The PN Junction

In analog electronics, understanding one of the most fundamental building blocks of electronics, the PN junction, is critical for a good foundation in electronics design. When I first was taught the basics of the PN junction in school, it went right over my head. This is the main reason I wanted to write this blog to help demystify the complexities of it.

The most common semiconductor is silicon; each silicon atom is bonded to its neighbors by four covalent bonds, a chemical bond that involves the sharing of electron pairs between atoms. At room temperature some of these electrons are able to gain thermal energy and are free to move and conduct thermal energy. For these electrons to move no additional energy is required. This broken bond behaves as a particle of positive charge.

Silicon doping is the intentional adding of impurities to silicon to modulate its electrical properties. New atoms are added to the silicon and if the new atom has five electrons in its outer shell the extra electron will be loosely bound to the impurity. At room temperature this electron becomes a conduction electron and gains a negative charge. The number of impurities implanted can be carefully controlled during manufacture. An N-semiconductor has negative charge carriers. The impurities are called donor impurities as they donate electrons.

A P-semiconductor has positive charge carriers. the impurities are called acceptor impurities as they accept electrons. An impurity with only three electrons in its outer shell will accept electrons. The three of the electrons complete three out of four bonds with its neighbor silicon atoms leaving one bond unoccupied. At room temperature the electrons from other bonds can move into occupy this free space creating a hole in the material and a positively charged impurity.

A PN Junction is a structure formed by neighboring regions with different dopings, a p type and an n type.  When a p-semiconductor is perfectly matched with an n-semiconductor. Holes from the p-region will diffuse in to the n-region and electrons from the n-region will diffuse into the p-region. This creates a region depleted of free charge particles. A region of positive ionized impurities and negative ionized impurities appear next to the region free of charged particles. This distribution of charges creates an electric field. The electric potential between these two regions of charged ionizes impurities acts as a barrier that prevents the displacement of electrons and holes.

Application of a positive voltage drop between the P and N regions causes this barrier to drop. A reduced barrier cannot prevent the displacement of electrons and holes. Electric current can then flow through the PN junction. This is called a forward bias. If the voltage potential is reversed, then the barrier height increases. The electron current is then negligible.

The PN junction can only conduct current in one direction.