Electronic instruments are being utilized in various fields like telecommunication, entertainment,
computers, nuclear physics and many more. Although the history started with the advent of vacuum tubes,
however the rapid advancement in electronics which we see today is due to the valuable contributions of
semiconductor devices
The electrons of an isolated atom are restricted to well defined energy levels. The maximum number of electrons
which can be accommodated in any level is determined by the Pauli exclusion principle. The electrons belonging
to the outermost energy level are called valence electrons. For example, the electronic configuration of sodium
This theory is based on the Pauli exclusion principle.
In isolated atom the valence electrons can exist only in one of the allowed orbitals each of a sharply defined
energy called energy levels. But when two atoms are brought nearer to each other, there are alterations in energy
levels and they spread in the form of bands
ENERGY BAND DESCRIPTION OF CONDUCTOR,
INSULATOR AND SEMICONDUCTOR
These are solids in which either the energy band containing valence band is partially filled or the energy band
containing valence electrons overlaps with next higher band to give a new band which is partially filled too. For
both these situations there are enough free levels available for electrons to which they can be excited by receiving
energy from an applied electric field.
It is a solid in which the energy band formation takes place in such a manner, that the valence band is completely
filled while the conduction band is completely empty. In addition to this, these two bands are separated by a
large energy gap called forbidden energy gap or band gap.
In case of semiconductors, the band structure is essentially of the same type as that for insulators with the only
difference that of a relatively smaller forbidden gap. In case of a semiconductor this is typically of the order of
1eV. At absolute zero temperature, the valence band is completely filled and the conduction band is completely
empty and consequently no electrical conduction can result. This is the same behaviour as observed in insulators.
i.e at absolute zero a semiconductor behaves like an insulator
A semiconductor free from impurities is called an intrinsic semiconductor. Ideally an intrinsic
semiconductor crystal should contain atoms of this semiconductor only but it is not possible in practice to
obtain crystals with such purities. However if the impurity is less than 1 in 108 part of semiconductor it can be
treated as intrinsic. For describing the properties of intrinsic semiconductor we are taking
examples of silicon and germanium. Both silicon and germanium are members of the group IV of periodic table
of elements and are tetravalent. Their electronic configuration is as follows:
A semiconductor, at room temperature, contains electrons in the conduction band and holes in the valence band.
When an external electric field is applied, the electrons move opposite to the field and the holes move in the
direction of the field, thus constituting current in the same direction. The total current is the sum of the electron
and hole currents
A semiconductor, at room temperature, contains electrons in the conduction band and holes in the valence band.
When an external electric field is applied, the electrons move opposite to the field and the holes move in the
direction of the field, thus constituting current in the same direction. The total current is the sum of the electron
and hole currents.
When a pentavalent impurity atom (antimony, phosphorus
or arsenic) is added to a Ge(or Si) crystal, it replaces a
Ge (or Si) atom in the crystal lattice. Four of the five valence
When a trivalent impurity atom (boron, aluminium,
gallium or indium) is added to a Ge (or Si) crystal, it
also replaces one of the Ge (or Si) atoms in the crystal
lattice . Its three valence electrons form covalent bonds
with one each valence electron of these Ge (or Si) atoms
A semiconductor, at room temperature, contains electrons
in the conduction band and holes in the valence band.
When an external electric field is applied, the electrons
move opposite to the field and the hole move in the direction
of the field, thus constituting current in the same direction.
The total current is the sum of the electron and hole
currents.
A junction diode is a basic semiconductor device. It is a semiconductor crystal having acceptor impurities in one
region (P – type crystal) and donor impurities in the other region (n–type crystal). The boundary between the two
regions is called ‘p–n junction
Due to concontration difference hole try to diffuse from p side to n side but due to depletion
layer only those hole are able to diffuse from p to n side which have high kinetic energy.
Similarly electron of high kinetic energy also diffuse from n to p so diffusion current flow from p
to n side
The junction diode can be connected to an external battery in two ways, called 'forward biasing’ and 'reverse
biasing' of the diode.It means the way of connecting emf source to P-N junction diode. It is of following two types
A junction diode is said to be reverse-biased when the positive terminal of the external battery is connected to the
n -region and the negative terminal to the p -region of the diode
The avalanche breakdown occurs in lightly doped junction. If the reverse bias is made very
high, the minority-carriers acquire kinetic energy enough to break the covalent bonds near the
junction, thus liberating electron-hole pairs. These charge-carriers are accelerated and produce,
in the same way, other electron-hole pairs
The avalanche breakdown occurs in lightly doped junction. If the reverse bias is made very
high, the minority-carriers acquire kinetic energy enough to break the covalent bonds near the
junction, thus liberating electron-hole pairs. These charge-carriers are accelerated and produce,
in the same way, other electron-hole pairs. The process is cumulative and an avalanche of
electron-hole pairs is produced. The reverse current then increases abruptly to a relatively
large value (part CD of the characteristic).
