# Estimate the number of electrons flowing in a semiconductor pn junction

## Homework Statement

The drift current in a p-n junction is 20μA . Estimate the number of electrons crossing a cross section per second in the depletion region.

## Homework Equations

Drift current is the current that occurs due to formation of holes and electrons in the depleted region from where the electrons and the holes are sucked away to the respective sides . This creates a current from n to p . Called the drift current The drift current is a result of flow of both holes and electrons.

## The Attempt at a Solution

So the electrons and the holes both equally contribute to the drift current . So the current due to the electrons would be 10μA . So that means
10 * 10^(-6) C flowing per second
charge on one electron = 1.6 * 10^-19 C

so we can write

n * 1.6 * 10^-19 C =10 * 10^(-6)
n = 6.25 * 10^13 electrons .

But the answer given is 3.125 * 10 ^ 13 , half of what I calculated .
Where am I wrong ?

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rude man
Homework Helper
Gold Member
I post this mainly since no one else has; it's not my specialty:

You can't assume that half the charges are electrons & the other half holes.

Each carrier current must be computed separately based on the respective charges' diffusion constants, minority carrier concentrations, and diffusion lengths (lifetimes); all figure separately for electrons and holes. Finally, you add the two to get the total junction current.

If the problem assumes equal n and p doping levels then one or more of the above parameters is such that the respective parameter ratio for holes & electrons accounts for the 3:1 ratio of hole current vs. electron current.