City of Light: The Story of Fiber Optics

RECIPES FOR GRAINS OF SALT 149
negative carrier) semiconductor. An impurity with one less outer electron
creates a hole, forming a semiconductor of the p-type (for positive carrier,
because the absence of an electron leaves a positive charge on the atomic
nucleus). Adding such dopants releases more carriers in the crystal, making
better electronic devices. Put layers of n-and p-type semiconductor together,
and you have a simple two-terminal device called a diode. 2
Things get interesting when you apply a voltage across a diode. A positive
electrical voltage attracts the negative electrons, while a negative voltage
attracts the positive holes. Apply a positive voltage to the n-type material
and a negative voltage to the p-type material, and the electrons and holes
move to the terminals and stay there, so no current flows through the junction
between the two materials. However, a current will flow if you switch
the voltages, because electrons in the n material move toward the positive
terminal on the p material, and holes move in the opposite direction. Thus,
a semiconductor diode conducts electricity in only one direction 3 (figure
12-1).
The real action happens at the junction between n and p material, where
electrons from the n material combine with holes from the p material. As
the electron drops into the hole, it releases its extra energy by a process called
recombination. In silicon, the energy typically is released as heat. However,
in gallium arsenide, indium phosphide, and certain other semiconductor
crystals, some energy is released as light. This is the basis of a light-emitting
diode or LED. Initially, the process was quite inefficient. The first LEDs turned
only about 0.01 percent of the input electrical energy into visible or infrared
light. 4
Birth of the Semiconductor Laser
When the laser hit the headlines, a few physicists thought of making lasers
from light-emitting semiconductors. However, nobody took the idea too
seriously until 1962, when Robert Rediker, Tom Quist, and Robert Keyes
changed the way they were processing gallium arsenide at the MIT Lincoln
Laboratory. LEDs made from the new material generated light surprisingly
efficiently. When Keyes announced their results at a July 1962 meeting in
New Hampshire, an astounded member of the audience stood up to say that
Keyes’s statement violated the second law of thermodynamics—a cardinal
principle of modern physics which holds that the degree of disorder or ‘‘entropy’’
always increases. Keyes returned to the microphone and deadpanned,
‘‘I’m sorry.’’ 5
Bob Hall, a semiconductor expert from the General Electric Research Laboratory,
immediately figured out what the MIT group had done. After everyone
stopped laughing, he stood up and explained why the results were possible.
He also realized that high efficiency opened the door to making a
semiconductor laser. He worked through the numbers on the train ride home,