction barrier is inversely proportional as the square root of impurity concentration.