Understanding Transistors: Types, Working and Applications, Assignments of Electronics

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TRANSISTORS AND

THEIR TYPES

PRESENTED BY

PRIYANKA

KUMAWAT

SONAM KULDEEP

BSC. 2

ND

YEAR

BASIC OF TRANSISTOR Earlier, the critical and important component of an electronic device was a vacuum tube; it is an electron tube used to control electric current. The vacuum tubes worked but they are bulky, require higher operating voltages, high power consumption, yield lower efficiency and cathode electron-emitting materials are used up in operation. So, that ended up as heat which shortened the life of the tube itself. To overcome these problems, John Bardeen, Walter Brattain and William Shockley were invented a transistor at Bell Labs in the year of 1947. This new device was a much more elegant solution to overcome many of the fundamental limitations of vacuum tubes. Transistor is a semiconductor device that can both conduct and insulate. A transistor can act as a switch and an amplifier. It converts audio waves into electronic waves and resistor, controlling electronic current. Transistors have very long life, smaller in size, can operate on lower voltage supplies for greater safety and required no filament current. The first transistor was fabricated with germanium. A transistor performs the same function as a vacuum tube triode, but using semiconductor junctions instead of heated electrodes in a vacuum chamber. It is the fundamental building block of modern electronic devices and found everywhere in modern electronic systems.

THREE TERMINALS OF TRANSISTORS

A transistor is a three terminal device. Namely,

Base: This is responsible for activating the transistor. This is the very

thin terminal of device. It has the very lowest doping concentration.

Collector: This is the positive lead. It is a very larger area of the

transistor. It is used to flow the current in transistor.

Emitter: This is the negative lead. It has very high doping

concentration. It passes the electron and holes in the base region.

BIPOLAR JUNCTION TRANSISTOR BJT is a current control device. It has current flow in both directions that’s why its called the bipolar junction transistor. A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. It comes with two types, P-N-P and N-P-N. P-N-P transistor, consisting of a layer of N-doped semiconductor between two layers of P-doped material. The base current entering in the collector is amplified at its output. That is when PNP transistor is ON when its base is pulled low relative to the emitter. The arrows of PNP transistor symbol the direction of current flow when the device is in forward active mode. N-P-N transistor consisting a layer of P-doped semiconductor between two layers of N- doped material. By amplifying current the base we get the high collector and emitter current.

That is when NPN transistor is ON when its base is pulled low relative to the emitter. When the transistor is in ON state, current flow is in between the collector and emitter of the transistor. Based on minority carriers in P-type region the electrons moving from emitter to collector. It allows the greater current and faster operation; because of this reason most bipolar transistors used today are NPN.

PNP TRANSISTOR In the PNP transistors, the emitter is more positive with base and also with respect to the collector. The PNP transistor is a three-terminal device that is made from the semiconductor material. The three terminals are collector, base, and emitter and the transistor is used for switching and amplifying applications. The operation of the PNP transistor is shown below. Generally, the collector terminal is connected to the positive terminal and the emitter to a negative supply with a resistor either the emitter or collector circuit. To the base terminal, the voltage is applied and it operates transistor as an ON/OFF state. The transistor is in the OFF state when the base voltage is the same as the emitter voltage. The transistor mode is in ON state when the base voltage decreases with respect to the

WORKING PRINCIPLE OF BJT The BE junction is forward bias and the CB is a reverse bias junction. The width of the depletion region of the CB junction is higher than the BE junction. The forward bias at the BE junction decreases the barrier potential and produces electrons to flow from the emitter to the base and the base is a thin and lightly doped it has very few holes and less amount of electrons from the emitter about 2% it recombine in the base region with holes and from the base terminal it will flow out. This initiates the base current flow due to the combination of electrons and holes. The leftover large number of electrons will pass the reverse bias collector junction to initiate the collector current. By using KCL we can observe the mathematical equation IE = IB + IC The base current is very less as compared to emitter and collector current IE ~ IC

CIRCUT DIAGRAM OF BJT

COMMON BASE CONFIGURATION The common base transistor configuration gives a low i/p while giving a high o/p impedance. When the voltage of the CB transistor is high, the gain of the current and overall gain of the power is also low compared to the other transistor configurations.

CIRCUIT DIAGRAM OF CB CONFIGURATION α = Output current/Input current α = IC/IE ( CURRENT GAIN)

COMMON COLLECTOR CONFIGURATION Offering a high input impedance and a low output impedance it is widely used as a buffer. The voltage gain is unity, although current gain is high. The input and output signals are in phase. In view of these characteristics, the emitter follower configuration is used as a buffer circuit providing a high input impedance to prevent loading of the previous stage, and a low output impedance to drive following stages.

CIRCUIT DIAGRAM OF CC CONFIGURATION Ai = output current/Input current A i

= I

E

/I

B

COMMON EMMITER CONFIGURATION In this configuration we use emitter as common terminal for both input and output. This common emitter configuration is an inverting amplifier circuit. Here the input is applied between base- emitter region and the output is taken between collector and emitter terminals. In this configuration the input parameters are VBE and IB and the output parameters are VCE and IC.

CIRCUIT DIAGRAM OF CE CONFIGURATION

Current gain (α) =

I

C

/I

E

Current gain (β) = ) =

I

C

/I

B

Collector current

I

C

I

E

= β) = I

B