In sum, in the first decades of the 20th century, scientists had already discovered the
physics that Andy Grove and his peers would need in order to create semiconductors
and lasers. In fact, the first patents for basic semiconductor transistors were granted in
Germany and the United States in the 1920s and 1930s. The practical manufacture of
these devices required progress in the material sciences, but transistors were
manufactured in the late 1940s, and the first modern-day field effect device, a type of
transistor, was proposed in 1952 and built in 1953.
Bohr, photons and spectral lines
Orbits (and energy levels) are quantized
Photons of light are quantized
·Only certain energy changes possible
·Energy = Planck’s constant times
frequency
·Observed spectral lines match energy
differences36.10 - Interactive checkpoint: photons and electron energy levels
A helium atom emits light of
wavelength 471 nm when an electron
makes a transition from the n = 4state
to the n = 3 state. If the energy of the
n = 3 state is -6.04 eV, what is the
energy of the n = 4 state?
Answer:E = eV
36.11 - Conduction in solids
This chapter started off by noting that the development of semiconductor-based devices, such as transistors, has been one of the most
important technological advances of the past 50 years. We also promised that understanding their functioning would require only a basic
understanding of some principles of quantum physics. With your introduction to quantized energy levels and photons essentially complete for
the purposes of this chapter, we can now start the discussion of semiconductors, and shortly, lasers.
To explain semiconductors, we need to discuss the conduction of electrical currents in solids. In terms of conducting electricity, semiconductors
lie between the extreme cases of conductors and insulators (hence the “semi” in their name). The ability of engineers to influence how readily
semiconductors conduct a current is a key to their utility.
To review some terminology: Some substances (like copper or aluminum) are considered conductors; current flows relatively easily in a
conductor. Others (like silicon laced with impurities) are considered semiconductors. Others, like silicon dioxide, are insulators, where it is very
difficult to cause a current to flow.
In this section, we discuss why current flows more í or less í easily in conductors, semiconductors and insulators. We start with an energy
diagram of two electrons in a single atom, as you see below. The electrons exist at distinct energy levels.