Page 28 - A Practical Guide from Design Planning to Manufacturing

4 Chapter One

an electric current when exposed to light. This type of effect had been
observed in other crystals but never to this degree, and it had never been
well understood. Upon examining the crystal more closely Ohl discov-
ered a crack that had formed in the crystal as it was made. This crack
had caused the impurities in the crystal to be distributed unevenly
between the two sides.
One side had impurities with electrons that were free to move
through the crystal. He called this side the N-type silicon because it had
negative charge carriers. The other side had impurities that produced
spaces that electrons could occupy but were empty. The spaces or holes
could move through the crystal as one electron after another moved to
fill the hole, like moving one car after another into an empty parking
space and causing the empty space to move. The holes acted as positive
charge carriers and so Ohl called this P-type silicon. The junction
formed by these two types of silicon allowed electricity to flow in only
one direction, which meant that the energy added by light could produce
a current in only one direction. The single-piece, solid-state diode had
been discovered.
Today we have a much better understanding of why some impurities pro-
duce N-type and others P-type semiconductors, and the operation of a
junction diode. Everything is made of atoms and since all atoms contain
electrons, anything can conduct electricity. Anyone who has seen a light-
ning storm has seen proof that with a sufficiently large electric field, even
air can conduct large amounts of electricity. Materials are classified by how
easily they carry electricity. Materials like copper that conduct easily are
called conductors, and materials like glass that do not conduct easily are
called insulators. Some materials, such as silicon, normally do not conduct
easily, but very small amounts of impurities cause them to become good
conductors. These materials are called semiconductors. The reason for
this behavior is electron energy bands and band gaps (Fig. 1-1).
Quantum mechanics tells us that electrons can occupy only a finite
number of discrete energy levels. In any noncrystalline material the

Band
gap
Allowed Band gap
energy
states
Energy Pure N-type P-type
Conductor Insulator
semiconductor semiconductor semiconductor
Figure 1-1 Energy bands.