PN junctions

When one n-type semi-conductor and one p-type semi- conductor are placed together, the resulting device has some very special properties. Due to the fact that each semi- conductor contains charge carriers of differing polarity, the negative electrons in the n-type semi-conductor will be drawn towards the positive holes in the p-type semi- conductor and vice versa. The charge carriers will subsequently diffuse into the neighboring area until a thermal equilibrium is reached in which the thermal energy of an individual charge carrier is no longer great enough to excite it over the newly formed depletion region around the np-junction. As shown in figure 1, this thermal equilibrium results in a depletion region surrounded by the remaining positive charge carriers in the p-type semi- conductor and the remaining negative charge carriers in the n-type semi-conductor. The resulting energy diagram is also shown in figure 1.

A fact that is often confusing to some is that, after the thermal equilibrium is reached, the n-type semi- conductor now possesses a net positive charge and the p-type side possesses a net negative charge. The explanation for this is fairly straightforward. Before the two semi- conductors were joined, each was independently neutral. After the two were joined, electrons in the n-type semi- conductor were attracted to the positive holes in the p-type semi-conductor and diffused over the junction. When the n- type semi-conductor lost some of its negative charge carriers, it was left with a resultant positive charge. The same argument holds for the p-type semi-conductor becoming negatively charged because it loses holes and gains electrons through diffusion.

The depletion region is thereby depleted of mobile charge carriers as a thermal equilibrium is reached, and a barrier is created between the n-type and p-type regions. As argued above, there is also an electric field produced in this region due to the produced net positive and negative charges. This field typically has a magnitude between 1,000 and 10,000 V/cm and is shown in figure 2.

The p-n junction is the basis of an electronic device called a diode, which allows electric current to flow in only one direction. Similarly, a third region can be doped n-type or p-type to form a three-terminal device, such as the bipolar junction transistor (which can be either p-n-p or n-p-n).