TABLE OF CONTENTS The junction photodiode that is the focus of this book has been described in detail in many other books and publications. Here only a few basics are given, so that you can use the photodiode effectively in real circuits. A simple model is presented that allows the main characteristics and limitations of real components to be understood. The ability to correctly derive the polarity of a photodiode's output, guess at what level of output current to expect, and have a feel for how detection speed depends on the attached load is necessary. The model we begin with has little to do with the typical real component fabricated using modern processing techniques; it is a schematic silicon pn-junction diode.
Figure 1.1a shows two separate blocks of silicon. Silicon has a chemical valency of four, indicating simplistically that each silicon atom has four electron bonds, which usually link it to neighboring atoms. However, the lower block has been doped with a low concentration (typically 10 13 to 10 18 foreign atoms per cm 3) of a five-valent element, such as arsenic or phosphorus. Because these dopants have one more valency than is needed to satisfy neighboring silicon atoms, they have a free electron to donate to the lattice and are therefore called donor atoms. The donor atoms are bound in the silicon crystal lattice, but their extra electron can be easily ionized by thermal energy at room temperature to contribute to...
