In physics, the field effect describes how an external electric field changes how well electricity can flow through a material.

In metals, there are so many electrons that respond to electric fields that the field only goes a short distance into the material. In semiconductors, there are fewer electrons (and possibly holes) that can respond to the field. This means the field can travel much farther into the semiconductor. When the field reaches the semiconductor’s surface, it changes the material’s conductivity. This change is called the field effect. The field effect helps devices like Schottky diodes and field-effect transistors, such as MOSFETs, JFETs, and MESFETs, work properly.

Surface conductance and band bending

The change in how electricity flows on the surface happens because the applied field changes the energy levels that electrons can occupy, even far from the surface. This change affects how many electrons are present in the surface area. A common way to explain this is through a band-bending diagram, which shows how the energy levels of the material change with depth.

In the diagram, energy is measured in electron volts (eV), and voltage is measured in volts. This avoids needing to use a factor for the elementary charge. The diagram shows a two-layer structure: an insulator on the left and a semiconductor on the right. An example of this is a MOS capacitor, a device with a metal gate, a semiconductor (like silicon), and an insulating layer (like silicon dioxide). The left side of the diagram shows the lowest energy level of the conduction band and the highest level of the valence band. These levels bend when a positive voltage is applied. By convention, the energy of electrons is shown, so a positive voltage lowers the conduction band’s energy. A dashed line in the diagram represents the Fermi level, which shows where electrons are most likely to be found. Below the Fermi level, states are more occupied, and the conduction band moves closer to the Fermi level, meaning more electrons are in the conduction band near the insulator.

In the example, the Fermi level in the semiconductor’s bulk (away from the surface) is near the valence band edge. This position is created by adding impurities called acceptors to the semiconductor. These acceptors take electrons from the valence band, becoming negatively charged ions. This leaves empty spots, or holes, in the valence band. In the region without an applied field, the material remains neutral because the negative acceptor ions balance the positive holes.

Band bending is described next. A positive charge is placed on the insulator’s left side (like a metal gate). Since the insulator has no charges, the electric field is constant, causing a linear change in voltage. This makes the insulator’s conduction and valence bands appear as straight lines in the diagram, separated by a large energy gap.

In the semiconductor, the positive charge on the insulator lowers the valence band’s energy. This causes the valence band states to fill up to a depth called the depletion depth, where the field can no longer affect the material. At this depth, the bulk’s normal electron distribution returns. Because the valence band near the surface is fully filled, only the immobile negative acceptor ions remain near the surface, creating an insulating region with no holes (the depletion layer). The depletion layer stops the field from penetrating further when the negative acceptor ions balance the positive charge on the insulator.

The conduction band’s energy also lowers, increasing the number of electrons in these states. At low voltages, this increase is small. However, at higher voltages, the conduction band’s energy lowers enough to create a layer of electrons near the surface, called an inversion layer. This layer forms when the applied voltage reaches a threshold value. Once this voltage is exceeded, the inversion layer holds most of the charge, rather than relying on the depletion layer expanding. At this point, the depletion layer’s depth remains constant because the electron density increases rapidly with further band bending, effectively stopping the field from penetrating deeper into the semiconductor.