This document discusses analog electronics and semiconductor devices. It describes the properties of conductors, insulators, and semiconductors. It also explains the different types of semiconductors such as N-type and P-type, and how a PN junction diode is formed between a P-type and N-type semiconductor. Additionally, it covers the operation and characteristics of a PN junction diode under forward and reverse bias conditions. The document concludes with discussions on Zener diodes, bipolar junction transistors (BJT), transistor biasing methods, and applications of semiconductor devices.
2. Conductors: Conducts
currents, Valence and
conduction band overlap
Insulators: Does not
Conducts currents, Valence
and conduction band
separated by forbidden
energy gap
Semiconductor: Conductivity
is less than conductor and
greater than insulators
3. Types of Semiconductors
N-type Semiconductor P-type Semiconductor
Intrinsic Extrinsic
Pure form of semiconductor
Conductivity is poor
Impure form of semiconductor
Conductivity is improved by
adding impurity
Created by adding
pentavalent impurity
Eg. Arsenic or antimony
Created by adding trivalent
impurity
Eg. Boron or Gallium
4. PN junction diode
• Junction is formed between p type and n type is called pn
junction.
• It is called diode because it has two electrodes one for P
region called anode and other is N type called cathode
• P type- majority carriers - holes
• N type- majority carriers -free electrons
5. Diffusion: Charge carrier is moving from high concentration
area to low concentration area.
Junction voltage or barrier voltage: Existence of immobile
ions develops the potential difference across the junction
under equilibrium condition.
For germanium = 0.3 V & For silicon = 0.7 V
6. Operation of PN junction
Forward bias:
+ to p and – to N
External voltage applied is > than barrier potential
and cancels the potential and permits current to flow.
7. Reverse bias:
- to p and + to N
External voltage is applied such that barrier
potential increases and does not permits current to flow.
Reverse Saturation Current: small amount of
current flows through it due to minority carriers.
8. V-I Characteristics of PN junction
Forward bias:
• On varying voltage slowly, the barrier potential is
eliminated and current starts flowing called threshold
voltage or cut in voltage or knee voltage.
• As forward voltage applied increases beyond threshold
voltage, forward current increases exponentially.
9. Reverse bias:
• Breakdown of junction occurs results sudden rise in
reverse current. This current is called reverse
saturation current.
Applications of PN junction diode
• Switches
• Rectifiers
• Power Supplies
• Digital systems
10. Zener Diode
• If p and n region is heavily doped, the breakdown
voltage is reduced.
• If doping is heavy, reverse voltage is low then
electron can break from its bond. This effect is
called zener effect
• Diode exhibits zener effect is called zener
diode(heavily doped PN junction operates in
breakdown region)
14. BJT configuration
• Signal transferred from low resistance circuit to high
resistance circuit
• Transfer + Resistor = Transistor
• It consists of two back to back pn junction joined
together to form single piece of semiconductor
25. 25
Transistor Biasing
The basic function of transistor is amplification. The process of raising the
strength of weak signal without any change in its general shape is referred as
faithful amplification. For faithful amplification it is essential that:-
1. Emitter-Base junction is forward biased
2. Collector- Base junction is reversed biased
3. Proper zero signal collector current
The proper flow of zero signal collector current and the maintenance of
proper collector emitter voltage during the passage of signal is called
transistor biasing.
26. 26
BIAS STABILITY
Through proper biasing, a desired quiescent operating point of the transistor
amplifier in the active region (linear region) of the characteristics is obtained. It is
desired that once selected the operating point should remain stable. The maintenance
of operating point stable is called Stabilisation.
The selection of a proper quiescent point generally depends on the following factors:
(a) The amplitude of the signal to be handled by the amplifier and distortion level
in signal
(b) The load to which the amplifier is to work for a corresponding supply voltage
The operating point of a transistor amplifier shifts mainly with changes in
temperature, since the transistor parameters — β, ICO and VBE (where the symbols carry
their usual meaning)—are functions of temperature.
27. 27
The DC Operating Point
For a transistor circuit to amplify it must be properly biased with dc
voltages. The dc operating point between saturation and cutoff is
called the Q-point. The goal is to set the Q-point such that that it
does not go into saturation or cutoff when an a ac signal is applied.
28. 28
Various Biasing Circuits
• Fixed Bias Circuit or Base Resistor method
• Fixed Bias with Emitter Resistor
• Collector to Base Bias or Collector Feedback
bias
• Potential Divider or Voltage Divider Bias
29. 29
VCC
RC
C
E
B
VCC
R1
RE
R2
IE
IC
Ib
Potential Divider Bias Circuit
Rth = R1*R2 & Vth = Vcc R2
R1+R2 R1+R2
VCC
RC
C
E
B
VCC
R1
RE
R2
Rth = R1*R2 & Vth = Vcc R2
R1+R2 R1+R2
Rth = R1*R2 & Vth = Vcc R2
R1+R2 R1+R2
Rth = R1*R2 & Vth = Vcc R2
R1+R2 R1+R2
30. 30
Summary
• The Q-point is the best point for operation of a
transistor for a given collector current.
• The purpose of biasing is to establish a stable
operating point (Q-point).
• The linear region of a transistor is the region of
operation within saturation and cutoff.
• Out of all the biasing circuits, potential divider bias
circuit provides highest stability to operating point.