SEMICONDUCTORS
Conductivity is the ease with which a given material conducts electricity. Conductors are regarded
as good conductors of electricity while insulators are regarded as poor conductors of electricity. A
semiconductor is a material whose conductivity lies between that of a perfect insulator and that of
a perfect conductor. It is the ability to control the conductivity that make semiconductors useful as
current/voltage control element. Current/Voltage control is the key to switches (digital logic
including microprocessor), amplifiers, LEDs, LASERs, photodetectors, etc.
Additionally, the conductivity of a semiconductor can be ‘controlled’ by various means such as:
intentionally introduceing impurities (doping)
externally applied electric fields
temperature variations
mechanical stress
radiation (light)
These methods of control have allowed for semiconductors to be used as thousands of electronic
devices, such as: diodes, bipolar transistors, field effect transistors (FET), temperature sensors,
force and pressure sensors, solar cells and photo-transistors
The conductivity of a semiconductor is determined by the number of ‘free charged particles’ in the
bulk, and their ability to move through the bulk (mobility). Electrons and holes are the two types
of charge carriers in semiconductors.
ENERGY BAND
According to Bohr’s theory, every shell of an atom contains a discrete amount of energy at
different levels. Energy band theory explains the interaction between the outermost shell and the
innermost shell. Based on the energy band theory, there are three different energy bands namely
the valence band, forbidden energy gap and the conduction band. The band of excited states is
called the conduction band because while valence electrons are tightly held to atoms, excited
electrons can move rather freely. In insulators, the energy gap is so large that electrons can’t get
to the conduction band. In conductors, if the valence band is not full, electrons can move in the
valence band. Also, if the conduction band broaden sufficiently to overlap the valence band, the
bands merge and electrons easily find new states in which to move. In semiconductors, the
electrons are bound. When the valence electrons receive energy, they become free electrons and
have energy to migrate into the unpopulated energy levels in the conduction band. The excited
electron which is free to move then conducts electricity.
The band gap energy is the minimum amount of energy required to dislodge an electron from the
covalent bond. It is the energy required for electrons to migrate from the filled valence band to the
vacant conduction band. The energy band gap also called forbidden gap in insulators is very high
up to 7eV. In semiconductors like Silicon and Germanium, they have an energy band gap of 1.1eV
and 0.72eV respectively. There are no forbidden gap between the valence band and conduction
band of a conductor.
INTRINSIC AND EXTRINSIC SEMICONDUCTORS
Semiconductors can be classified into intrinsic semiconductors and extrinsic semiconductors. An
‘intrinsic semiconductor’ is a perfect semiconductor crystal with no impurities or lattice defects.
Examples include Silicon and Germanium. For equilibrium conditions, for an intrinsic
semiconductor, the number of electrons equals the number of holes i.e. n = p ≡ ni. An ‘extrinsic
semiconductor’ is an intrinsic semiconductor that has been doped with impurities called dopants.
Usually, a group three metal or group five metal is used to dope the intrinsic semiconductor. The
process of doping an intrinsic semiconductor is called doping. Usually, a semiconductor is
intentionally doped to create either:
a surplus of electrons in the bulk (n-type material) usually done with phosphorus (group V
element) doping
a surplus of holes in the bulk (p-type material) usually done with boron (group III metal)
doping.
Silicon Structure and bonding
EXTRINSIC SEMICONDUCTORS
N-Type
Pentavalent impurities such as phosphorus, arsenic, antimony, and bismuth have 5 valence
electrons. When phosphorus impurity is added to Si, every phosphorus atom’s four valenc
eelectrons are locked up in covalent bond with valence electrons of four neighboring Si atoms.
However, the 5th valence electron of phosphorus atom does not find a binding electron and thus
remains free to float. When a voltage is applied across the silicon-phosphorus mixture, free
electrons migrate toward the positive voltage end. When phosphorus is added to Si to yield the
above effect, we say that Si is doped with phosphorus. The resulting mixture is called N-type
silicon (N: negative charge carrier silicon). Here, the electrons are the majority charge carrier while
holes are the minority charge carrier. The pentavalent impurities are referred to as donor impurities.
N-Type Si Semiconductor
P-Type
Trivalent impurities e.g., boron, aluminum, indium, and gallium have 3 valence electrons. When
boron is added to Si, every boron atom’s three valence electrons are locked up in covalent bond
with valence electrons of three neighboring Si atoms. However, a vacant spot “hole” is created
within the covalent bond between one boron atom and a neighboring Si atom. The holes are
considered to be positive charge carriers. When a voltage is applied across the silicon-boron
mixture, a hole moves toward the negative voltage end while a neighboring electron fills in its
place. When boron is added to Si to yield the above effect, we say that Si is doped with boron. The
resulting mixture is called P-type silicon (P: positive charge carrier silicon). The trivalent
impurities are referred to as acceptor impurities. Here, the holes are the majority charge carrier
while electron are the minority charge carrier.
P-Type Si Semiconductor
