Page 8 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
1

Photodetection Basics

1.1 Introduction

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 fab-
ricated using modern processing techniques; it is a schematic silicon pn-
junction diode.

1.2 Junction Diodes/Photodiodes
and Photodetection

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
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doped with a low concentration (typically 10 to 10 foreign atoms per cm ) of
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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 con-
tribute to electrical conductivity. The extra electrons then in the conduction
band are effectively free to travel throughout the bulk material. Because of the
dominant presence of negatively charged conducting species, this doped mate-
rial is called n-type.
By contrast, the upper block has been doped with an element such as boron,

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