# 4.2 Structure and Principle of Operation

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## 4.2.1 Structure of a p-n junction

A p-n junction consists of two semiconductor regions with opposite doping type as shown in the figure below. The region on the left is p-type with an acceptor density NA, while the region on the right is n-type with a donor density ND. The electron (hole) density in the n-type (p-type) region is approximately equal to the donor (acceptor) density.

pnstruct.gif

Fig.4.2.1 Crosssection of a p-n junction

We will assume, unless stated otherwise, that the doped regions are uniformly doped and that the transition between the two regions is abrupt. We will refer to this structure as being an abrupt p-n junction.

Frequently we will deal with p-n junctions in which one side is distinctly higher doped than the other. We will find that in such a case only the low doped region needs to be considered, since it primarily determines the device characteristics. We will refer to such a structure as a one-sided abrupt p-n junction.

The junction is biased with a voltage Va as shown in the figure. We will call the junction forward-biased if a positive voltage is applied to the p-doped region and reversed-biased if a negative voltage is applied to the p-doped region. The contact to the p-type region is also called the anode, while the contact to the n-type region is called the cathode, in reference to the anions or positive carriers and cathions or negative carriers in each of these regions.

## 4.2.2 Principle of operation

The principle of operation will be explained using a gedanken experiment, were we imagine that one can bring both regions simply together, aligning both the conduction and valence band energies of each region as shown in the figure below.

pnband.gif

Fig.4.2.2 Energy band diagram of a p-n junction just after merging the n-type and p-type regions
Note that this does not automatically align the fermi energies. The electrons (holes) close to the metallurgical junction diffuse across the junction into the p-type (n-type) region where hardly any electrons (holes) are present. This process leaves the ionized donors (acceptors) behind, creating a region around the junction which is depleted of mobile carriers. We call this region the depletion region, indicated by the symbol w as shown on the above figure. The charge due to the ionized donors and acceptors causes an electric field which in turn causes a drift of carriers in the opposite direction. The diffusion of carriers continues until the drift current balances the diffusion current, thereby reaching equilibrium. This situation is shown below:

pnband1.gif

Fig.4.2.3 Energy band diagram of a p-n junction in thermal equilibrium
While in thermal equilibrium no voltage difference exists between the n-type and p-type material, there is an internal potential which is caused by the fermi energy difference between the n-type and p-type semiconductors. This built-in potential is discussed in the next section
4.1 ¬ ­ ® 4.3

© Bart J. Van Zeghbroeck, 1996, 1997