The current-voltage curve of junction diode shows that the current does not vary linearly with the
voltage, that is, Ohm's law is not obeyed. In such situation, a quantity known as 'dynamic resistance'
(or a.c. resistance) is defined
An electronic device which converts alternating current / voltage into direct current
The half-wave rectifier circuit is shown in Fig. (a) and the input and output wave forms in Fig. (b). The alternating
input voltage is applied across the primary P1P2 of a transformer. S1S2 is the secondary coil of the same transformer.
S1 is connected to the p -type crystal of the junction diode and S2 is connected to the n -type crystal through a
load resistance RL.
In a full-wave rectifier, a unidirectional, pulsating output current is obtained for both halves of the a.c. input voltage.
Essentially, it requires two junction diodes so connected that one diode rectifies one half and the second diode
rectifies the second half of the input
In a full-wave rectifier, a unidirectional, pulsating output current is obtained for both halves of the alternating input
voltage. Essentially, it requires two junction diodes so connected that one diode rectifies one half and the second
diode rectifies the second half of the input
The rectified voltage is in the form of pulses of the shape of half sinusoids. Though it is unidirectional it does not
have a steady value. To get steady dc output from the pulsating voltage normally a capacitor is connected across
the output terminals (parallel to the load RL). One can also use an inductor in series with RL for the same
purpose. Since these additional circuits appear to filter out the ac ripple and give a pure dc voltage, so they are
called filters
Full wave bridge rectifier
The junction diodes are of many types. The important types are Zener diode, photodiode, light-emitting diode
(LED) and solar cell
Transistor structure and action :
A transistor has three doped regions forming two p–n junctions between them. There are two types of transistors,
as shown in figure
JUNCTION TRANSISTOR :
Transistor structure and action :
A transistor has three doped regions forming two p–n junctions between them. There are two types of transistors,
as shown in figure.
There are four possible ways of biasing the two P-N junctions (emitter junction and collector junction) of
transistor.
A transistor can be connected in a circuit in the following three different configurations.
Common base (CB), Common emitter (CE) and Common collector (CC) configuration.
(1) CB configurations : Base is common to both emitter and collector
The transistor is most widely used in the CE configuration.
When a transistor is used in CE configuration, the input is between the base and the emitter and the output is
between the collector and the emitter. The variation of the base current ?B with the base–emitter voltage VBE is
called the input characteristic. The output characteristics are controlled by the input characteristics. This implies
that the collector current changes with the base current
The transistor can be used as a device application depending on the configuration used (namely CB, CC and
CE), the biasing of the E-B and B-C junction and the operation region namely cutoff, active region and saturation.
When the transistor is used in the cutoff or saturation state it acts as a switch. On the other hand for using the
transistor as an amplifier, it has to operate in the active region.
To operate the transistor as an amplifier it is necessary to fix its operating point somewhere in the middle of its
active region. If we fix the value of VBB corresponding to a point in the middle of the linear part of the transfer curve
then the dc base current IB would be constant and corresponding collector current IC will be constant The dc
In an oscillator, we get ac output without any external input singnal. A portion of the output power is returned back
(feedback) to the input in phase with the starting power (this process is termed positive feedback) as shown in
figure(a). The feedback can be achieved by inductive coupling (through mutual inductance) or LC or RC networks.
There are two types of electronic circuits : analogue circuits and digital circuits :
In analogue circuits, the voltage (or current) varies continuously with time (figure a). Such a voltage
(or current) signal is called an ‘analogue signal’.Figure shows a typical voltage analogue signal varying
sinusoidally between 0 and 5V.
A logic gate is a digital circuit which works according to some logical relationship between input and output
voltages. It either allows a signal to pass through or stops it.
Various combinations of the three basic gates, namely, OR, AND and NOT, produce complicated digital circuits,
which are also called ‘gates’. The commonly used combinations of basic gates are NAND gate, NOR, gate.
These are also called universal gates.
