Physics · Ch 14 — Semiconductors
Biasing a p-n junction
Biasing a p-n junction
Because of the potential barrier that exists across the depletion region, charge carriers need some extra energy to overcome it and cross the junction; a suitable external voltage applied to the junction supplies exactly this. There are two distinct ways to apply this external voltage (Fig. 14.19), and the junction behaves very differently in each.
FORWARD BIAS: the p-region is connected to the positive terminal and the n-region to the negative terminal of the external voltage source. This external voltage effectively OPPOSES the junction's own built-in potential, which reduces the width of the potential barrier. At the same time, this external field pushes the negative charge carriers (electrons) in the n-region, and the positive charge carriers (holes) in the p-region, TOWARD the junction. Both effects together -- a narrower barrier, and carriers actively pushed toward it -- make it easy for both carrier types to cross over and contribute to a current. This arrangement, where a p-n junction carries a large current (of order a few milliamperes, ), is called FORWARD BIAS. …
What this figure shows. Two circuit diagrams of the same p-n junction connected to an external battery in the two opposite possible orientations. In panel (a), FORWARD BIAS: the p-region is connected to the positive terminal and the n-region to the negative terminal of the external source; arrows on the diagram show the depletion region drawn NARROWER than at zero bias, with electrons in the n-region and holes in the p-region both shown being pushed TOWARD the junction, indicating the barrier is opposed and reduced. In panel (b), REVERSE BIAS: the p-region is connected to the negative terminal and the n-region to the positive terminal instead; arrows show the depletion region drawn WIDER than at zero bias, with electrons in the n-region and holes in the p-region both shown being pulled AWAY f …