Semiconductors: Controlling Current Flow

The conductivity of materials is a spectrum; on one end lies conductors, materials that readily allow the flow of electrical current, and at the other lie insulators, which block current flow. Between these extremes lie semiconductors that only conduct under certain conditions, making them perfect for controlling electric current.

Pure Silicon (Si) and Germanium (Ge) are intrinsic semiconductors. They are poor electrical conductors because the 4 outer (valence) electrons within each atom are covalently bonded to 4 adjacent atoms, leaving few delocalised electrons available to conduct. Despite their diamond-like lattice, the conductivity of these materials increases with temperature because far less heat energy is required to free valence electrons from their bonds than is required to release electrons from their bonds in insulating materials.

The composition of charge carriers within these pure semiconductors can be tailored through the addition of impurities to the material, otherwise known as doping. Doped semiconductors are more conductive than their respective pure semiconductors. The impurity that is added either contains an excess of electrons or an excess of holes, to produce n-type or p-type semiconductors respectively.

Adding a small amount of phosphorus (P) dopant to Si will produce an n-type semiconductor. P atoms have 5 valence electrons, 4 of which bond with the Si atoms as previously described, and 1 of which becomes a part of a sea of delocalised electrons; as such, it is known as an electron donor.

N-doped semiconductors thus have an abundance of electrons as their majority charge carriers, and holes as their minor charge carriers. Conversely, p-doping involves the addition of an impurity which contains 3 valence electrons, to create a surplus of holes (the majority charge carriers) and a minority of electron charge carriers. Increasing the proportion of dopant in either type of semiconductor will increase the conductivity of the material, however very small amounts of impurity are required to see significant change.

These impure semiconductors are of little use to use separately, but when brought together to form a p-n junction, they behave in a manner which makes them extremely useful. The p-n junction is now a fundamental building block used to create diodes and solar panels.

The p-type material contains bound electrons, and free holes, whilst the n-type material contains bound holes and free electrons; overall, there is no net charge in either material. Due to random motion, the free particles in each side diffuse across the gap. Consequently, the p-doped material gains a net negative charge and the n-region becomes positively charged as its electrons are removed.

The free majority carriers in each semiconductor material move about with random motion, diffusing across the junction into the opposite region. This causes a net negative charge to develop on the left and a net positive charge on the right.

Due to the net negative and positive charge that develops in either side, an electric field develops across the junction, pushing the free positive charges in the p-type semiconductor, and free negative charges in the n-type material, away from the centre and toward the outer edges of the material. This creates a central region which is depleted of all charge carriers, known as the depletion region. This behaves like an insulator that separates the far ends of the p and n type regions, effectively acting as a potential barrier.

The electric field across the p-n junction pulls the mobile charge carriers to the outer regions of the material, causing a depletion region to form in the centre.

This barrier is approximately 0.6-0.7V in Si; once this voltage is surpassed, majority carriers can overcome the barrier and the material can conduct once again. The diode, one of the simplest semiconductor devices, only allows current to travel through it in one direction, by exploiting this property. The direction in which the diode allows the flow of current – provided that there is a sufficient voltage supply to overcome the potential barrier – is known as the forward bias. In reverse bias, majority carriers are pulled further away from the p-n junction and thus the depletion region increases in width and current flow is blocked.

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