Charge supply by a Forward-biased PN Junction
In the following applet, use the scroll bar to set a positive bias voltage (Forward bias), say 0.2 Volts. Now, suppose that you are sitting inside the p-type Semiconductor (red rectangle in the top diagram). As you look at the junction, what kind of carriers are moving into the p-type region ? Answer: lots of electrons and a small amount of holes.
Question-1: At a forward bias of 0.2 V, calculate the minority electron concentration at the junction edge (or depletion edge) of the p-type. Assume that the minority electron concentration at equilibrium is np0 = 1000 electrons/cm3 and use the formula np = np0 exp(V/VT) where VT = 0.025 V at room temperature. see the answer. Calculate the minority electron density at the depletion edge at 0.4, 0.55, and 0.7 V of forward bias. see the answer.
What you see in this applet is the log of the number of carriers moving. So, even though the applet shows a small difference in the numbers of carriers, it is actually a big difference. Not worrying about the carriers leaving the p-region, and worrying only about those moving in, we conclude the following: To the p-side, a forward-biased junction sends in lots of electrons (minority carrier), and leaking in a small amount of holes. The small amount holes are the thermally generated minority holes within a diffusion-length distance from the junction. A very small number indeed. negligible.
Question-2: For Nd = 1E17 cm-3 (or n = 1E17 cm-3), calculate the minority hole concentration at equilibrium in Si at 300K ? see the answer. Use this applet to convince yourself of the small number of minority holes in the n-side.
A forward-biased PN Junction supplies a large amount of minority carrier charge into the semiconductor (injection), and supplies a very small amount of majority charge (leakage).