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Learner’s Guide, Mathematics (311)
ENERGY BANDS IN SOLIDS
When two atoms come closer to form a stable
structure, such that the separation between them
tends to be lesser than their diameter (d), the
energy states tend to overlap, which is forbidden
by Pauli's exclusion principle.
Hence, they get modified and corresponding to
each of the interaction energy states, two energy
states are created: one slightly lower than the
normal state which is called the bonding state and
the other slightly higher than the normal state
called the antibonding state
Quasi continuous distribution of energy states,
which are though separate but practically in
discriminable, is called energy band.
The process of interaction of energy states (and
thereby energy band formation) starts from outer
unfilled energy states and then proceeds to valence
level. The band formed of unfilled energy levels is
called conduction band and the one formed of
filled valence levels is called valence band.
Classification of Solids as Conductors,
Semiconductors and Insulators on the
basis of Energy Bands
Conductor Semiconductor Insulator
INTRINSIC AND EXTRINSIC
SEMICONDUCTORS
An Intrinsic Semiconductor
Pure silicon and germanium are intrinsic
semiconductors as the electrons in these
elements are all tightly held in their
crystalline structure, i.e., they do not have
free electrons.
When energy is added to pure silicon in the form of heat, say, it can cause a few electrons to break free of their bonds, leaving behind a hole in each case. These electrons move randomly in the crystal. These electrons and holes are called free carriers, and move to create electrical current
when a free electron moves in a crystal because
of thermal energy; its path deviates whenever it collides with a nucleus or other free electrons. This gives rise to a zig-zag or random motion, which is similar to that of a molecule in a gas
SEMICONDUCTORS AND SEMICONDUCTING DEVICES
Learner’s Guide, Mathematics (311)
An Extrinsic Semiconductor
The process in which some atoms of a pure or intrinsic semiconductor are replaced by impurity atoms from their lattice-sites is called doping and the impurity so added is called dopant.
Such doped semiconductors are called extrinsic semiconductors.
n-and p-type Semiconductors
n-type Semiconductor
When silicon (or germanium) is doped with a pentavalent (five electrons in the outermost orbit) atom like phosphorus, arsenic or antiomony, four electrons form covalent bonds with the four neighbouring silicon atoms, but the fifth (valence) electron remains unbound and is available for conduction
when a silicon (or germanium) crystal is doped with a pentavalent element, it develops excess free electrons and is said to be an n-type semiconductor. Such impurities are known as donor impurities.
p-type Semiconductor
If silicon (or germanium) is doped with a trivalent (three electrons in the outermost shell) atom like boron, aluminium, gallium or indium, three valence electrons form covalent bonds with three silicon atoms and deficiency of one electron is created. This deficienty of electron is referred to as hole.
Such a semiconductor is said to be a p-type semiconductor and the impurities are known as acceptor impurities.
A p-n JUNCTION
When a n-type material is placed in contact with a p- type material. The formation is known as p-n junction.
Formation of a p-n Junction
Learner’s Guide, Mathematics (311)
As the applied voltage is increased beyond knee voltage, the current through the diode increases linearly.
Reverse Bias Characteristics
To draw reverse bias characteristics of a p-n junction
(i) The terminals of the junction are reversed.
(ii) (ii) Instead of milliammeter, microammeter
(μ A) is used
(iii) The unidirectional conducting property of
a diode is used to convert ac voltage into dc voltage as a rectifier.
(iv) Diodes are also used in adaptors to
recharge batteries of cell phones, CDplayers, laptops, etc.
(v) A device that uses batteries often contains a
diode as it simply blocks any current from leaving the battery, if it is reverse biased. This protects the sensitive electronics in the device.
TYPES OF DIODES
I–V Characteristics of Zener diode
Zener diode is fabricated by heavily doping both the p- and n-sides of the junction. Hence depletion layer formed is very thin 6 (< 10 m) −. And the electric field across the depletion layer is extremely high 6 – 1 ( 5× 10 N C ) ∼ even for a small reverse bias voltage of 5 V.
Characteristics of light-emitting diode
In light-emitting diode (LED) when the forward
current of diode is small the intensity of light emitted is small.
As the forward current increases, the intensity
of the emitted light increases and reaches a maximum value. Further increase in the forward current results in a decrease of light intensity.
LEDs are biased such that the light emitting
efficiency is maximum.
Learner’s Guide, Mathematics (311)
I-V Characteristics of Photo diode
The photo diode is fabricated such that the generation of electron – hole pairs takes place in or near the depletion region in the diode. Due to the electric field of the junction, the electrons and holes are separated before they recombine. The direction of the electric field is such that the electrons reach the n-side and the holes reach the p-side. The electrons are collected on the n-side and the holes are collected on the p- side giving rise to an emf. When an external load is connected, the current flows. The magnitude of the photocurrent depends on the intensity of the incident light.
I-V Characteristics of Solar Cell
The generation of emf, when the light falls on a solar cell is due to the following three basic processes: generation, separation and collection. Generation of electron – hole pairs is due to the light (with hν> Eg)
close to the junction. Separation of electrons and holes is due to the electric field of the depletion layer. Electrons are swept to the n-side and the holes to the p- side.
TRANSISTORS – pnp and npn
A transistor is basically a silicon or germanium crystal containing three alternate regions of p and n-type semiconductors. These three regions are called emitter(E), base(B) and collector(C). The middle region is the base and the outer two regions are emitter and collector. the emitter and collector are of the same type (p or n) and collector is the largest of the three regions.
Learner’s Guide, Mathematics (311)
Common Emitter (CE) Configuration of a pnp Transistor
Input Characteristics
Output Characteristics
1. For rectifying action we use
A. Choke
B. Transformer
C. Condenser
D. Diode
2. The saturation current in a diode can be
increased by
A. Lowering plate potential
B. Raising plate potential
C. Increasing cathode temperature
D. Decreasing cathode temperature
3. Can a diode valve be used as an oscillator
A. Yes
B. No
C. Some times with another diode
D. None of the above
4. A semiconductor is the substance which
contains
A. A large number of free electron
B. Only one electron in the outermost
electron
C. Only few electronat room
temperature
Check Yourself
Learner’s Guide, Mathematics (311)
D. No free electon at o^0 K and
atomforms covalent bond with
neighbouring bond
5. Which one of the followingis
semiconductor
A. Plastic
B. Aluminium
C. Wood
D. Cesium
1. Describe the most important characteristic
of a p-n junction diodes.
2. Explain the formation of depletion region
in a p-n junction diode.
3. Which charge carriers conduct forward
current in a p-n junction diode?
4. Differentiate between (i) Forward bias and
reverse bias (ii) Avalancehe and zener
breakdown
5. Explain the working of p-n-p and n-p-n
transistors
1C 2C 3B 4D 5D
Stretch Yourself
Hint to Check Yourself
