• Valence band. The range of energies (i.e. band)possessed by
valence electrons
• Conduction band. In certain materials (e.g. metals), the valence
electrons are loosely attached to the nucleus
• Forbidden energy gap. The separation between conduction band
and valence band on the energy level diagram
• Insulators. Insulators (e.g. wood, glass etc.)are those substances
which do not allow the passage of electric current through them.
• Conductors. Conductors (e.g. copper, aluminium) are those
substances which easily allow the passage of electric current
through them
• Semiconductors. Semiconductors are those substances whose
electrical conductivity lies in between conductors and insulators.
(e.g. germanium, silicon etc.) resistivity (10−4 to 0.5 Ωm)
• Properties of Semiconductors
1. The resistivity of a semiconductor is less than an insulator
but more than a conductor
2. Semiconductors have negative temperature co-efficient of
resistance
• Crystal is a substance in which the atoms or molecules are
arranged in an orderly pattern
• Effect of Temperature on Semiconductors
1. At Absolute zero. At absolute zero temperature, all the
electrons are tightly held by the semiconductor atoms.
2. Above absolute zero. When the temperature is raised,
some of the covalent bonds in the semiconductor break
due to the thermal energy supplied
• intrinsic semiconductor is when semiconductor in an extremely
pure form
• extrinsic semiconductor or impurity is the adding of a small
amount of suitable impurity to a semiconductor. Because
increases its conducting properties
• doping is the process of adding impurities to a semiconductor
• n-type semiconductor is when a small amount of pentavalent
impurity is added to a pure semiconductor
• Typical examples of pentavalent impurities are arsenic and
antimony
• Such impurities which produce n-type semiconductor are known
as donor impurities
• n-type conductivity. The current conduction in an n-type
semiconductor is predominantly by free electrons i.e. negative
charges and is called n-type or electron type conductivity
• p-type semiconductor when a small amount of trivalent impurity
is added to a pure semiconductor
• The addition of trivalent impurity provides a large number of
holes in the semiconductor.
• Typical examples of trivalent impurities are gallium and indium
• Such impurities which produce p-type semiconductor are known
as acceptor impurities
• p-type conductivity. The current conduction in p-type
semiconductor is predominantly by holes i.e. positive charges and
is called p-type or hole-type conductivity.
• pn junction when a p-type semiconductor is suitably joined to ntype semiconductor, the contact surface .
• Forward biasing. Is when external d.c. voltage applied to the
junction is in such a direction that it cancels the potential barrier,
thus permitting current flow,
• Reverse biasing. When the external d.c. voltage applied to the
junction is in such a direction that potential barrier is increased
• Breakdown voltage. It is the minimum reverse voltage at which
pn junction breaks down with sudden rise in reverse current
• Knee voltage. It is the forward voltage at which the current
through the junction starts to increase rapidly
• Maximum forward current. It is the highest instantaneous
forward current that a pn junction can conduct without damage
to the junction.
• Peak inverse voltage (PIV). It is the maximum reverse voltage that
can be applied to the pn junction without damage to the junction
• Maximum power rating. It is the maximum power that can be
dissipated at the junction without damaging it
• Forward resistance. Is the resistance offered by the diode to
forward bias
1. d.c. forward resistance. It is the opposition offered by
the diode to the direct current
2. a.c. forward resistance. It is the opposition offered by
the diode to the changing forward current.
• Reverse resistance. The resistance offered by the diode to the
reverse bias
• Forward current. It is the current flowing through a forward
biased diode.
• Peak inverse voltage. It is the maximum reverse voltage that a
diode can withstand without destroying the junction
• ripple factor Is the ratio of r.m.s. value of a.c. component to the
d.c. component in the rectifier output
• A filter circuit is a device which removes the a.c. component of
rectifier output but allows the d.c. component to reach the load
• A zener diode is a special type of diode that is designed to
operate in the reverse breakdown region
• A light-emitting diode (LED) is a diode that gives off visible light
when forward biased
• Advantages of LED
1. Low voltage
2. Longer life (more than 20 years)
3. Fast on-off switching
• multicolour LED emits one colour when forward biased and
another colour when reverse biased
• Applications of LEDs
1. As a power indicator
2. Seven-segment display
• photo-diode is a reverse-biased silicon or germanium pn junction
in which reverse current increases when the junction is exposed
to light
• Principle. When a rectifier diode is reverse biased, it has a very
small reverse leakage current. The same is true for a photo-diode
• dark current when no light is incident on the pn junction of
photo-diode, the reverse current Ir is extremely small.
• saturation current. Is the intensity of light increases, the reverse
current IR goes on increasing till it becomes maximum.
• Characteristics of Photo-diode
1. Reverse current-Illumination curve
2. Reverse voltage-Reverse current curve.
• Applications of Photo-diodes
1. Alarm circuit using photo-diode.
2. Counter circuit using photo-diode
• opt isolator (also called opt coupler) is a device that uses light to
couple a signal from its input (a photo emitter e.g., a LED) to its
output (a photodetector e.g., a photo-diode)
• A tunnel diode is a pn junction that exhibits negative resistance
between two values of forward voltage (i.e., between peak-point
voltage and valley-point voltage).
• Theory. The tunnel diode is basically a pn junction with heavy
doping of p-type and n-type semiconductor materials.
• Tunneling effect. The heavy doping provides a large number of
majority carriers
• tunneling the movement of valence electrons from the valence
energy band to the conduction band with little or no applied
forward voltage