2.6. THE P-N JUNCTION55• Most semiconductors are based on elements from group IVA of the periodic table, siliconbeing the most prevalent. Germanium is all but obsolete. Carbon (diamond) is beingdeveloped.• Compound semiconductors such as silicon carbide (group IVA) and gallium arsenide (groupIII-V) are widely used.2.6The P-N junctionIf a block of P-type semiconductor is placed in contact with a block of N-type semiconductor inFigure 64,2.28(a), the result is of no value. We have two conductive blocks in contact with eachother, showing no unique properties. The problem is two separate and distinct crystal bodies.The number of electrons is balanced by the number of protons in both blocks. Thus, neitherblock has any net charge.However, a single semiconductor crystal manufactured with P-type material at one end andN-type material at the other in Figure 64,2.28 (b) has some unique properties. The P-type materialhas positive majority charge carriers, holes, which are free to move about the crystal lattice.The N-type material has mobile negative majority carriers, electrons. Near the junction, theN-type material electrons diffuse across the junction, combining with holes in P-type material.The region of the P-type material near the junction takes on a net negative charge becauseof the electrons attracted. Since electrons departed the N-type region, it takes on a localizedpositive charge. The thin layer of the crystal lattice between these charges has been depletedof majority carriers, thus, is known as the depletion region. It becomes nonconductive intrinsicsemiconductor material. In effect, we have nearly an insulator separating the conductive Pand N doped regions.(a)crystal latticeNPNP(b)intrinsicno chargeseparationchargeseparationholeelectronFigure 2.28:(a) Blocks of P and N semiconductor in contact have no exploitable properties. (b)Single crystal doped with P and N type impurities develops a potential barrier.This separation of charges at the PN junction constitutes a potential barrier. This potentialbarrier must be overcome by an external voltage source to make the junction conduct. Theformation of the junction and potential barrier happens during the manufacturing process.The magnitude of the potential barrier is a function of the materials used in manufacturing.Silicon PN junctions have a higher potential barrier than germanium junctions.