Semiconductor Basics

1The Bohr Atom

OBJECTIVES OF THIS CHAPTER

To understand how semiconductors work and how they are used, we need to be familiar with the concept of allowed energy levels first proposed by Niels Bohr. How Bohr came up with the idea of a planetary model of the atom is very interesting. Science is a continuum: one observation leads to a hypothesis that leads to a theory that leads to more observations, and so on. Bohr did not come up with his model of the atom out of the blue – no apple fell on his head. A lot of observations and theories going back to the 1700s were proposed before he put them together in a now well‐known theory.

In this chapter, I discuss the experimental evidence and the scientific observations that led to Bohr's planetary model of the atom and the discrete energy levels it postulated. This brief journey will help us understand the significance of the Bohr atom that explains the strange behavior of light spectra.

1.1 Sinusoidal Waves

Before I start, I want to clarify the terms used to define a wave, which I use in the next few sections, and the relations between these terms (see Figure 1.1). There are four variables that we use to define any sinusoidal wave:

• The amplitude, A: How high each peak of the wave is in relation to the middle, its zero value.
• The frequency, f: The number of ups and downs in the wave in a given time. The units are Hertz or number of ups and downs per second: a number/s.
• The wavelength, λ: The distance between two peaks, in meters ...