Page 13 - Photonics Essentials an introduction with experiments

Source: Photonics Essentials

Chapter
2

Electrons and Photons

2.1 Introduction

You will discover by measurement that all p-n diodes are sensitive to
light, even if they are intended for some other application. A photodi-
ode is a simple and inexpensive component that you will use to meas-
ure the particle behavior of light. This is a fundamental quantum-
mechanical property of matter, and is the effect for which Albert Ein-
stein was awarded the Nobel Prize in physics in 1921.
Photonic devices are used to convert photons to electrons and vice-
versa. Photons and electrons are two of the basic quantum-mechani-
cal particles. Like all quantum-mechanical particles, electrons and
photons also behave like waves.
In this chapter, you will learn about the wave-like and particle-like
aspects of the behavior of electrons and photons. Each electron that
carries current in a semiconductor is spread out over many thousands
of atoms; that is, it is delocalized. Trying to specify its position or its
velocity is a hopeless task. Furthermore, the semiconductor is full of
many absolutely identical electrons. They are all moving around at a
frenetic pace. Clearly, a different approach is needed.
An important new idea in this chapter is to introduce a “road map”
for electrons in a semiconductor. It tells you what states the electrons
are allowed to occupy, just as a road map tells you where the roads
are located that cars may travel on. The road map for electrons does
not tell you where the electrons are or how fast they are moving, just
as a roadmap for cars does not tell you where the cars are or how fast
they are moving. This road map is called a band structure.
Position and velocity are not very useful ideas for describing either

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