We’ll now take a quick look at the basic semiconductor physics behind the pn junction, and then follow this up with examples and applications.
For T > 0, random thermal vibration provides enough energy for some of the e- to break their covalent bonds. When electrons are thermally excited out of covalent bonds, they leave a “vacancy” at the bond site, as illustrated above in Fig.
The concentration of holes and free electrons can be changed in a silicon crystal by adding small amounts of impurities called dopants.
These impurity atoms displace silicon atoms (having four electrons) with boron atoms (having three electrons). Consequently, the regular silicon lattice has “holes,” or locations in the lattice that can accept a free electron. Something very special occurs when we place p-type material in contact with n-type material. Through the mechanism of diffusion (random motion due to thermal agitation), excess holes will migrate to the n-type region while excess free electrons will migrate to the p-type region.


With this electron now “gone,” we have “uncovered” a positive charge from the dopant atom in the n-type region.
This contact region between the p and n regions now has a bound volume electric charge density. For the reversed biased state of the pn junction, the electric field produced by the battery Ebattery adds to this electric field of the space charge E in the depletion region. Consequently, the “majority carriers” cannot flow through the region: holes in the p material are opposed by E in the depletion region, as are electrons in the n material.
When V is large enough so that Ebattery > E, then (i) holes are swept from the p to n regions, and (ii) electrons are swept from the n to p regions. Breakdown – in this region v » VZK ? - for all I, where VZK is called the breakdown knee voltage. For such “p-type” semiconductors, the silicon is doped with trivalent impurity elements such as boron. In n-type materials we then have free electrons while in p-type materials we have free holes.


There now appears to be an “excess” of holes in the p-type material and an “excess” of free electrons in the n type.
Interestingly, holes can also contribute to conduction current in a semiconductor material (see the figure below). This may seem an unexpected name since only in this region is there a net volume charge density (aka space charge)!
This occurs when Ebattery is strong enough to strip electrons from the covalent bonds of the atoms, which are then swept across the junction.



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