Biasing a Diode: Forward and Reverse Bias

Biasing means supplying external energy (a bias voltage) to the diode's charge carriers to help them overcome -- or be blocked by -- the barrier potential. There are exactly two ways to connect an external source across a p-n junction, giving two biasing modes. In FORWARD bias, the source's positive terminal connects to the p-side and negative terminal to the n-side; this pushes electrons in the n-side and holes in the p-side TOWARDS the junction, recombination near the junction consumes some of the immobile ions, and the depletion region -- and hence the barrier potential -- SHRINKS. As the applied forward voltage is raised further, the reduced barrier lets progressively more carriers cross, and the forward current rises exponentially. In REVERSE bias, the source's positive terminal connects to the n-side and negative to the p-side instead; now majority carriers on both sides are pulled AWAY from the junction, more immobile ions are exposed, the depletion region WIDENS, and the barrier potential INCREASES, effectively choking off the majority-carrier diffusion current almost completely. A small reverse current still flows, however, carried by the MINORITY carriers on each side, for which the applied reverse bias is actually a forward bias -- this is the reverse saturation current $ …