Junctions and Contacts

Abstract

A junction is formed when two dissimilar materials come in contact with each other. The junction between a p-type and n-type semiconductor is called a pn junction. A heterojunction is formed when the semiconductors on both sides of a pn junction are not the same. An example of a heterojunction is when one side is made of silicon and the other of a silicon–germanium alloy. A junction formed between a metal, or a material of metallic character, and a semiconductor is called a contact. The contact is ohmic if it exhibits no barrier to majority carriers in either direction, resulting in a symmetrical current–voltage characteristic with respect to the zero origin. A rectifying contact is asymmetrical, the resistance to current being much larger in one direction than in the other. The pn junction is the fundamental building block for other silicon devices. The junction shape, profile and characteristics have a direct impact on device parameters. A thorough understanding of the properties of pn junctions is therefore essential to the understanding of the operation of transistors and integrated circuits.