= Solution
At equilibrium the <diffusion currents> and <drift currents> cancel separately for each carrier species. Electron diffusion is from n to p, while the <electric field> drives electrons from p to n; hole diffusion is from p to n, while hole drift is from n to p. The net current is zero, although these opposing microscopic fluxes are present. In conventional-current signs, both majority diffusion contributions point p to n and the balancing drift contributions point n to p.
<Forward bias>, $V_{\rm ext}>0$, reduces the barrier and enhances majority-carrier diffusion and <minority carrier> injection. The ideal low-injection <Shockley diode equation> gives
$$
\boxed{J=J_s\left[e^{eV_{\rm ext}/(k_BT)}-1\right],}
$$
where positive current is p to n. For <reverse bias>, majority diffusion is suppressed and the field sweeps thermally supplied minority carriers across the junction, giving approximately $J=-J_s$. This saturation is limited by carrier supply rather than a current proportional to the field. Depletion-layer generation, recombination and eventual <diode reverse breakdown> modify the ideal result.
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