Chapter 7 - Semiconductor Selection

Abstract

Diodes are manufactured in a variety of technologies and materials. This chapter discusses the strengths and weaknesses of a wide variety of diode technologies based on application in switchmode power supplies. One of the diodes is “PN” junction diodes that consist of a piece of P material (current flow is from holes) on one terminal and a piece of N material (current flow is from electrons) on the other terminal. The area between the two doped regions is an area called the depletion layer, where very few holes or free electrons exist. The depletion layer acts very much like intrinsic semiconductor material. When the PN junction is forward biased, holes and electrons are injected into the depletion layer where they recombine and allow current to flow. The thickness of the depletion layer decreases to a very small size when forward biased. Likewise, when the PN junction is reverse biased, the applied voltage pulls holes from one side and electrons from the other side of the junction. Another diode discussed is Schottky diodes. The Schottky effect occurs when metal is in contact with semiconductor material. The very oldest diodes (cat's whisker diodes) used metal with a sharp point in contact with a semiconductor material. The metal in contact with the semiconductor material creates a space charge region that allows current to flow in one direction but not the other. Schottky diodes are an extension of that technology. The chapter also presents the structure of modern Schottky diodes.