Semiconductor Junctions

When an interface is formed between materials with dissimilar band structures and electrical properties the interaction between them can cause short-scale movement of electronic charges across the interface as the Fermi levels attempt to reach thermal equilibrium. This process can be utilised to produce semiconductor devices with electronic properties not found in any of the materials if used individually. These junctions form the basis of numerous electronic components such as MOSFETs, Schottky diodes, LEDs, etc.. Multiple devices can be printed onto single integrated circuits to exhibit more complicated

61 device behaviours such as those seen in microprocessors, RAM, charge-coupled devices, etc..

2.3.6.1 Metal-Semiconductor (M-S) Junctions

Figure 14: Left: band structures of a metal (with work function = qΦm) and an n-type semiconductor (electron

affinity = qΧ) before electrical contact is made between them. Right: After contact is made and the layers reach thermal equilibrium by electrons from the semiconductor travelling into the metal to lower their energy. This leaves a sea of positive ions which in turn creates a negative electric field, lowering the band edges of the semiconductor. Electrons continue to flow across the barrier until the Fermi energies of the two

materials are equal.

Figure 14 shows the band structure representation of a metal and an n-type semiconductor before and after they have been brought into contact. Once contact between the two is made and they have reached thermal equilibrium a potential barrier exists that electrons can tunnel through or otherwise be given enough energy to pass over before conduction can occur. Under positive bias, whereby the positive terminal of a current source is connected to the metal, the Fermi energy of the metal is lowered with respect to the semiconductor resulting in a smaller potential drop. This allows a positive current to pass through the circuit from the metal to the semiconductor. When subjected to a negative bias, the Fermi energy of the metal raises with respect to the semiconductor. This has the effect of increasing the barrier potential and increasing the size of the depletion region, effectively stopping the flow of current and behaving as a rectifying junction.

For an n-type semiconductor this barrier height can be calculated:

62 For a p-type semiconductor the barrier height is:

The built-in potential ( ), i.e. the difference between the Fermi energies of the metal and semiconductor can also be calculated (114)_:

For n-type junctions:

And for p-type junctions:

From these results the potential across the semiconductor can be calculated as the built-in potential ( ) minus the applied voltage:

2.3.6.2 P-N Junctions

When two semiconductors with different majority carriers are brought into contact, or when adjacent regions of a semiconductor are doped n and p-type the resultant device can show rectifying characteristics. This device property forms the basis of MOSFETs, BJTs, solar cells, LEDs, and many other types of widely used devices.

63 Figure 15: Left: P and n-type semiconductors before electrical contact is made between them. Right: An

abrupt p-n junction in thermal equilibrium.

Figure 15 shows the electronic band structures for p-type and n-type semiconductors before and after thermal equilibrium is achieved. To reach thermal equilibrium, electrons from the n-type region diffuse across the boundary into the p-type region while holes from the p-type transfer into the n-type material. With no externally applied potential, over a short distance on either side of the interface both semiconductors lose their majority carriers. The area is known as the depletion region (Figure 16).

64 Figure 16 shows schematically the effect of applying forward and reverse biases to a p-n junction structure. In the first image no bias is applied and a depletion region is formed across the interface. The size of this zone is proportional to the built-in potential of the device. The bottom left image shows the device under forward bias. Electrons from the negative terminal are able to fill the empty spaces formed at the n-type side of the depletion region, while holes from the positive terminal fill up the holes on the p-type side. The size of the depletion region is reduced allowing current to flow across the junction. Referring back to the equilibrium band structure in Figure 15, an applied forward bias will have the effect of reducing the energy difference between the conduction and valences bands at both sides, i.e. raising both band edges on the n-type side and lowering both band edges on the p-type side. Majority carriers on either side of the interface are then able to cross the much reduced depletion zone via quantum tunnelling. The bottom right image shows the effect of reverse bias on the p-n junction. In this case no current is able to flow across the junction as the size of the depletion region is increased by the applied electric field.

It is possible to increase the minimum width of the depletion region by placing a third layer of intrinsic semiconductor material between the n and p-type materials. This is a useful technique when constructing solar cells or photodiodes as it enables absorption of electromagnetic radiation to be maximised within the depletion zone, yielding a higher photocurrent (2,114,115,116,117,118).