Shot noise is a common noise source in some electronic devices. Shot noise occurs when there is direct current (DC) through a potential barrier. It was first explained by Walter Schottky in 1918.
Shot noise occurs because the current carriers impinging on a potential barrier, such as that found in a p-n junction, have varying kinetic energies. Although all of the current carriers are subject to the same electric field, they suffer from random collisions, and thus have random velocities and kinetic energies (see conductivity). Neglecting tunneling effects, only carriers with kinetic energies greater than the potential barrier can cross the barrier. Since the current carriers arrive at the junction as discrete packets of charge, the current through the junction is noisy--the actual current varies from the average current over time.
A simple approximation for the root-mean-square shot noise current is (2qIDCΔf)1/2, where q is the electron charge, IDC is the average DC current, and Δf is the bandwidth of the electrical circuit. Since shot noise requires a potential barrier, the problem is much larger in bipolar junction transistors than it is in MOSFET's.