3.3 Current mechanisms in a Metal-Semiconductor junction

Table of Contents - 1 2 3 4 5 6 7 8 9 R S ®

In this Section

  1. Diffusion current
  2. Thermionic emission
  3. Field emission

The current across a metal-semiconductor junction is mainly due to majority carriers. Three distinctly different mechanisms exist: diffusion of carriers from the semiconductor into the metal, thermionic emission of carriers across the Schottky barrier and quantum-mechanical tunneling through the barrier. The diffusion theory assumes that the driving force is diffusion of carriers due to the density gradient in the depletion layer. The thermionic emission theory on the other hand postulates that only energetic carriers, those having an energy equal to or larger than the conduction band energy at the metal-semiconductor interface, contribute to the current flow. Quantum-mechanical tunneling through the barrier takes into account the wave-nature of the electrons allowing them to penetrate through thin barriers. In a given junction one finds that a combination of all three mechanisms could exist. However one finds that typically one limits the current, making it the dominant current mechanism.

The analysis reveals that the diffusion and thermionic emission currents can be written in the following form:

This expression states that the current is the product of the electronic charge, a velocity and the density of available carriers in the semiconductor located next to the interface. The velocity equals the mobility multiplied with the field at the interface for the diffusion current and the Richardson velocity for the thermionic emission current. The minus one term ensures that the current is zero if no voltage is applied as in thermal equilibrium any motion of carriers is balanced by a motion of carriers in the opposite direction.

The tunneling current is of a similar form, namely:

where vR is the Richardson velocity and n is the density of carriers in the semiconductor. The tunneling probability term, Q, is added since the total current depends on the amount of carriers arriving at the tunnel barrier multiplied with the probability that they tunnel through the barrier.

3.2 ® 3.4

© Bart Van Zeghbroeck 1997