INTRODUCTION TO BIPOLAR JUNCTION TRANSISTOR:
Ø A Bipolar
junction transistor (BJT) is a three terminal semi conductor device in which the operation depends on the interaction of both
majority and minority carriers.
it is called as Bi polar junction transistor.
has wide applications in computers, satellites and other modern communication systems.
CONSTRUCTION OF BJT:
BJT consists of a silicon or (germanium) crystal in which a thin layer of N-type silicon is sandwiched between two layers
of P-type silicon.
transistor is referred to as P N P.
three portions of BJT are Emitter, base, and collector represented as E, B, and C respectively.
TYPES OF CONFIGURATION:
a transistor is to be connected in a circuit, one terminal is used as an input terminal, the other terminal is used as an
output terminal and the third terminal is common to both input and output terminal.
on the input, output and common terminal, a transistor can be connected in three configurations.
base configuration (CB)
emitter configuration (CE)
collector configuration (CC)
is also called grounded base configuration.
this configuration, emitter is the input terminal, collector is the output terminal and
base is the common terminal.
is called as grounded emitter configuration.
this configuration, base is the input terminal, collector is the output terminal, and emitter is the common terminal.
is called as grounded collector configuration.
this configuration base is the input terminal, emitter is the output terminal and collector is the common terminal.
Current amplification factor:
a transistor amplifier with a. c. input signal, the ratio of change in output current to the change in input current is known
as the current amplification factor.
BREAKDOWN IN TRANSISTORS:
down is defined as the sudden change in reverse current.
are two types of breakdowns.
multiplication or Avalanche Break down.
- through or punch - through
is possible to raise the punch-through voltage by increasing the doping concentration in the base, but this automatically
reduces the emitter efficiency.
takes place at a fixed voltage between collector and base and is not dependent on circuit configuration, where as Avalanche
multiplication takes place at different voltages depending upon the circuit configuration.
the voltage limit of a particular transistor is determined by either of the two types of Break down, which ever occurs at
INTRODUCTION TO FIELD EFFECT TRANSISTOR:
FET is a device in which the flow of current through the conducting region is controlled by an electric field.
current conduction is carried only by majority carriers, the FET is said to be an uni polar device.
on the construction, the FET can be classified into two types as junction
(JFET) and metal oxide semiconductor FET (MOSFET) or insulated gate
(IGFET) or metal oxide silicon transistor (MOST).
upon the majority carriers, JFET has been classified into two types, namely,
1. N-channel JFET
with electrons as the majority carriers
2. P-channel JFET
with holes as the majority carriers.
CONSTRUCTION OF N-CHANNEL JFET:
consists of a N-type silicon bar which is made of silicon.
contacts (terminals), made at the two ends of the bar, are called source and drain.
terminal is connected to the negative pole of the Battery.
which are the majority charge carriers in the N-type bar enter the bar through this terminal.
terminal is connected to the positive pole of the battery.
majority carriers leave the bar through this terminal.
doped P-type silicon is diffused on both sides of the N-type silicon bar by which PN junctions are formed.
layers are joined together and called Gate (G).
APPLICATIONS OF JFET:
is used as a buffer in measuring instruments, receivers since it has high input impedance and low output impedance.
are used in RF amplifiers in FM tuners and communication equipment for the low noise level.
the device is voltage controlled, it is used as a voltage variable resistor in operational amplifiers and tone controls.
the input capacitance is low, FET s are used in cascade amplifiers in measuring and testing equipments.
s are used in mixer circuits in FM and TV receivers, and communication equipment because inter modulation distortion is low.
is used in oscillator circuits because frequency drift is low.
the coupling capacitor is small, FET s are used in low frequency amplifiers in hearing aids and inductive transducers.
s is used in digital circuits in computers, LSD and memory circuits because of its small size.
METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR
MOSFET is the common term for the Insulated Gate Field Effect Transistor (IGFET).
