The Schottky effect is the image force induced lowering of the potential energy for charge carrier emission when an electric field is applied. The attractive force called image force is: 

In metal-semiconductor contacts, the Schottky effect is the image force-induced lowering of the potential energy for charge carrier emission when an electric field is applied. This image force is:

Key Points

•  Image force barrier lowering is a dominant mechanism in the case of transistors with Schottky contacts, governed by both the drain and source bias and the gate voltage.
• In the presence of this mechanism, the barrier height, governing the threshold voltage, and consequently its characteristics, will be modulated.
• Schottky diode performance is also affected by the barrier height, as it dictates the reverse saturation current, governed by diffusion, thermionic emission, and tunneling, and hence causes ideality factor (η) variations.
• Moreover, in the case of two-dimensional hetero-structures, such as TMD/graphene, the barrier height lowering can be attributed to both graphene quantum capacitance and the image force barrier lowering effect, affecting the on-off ratio of such FET devices.