= Diode minority-carrier diffusion current
{title2=$I_S=e\mathcal A n_i^2[D_n/(L_nN_A)+D_p/(L_pN_D)]$}
For a long, nondegenerate, ideal <p-n junction> under low injection, the <diode saturation current> is set by <minority carriers> supplied from the neutral regions. Their equilibrium densities are $n_{p0}=n_i^2/N_A$ and $p_{n0}=n_i^2/N_D$. Solving the steady diffusion and recombination equations gives $I_S=e\mathcal A(D_nn_{p0}/L_n+D_pp_{n0}/L_p)$, where $\mathcal A$ is junction area, $D_n,D_p$ are diffusion coefficients, and $L_n,L_p$ are diffusion lengths. Reverse bias suppresses opposite majority-carrier injection exponentially while collecting this minority supply. Thus $I_S$ is approximately bias independent before breakdown, but depends strongly on <temperature> through $n_i^2$ and on transport, doping, and geometry.
Back to article page
