1. SEMICONDUCTOR
MATERIALS
BY
GROUP 8
16271-16278

2. Classification of semiconductors :-
Semiconductors are classified in to two types
(i) Intrinsic semiconductors (ii) Exterinsic semiconductors
n – type p – type
semiconductor semiconductor

4. INTRINSIC SEMICONDUCTOR
o A semiconductor in an extremely pure form is known as an
intrinic semiconductor.
o An Intrinfic semiconductor, even at room temperature, hole-
electron pairs all created
o When electric field is applied across an semiconductor
intrinisic semiconductor, the current conduction takes place by
two process, namely by free electrons and holes.

6. EXTRINSIC SEMICONDUCTOR
 The current conduction capability of intrinisic semiconductor is very low
at rom temperature. So we can not use it in electric devices.
 Hence the current conduction capability must be increased. This can be
achieved by adding impurities to the intrinisic semiconductor. So that it
become impurity semiconductor (or) Extrinisic semiconductor.
 The process of adding impurity is known as doping.

7.  If the pentavalent impurity is adding to the semiconductor, a large
number of free electrons are produced in the semiconductor.
 On the other hand if the trivalent impurtiy is added it introdued
large number of holes.
 Depending upon the type of impurity added, extrisic semiconductors
are classified in to
a) n – type Semiconductor
b) p – type Semiconductor

9. NPN JUNCTION TRANSISTOR
A Bipolar Junction Transistor (BJT) has three terminals connected to three doped
semiconductor regions. In an NPN transistor, a thin and lightly doped P-type base is
sandwiched between a heavily doped N-type emitter and another N-type collector
The flowing of these electrons from emitter to collector forms the current flow in the
transistor. Generally the NPN transistor is the most used type of bipolar transistors
because the mobility of electrons is higher than the mobility of holes. The NPN
transistor is mostly used for amplifying and switching the signals.

10. The above figure shows the NPN transistor circuit with supply voltages and resistive loads.
Here the collector terminal always connected to the positive voltage, the emitter terminal
connected to the negative supply and the base terminal controls the ON/OFF states of
transistor depending on the voltage applied to it.
Working;
The working of NPN transistor is quite complex. In the above circuit connections we
observed that the supply voltage VB is applied to the base terminal through the load RB.
The collector terminal connected to the voltage VCC through the load RL. Here both the
loads RB and RL can limit the current flow through the corresponding terminals. Here the
base terminal and collector terminals always contain positive voltages with respect to
emitter terminal

11. If the base voltage is equal to the emitter voltage then the transistor is in OFF state. If the base
voltage increases over emitter voltage then the transistor becomes more switched until it is in
fully ON state. If the sufficient positive voltage is applied to the base terminal i.e. fully-ON state,
then electrons flow generated and the current (IC) flows from emitter to the collector. Here the
base terminal acts as input and the collector-emitter region acts as output.
To allow current flow between emitter and collector properly, it is necessary that the collector
voltage must be positive and also greater than the emitter voltage of transistor. Some amount of
voltage drop presented between base and emitter, such as 0.7V. So the base voltage must be
greater than the voltage drop 0.7V otherwise the transistor will not operate. The equation for
base current of a bipolar NPN transistor is given by,
IB = (VB-VBE)/RB
Where,
IB = Base current
VB = Base bias voltage
VBE = Input Base-emitter voltage = 0.7V
RB = Base resistance

12. The equation for collector supply voltage is given as
VCC = ICRL + VCE
From the above equation the collector current for common emitter NPN transistor is given as
IC = (VCC-VCE)/RL
In a common emitter NPN transistor the relation between collector current and emitter
current is given as
IC = β IB
In active region the NPN transistor acts as a good amplifier.

15. CHEMICAL BONDING IN SILICON AND
GERMANIUM
Covalent bonding in silicon
The outermost shell of atom is capable to hold up to eight electrons. The atom which
has eight electrons in the outermost orbit is said to be completely filled and most
stable. But the outermost orbit of silicon has only four electrons. Silicon atom needs
four more electrons to become most stable. Silicon atom forms four covalent bonds
with the four neighboring atoms. In covalent bonding each valence electron is shared
by two atoms.
When silicon atoms comes close to each other, each valence electron of atom is
shared with the neighboring atom and each valence electron of neighboring atom is
shared with this atom. Likewise each atom will share four valence electrons with the
four neighboring atoms and four neighboring atoms will share each valence electron
with this atom. Therefore, total eight electrons are shared.

16. Same bonding is experienced in Germanium atom also
The outermost shell of silicon and germanium is completely filled and valence electrons
are tightly bound to the nucleus of atom because of sharing electrons with neighboring
atoms. In intrinsic semiconductors free electrons are not present at absolute zero
temperature. Therefore intrinsic semiconductor behaves as perfect insulator.

18. NP JUNCTION RECTIFIERS
 The electricity supply in homes provides ac voltage. It is a sinusoidal signal of
frequency 50 Hz . It means that voltage (or current) becomes zero twice in one
cycle, i.e., the waveform has one positive and other negative half cycle varying
symmetrically around zero voltage level. The average voltage of such a wave is
zero.
 A rectifier is an electrical device that converts alternating current (AC), which
periodically reverses direction, to direct current (DC), which flows in only one
direction. The process is known as rectification.
 There are two rectifiers using of diodes
(a) half wave rectifier
(b) full wave rectifier

19. HALF WAVE RECTIFIER
The signal from ac mains is fed into a step down transformer which makes it available
at the terminals. The load resistance RL is connected to these terminals through a p-n
junction diode D.
In the positive half cycle, during the time interval 0 to T/2, diode D will be forward
biased and conduct, i.e., current flows through RL
However, during the negative half cycle, i.e., in the interval T/2 to T, D is reverse
biased and the junction will not conduct, i.e. no current flows through RL.
Since the p-n junction conducts only in one-half cycle of the sine wave, it acts as a half-wave
rectifier

20. The dc voltage, Vdc across RL, as measured by voltmeter in case of half-wave rectifier, is given
by
Vdc = Vm/π
The dc current Idc through the load resistance RL is given by
Idc = Vdc/RL
During the non-conducting half cycle, the maximum reverse voltage appearing across
the diode is equal to the peak ac voltage Vm. The maximum reverse voltage that a diode
can oppose without breakdown is called its Peak Inverse Voltage (PIV)

21. FULL WAVE RECTIFIER
This type of single phase rectifier uses four individual rectifying diodes connected in a
closed loop “bridge” configuration to produce the desired output. The main advantage
of this bridge circuit is that it does not require a special centre tapped transformer,
thereby reducing its size and cost. The single secondary winding is connected to one
side of the diode bridge network and the load to the other side as shown below.
The four diodes labelled D1 to D4 are arranged in “series pairs” with only two diodes
conducting current during each half cycle.
During the positive half cycle of the supply, diodes D1 and D2 conduct in series while
diodes D3 and D4 are reverse biased and the current flows through the load as shown
below.During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but
diodes D1 and D2 switch “OFF” as they are now reverse biased. The current flowing
through the load is the same direction as before.

22. The maximum ripple voltage present for a Full Wave Rectifier circuit is
not only determined by the value of the smoothing capacitor but by the
frequency and load current, and is calculated as:
Bridge Rectifier Ripple Voltage
Where: I is the DC load current in amps, ƒ is the frequency of the ripple or
twice the input frequency in Hertz, and C is the capacitance in Farads