Moore’s law and the silicon revolution - The Computing Universe: A Journey through a Revolution - page 123

Fig. 7.4. Semiconductors doped with
impurity atoms. (a) n-type semicon-
ductor in which the impurity atoms
have an extra electron. This results
in the effective energy-level diagram
shown here with a “donor level” just
below the conduction band. (b) p-type
semiconductor doped with impu-
rity atoms with one fewer electron,
resulting in electron “holes.” The
equivalent energy-level diagram has an
empty “acceptor level” just above the
valence band.

Electron
conduction
band

Impurity donor
level

Electrons in acceptor level
from valence band

Electrons in conduction
band from donor atoms

Acceptor
energy level

Hole

Filled valence
band

Semiconductor
atom

Hole

Impurity atom
such as
phosphorus

Impurity atom
such as boron

Extra
electron

n-type semiconductor p-type semiconductor

below the conduction band, and these electrons need only gain a small amount
of energy to jump into the conduction band. Semiconductors doped with boron
are called p-type semiconductors . The boron atoms give rise to electron acceptor
states just above the nearly full valence band and at room temperatures elec-
trons are readily excited into these levels. Compared to an undoped semicon-
ductor, the boron impurity site is missing a negatively charged electron. This
is equivalent to the p-type semiconductor having a positive charge compared
to the undoped material. Conductivity in the nearly full valence band is possi-
ble because electrons can move into the unoccupied “hole” states. In a p-type
semiconductor, instead of thinking of a negatively charged electron moving
in response to a voltage, we can equally well think of a positively charged hole
moving in the opposite direction. Because moving a negative charge to the left
has the effect of increasing the charge on the right, we can alternatively think
of the current as a flow of positively charged holes moving to the right.
Why is all this useful? Russell Ohl ( B.7.1 ) at Bell Labs had discovered that
p- and n-type semiconductors could be put together to form interesting semi-
conductor devices. The simplest device is the p-n junction diode , which prevents
current from flowing in one direction but not the other. This p-n junction
device is able to convert an alternating current into a unidirectional current - a
property called rectification. . The development of the p-n junction diode was the
first step toward the invention of the transistor, a semiconductor device that
could be used either to amplify a signal or to switch a circuit on or off. John
Bardeen, Walter Brattain, and William Shockley ( B.7.2 ) were awarded the 1956
Nobel Prize for physics for their invention of the transistor. The transistor was
not discovered by accident - it was the culmination of an extensive research
program at Bell Labs. As Bardeen later said in his Nobel Prize lecture: “The

B.7.1. Russell Shoemaker Ohl
(1898-1987) was a researcher inves-
tigating the behavior of semiconduc-
tors at AT&T's Bell Labs in Holmdel,
New Jersey. In 1939, Ohl discovered
the “p-n junction” by which he was
able to manipulate current flows. He
also recognized the importance of
using exceptionally pure semicon-
ductor crystals to make repeatable
and usable semiconductor diodes.
His work with these devices led him
to develop and patent the first sili-
con solar cells.