Engineering Chemistry S Datta

492 ENGINEERING CHEMISTRY
Role of Silicon (Si) and Germenium (Ge) in the field of semiconductors
Semiconductors are the materials that exhibit conductivities considerably
lower than those of the metals. Semiconductors are defined as inorganic crystals responding
to electronegativity when excited by heat or electromagnetic radiation. Si and Ge in pure state
are very poor conductors of electricity. However, impurity doped Si and Ge exhibit semiconductivity.
Doping of small amounts of group IV or V elements improves their electrical
conductivity appreciably. Both in Si and Ge, each atom is covalently bonded to four neighbours
such that all four outer electrons of each are involved in bonding.
Let us suppose an atom of group V element like P, As, Sb or Bi is introduced in place of
Ge, or Si atoms, then the four electrons of this atom will be utilized in the formation of covalent
bonds and the remaining one will be free to move and thus induces an enhanced conductivity.
Similarly, if a group III element like B, Al etc, is introduced in place of Ge or Si, an electron
vacancy in covalently bonded structure will be generated and such type of electron vacancy is
known as ‘hole’. This hole instead of remaining confined to the impurity atom, migrates
throughout the structure when filled by an electron. So, electrons are able to move in a crystal
having such electron vacancies or hole and thus the crystal exhibits electrical conductivity.
Semiconductors that exhibit conductivity due to excess electrons are called n-type
semiconductors where n stands for negative hole. In other cases, there are some semiconductors
in which electrical conductivity is imposed due to presence of positive holes, they are known as
p-type semiconductors, discussed later on.
With rise in temperature the conductivity of semiconductors increase because at high
temperature the excess electrons or holes become more free to move and so the conductivity
increases.
Highlight:
It may be noted that both type of semiconductors (n- and p-type) are electrically
neutral.
Transistors (semiconductor triodes)
The word transistor is derived from two words—transfer and resistor. A triod transistor
contains two P-N junction diodes placed back to back. Junction transistors are of two types:
• Grown junction type and
• Alloy junction type.
The following Fig. 22.16(a) and (b) show a grown P-N-P junction triode transistor and
an N-P-N junction transistor respecttively
Emitter Base Collector
(Input) E
P
N
P
C
(Output)
(Input) E
N
P
N
C
(Output)
B (Base)
(a) A PNP transistor
B (Base)
(b) An NPN transistor
Fig. 22.16 Transistors.