A Schottky barrier and pn junction are very similar.   If the channel length of a FET is small enough, then the transistor can be viewed as two pn junctions (i.e. diodes) placed back to back.   Examining the energy band diagrams of a pn junction under certain condition shows how the pn junction operates.   The p-doped region causes the Fermi energy level to move closer to the valence band, and the n-doped region moves the Fermi level closer to the conduction band.   The doping presents more holes in the valence band on the p-doped side but more electrons in conduction band on the n-doped side.   When no voltage is applied (no bias involved), there is an intrinsic built-in energy barrier (Vbi) which prevents electrons from moving from the n-side to the p-side.   The built-in barrier depends on the doping densities of both the n-side and p-side.   Less dopant concentration decreases the energy barrier seen by electrons in the n-region.   If a positive voltage is applied to the n-side of the junction, an even larger barrier exists as seen by the reverse bias condition.   The majority carriers on the p-side (holes) are repelled by the positive voltage on the opposite side of the junction.   Likewise the electrons are also repelled in the other direction by the same electric field.   Thus, no charge carriers can overcome the barrier to create a flow of current.   Changing the applied voltage to the p-side creates a forward-bias condition.   The built-in energy barrier is reduced by VF.   The electric field across the center of the junction, W, attracts electron to the p-side and repels holes to the n-side resulting in a flow of current.

The band diagrams of an abrupt metal-semiconductor junction under reverse and forward bias conditions are give below.   No energy bandgap is present to the left of the junction since the material is metal.   Instead, the bottom of the conduction band and the top of the valence conincide.   The band diagram indicates that an electron in a metal can jump to the conduction band with ease.   Just as with the pn junction, the energy barrier of the metal-semiconductor junction is large under a reverse bias condition.   The width of the barrier is also large, so it is extremely improbable for an electron to traverse the large barrier as well as a large energy barrier height.   However, under forward bias conditions, the energy barrier height and width decrease.   If the width of the schottky barrier is extremely small (on the order angstroms), then tunneling becomes a very definite possibilty.   The probability that an electron crosses the schottky barrier is determined by Schrödinger's wave equation.   The probability can be viewed as a transmission coefficient indicating how many electrons cross for every electron approaching the barrier.   Even though the transmission coefficient may be small in most cases, if a many electrons approach the barrier, a tunneling current still occurs.   Read further on the main page for specifics on the tunneling probability.