There are two basic forms of MOSFET:
By applying a transverse electric field across an insulator, deposited on the semi conducting material, the thickness and
hence the resistance of a conducting channel of a semi conducting material can be controlled.
a depletion MOSFET, the controlling electric field reduces the number of majority carriers available for conduction, where
as in the enhancement MOSFET, application of electric field causes an increase in the majority carrier density in the conducting
region of the transistor.
MOSFET s are widely used in digital VLSI circuits than JFET s because of the following advantages:
input resistance of MOSFET is very high.
drain resistance of MOSFET is low.
transverse electric field induced across an insulating layer deposited on the semiconductor material controls the conductivity
of the channel.
is very susceptible to overload voltage and needs special handling during
installation. It gets damaged easily if it is properly handled.
has zero offset voltage. As it is a symmetrical device, the source and drain can be interchanged. This is very useful in analog
UJT (UNI JUNCTION TRANSISTOR):
is a three terminal semi conductor Switching device. As it has only one PN junction and three leads, it is called as uni junction
basic structure of UJT is it consists of a lightly doped N-type silicon bar with a heavily doped P-type material alloyed to
its one side closer to base (B2) for producing single PN junction.
Ø A unique
characteristic of UJT is, when it is triggered, the emitter current increases regeneratively until it is limited by emitter
to this negative resistance property, a UJT can be employed in a variety of applications, viz. saw tooth wave generator, pulse
generator, switching, timing, and phase control circuits.
PNPN DIODE (SHOCKELY DIODE):
is a semiconductor device having three or more junctions.
family of thyristors consists of PNPN diode (Shockely diode), SCR, LASCR, TRIAC,DIAC, and UJT etc.
diode is a Four Layer PNPN silicon device with two terminals.
diode acts as a switch during forward bias condition.
SILICON CONTROLLED RECTIFIER (SCR):
is a four layer three terminal device in which the end P-layer acts as anode, the end
N-layer acts as cathode and P-layer nearer to the cathode acts as gate.
leakage current in silicon is very small compared to germanium, SCR s are made of silicon
and not germanium.
acts as a switch when it is forward biased.
is used in relay control, motor control, phase control, heater control, battery charges, inverters, regulated power supplies
and as static switches.
is basically a switch; it can be used in linear applications like rectification.
s are much superior in performance than ordinary diode rectifiers.
main advantage of SCR full wave rectifier over ordinary full wave rectifier is that
any voltage can be made available at the output by simply changing the firing angle of the SCR s.
current is the minimum value of current to hold the device in ON-state.
Ø For turning the device OFF, the anode current should be lowered below holding current
by increasing the external circuit resistance.
current is the current applied to the gate of the device for control purposes.
minimum gate current is the minimum value of current required at the gate for
triggering the device.
the gate current, easier is the triggering of the device and device and vice-versa.
LASCR (LIGHT ACTIVATED SCR):
LASCR is triggered by irradiating with light.
incident light generates electron- hole pairs in the device thus increasing the number of charge carriers.
leads to the instantaneous flow of current within the device and the device turns ON.
TRIAC (TRIODE A C SWITCH):
is a three terminal semi conductor Switching device which can control alternating current in a load.
is nothing but two SCRs connected in parallel but in the reverse direction.
also acts as a switch for both directions.
is used for illumination control, temperature control, liquid level control, motor speed
control and as static switch to turn a. c. power ON and OFF.
main limitation in comparison to SCR is its low power handling capacity.
DIAC (DIODE A. C. SWITCH):
is three layers, two terminal semi conductor devices.
acts as a Bi -directional avalanche diode. It has two junctions J1 and J2.
exhibits negative resistance characteristics, i.e. the current flowing in the device starts increasing and the voltage across
it starts decreasing.
DIAC is not a control device.
is used as triggering device in TRIAC phase control circuits used for light dimming,
motor speed control and heater control.