Current flow in a pn-junction diode under forward bias.

Under the forward bias (p-side is more positive w.r.t. n-side, or n-side is more negative w.r.t. p-side), holes are injected from the p-side (red), across the depletion region (around the junction, a region depleted of mobile carriers) into the n-side (blue). The holes injected into the n-side are minority carriers. Near the depletion region boundary, the minority-carrier hole concentration is above the thermal equilibrium concentration because of these electrically injected holes. There are two processes that the injected holes go through: (1) diffusion and (2) recombination.

(1)Diffusion: There are more holes near the depletion boundary than deeper into the n-region. Therefore they undergo a thermal diffusion, resulting in a net flux of holes (red) away from the depletion boundary.

(2)Recombination: The Nature dictates that the hole concentration try to recover its thermal equilibrium value, and thus try to get rid of the excess holes (dp). These excess holes thus undergo a recombination with a majority-carrier electron which process annihilates holes and electrons in pairs (the vertical flow of red dots). Any of the lost (majority-carrier) electrons are quickly re-supplied from farther-into the n-side, through the Ohmic-contact and metal wire at far side of the n-type region.

The diffusion process and the recombination process together produce an exponential concentration profile for the minority holes, as can be seen in the second diagram (lower-right, red). At a constant forward-bias, the number of holes injected across the junction per unit time equals the number of holes lost by recombination, thus establishing a steady state (i.e., constant in time).

The hole component of the total current is shown by the red curve in the third figure. Going from left to right, the hole current in p-type region is drift current (due to small electric field in the neutral p-region, red), drift current in the depletion region (due to the built-in field), and diffusion current in the n-side (blue) due to the concentration gradient. The excess minority hole concentration reaches zero eventually.

Electrons (blue) are injected from n-side (blue) to the p-side (red) across the junction (or depletion region, gray). They go through the same process as holes.

Moving dots: red = hole, blue = electron. Colored regions: red = p-type, blue = n-type. dp = excess hole concentration at the depletion boundary, dn=excess electron concentration at the depletion boundary. Lp = hole diffusion length, Ln = electron diffusion length. I = total current, In(x) = electron current (blue curve), Ip(x) = hole current (red curve). The diagram at middle may be viewed as a band diagram, but the band banding across the depletion region is not shown here. The numerical values are for a Si pn junction diode.

Program implemented by Jia Wang
Content, text, and programming supervision by C. R. Wie