Transistors are also comes under the category of semiconductors. These are responsible for the revolutionary changes in the field of electronics. The first practical transistor was introduced in the year 1927 is known as point contact transistor by John Bardeen, Walter Brattain and William Shockley.
The compression of the sizes of the electronic devices is only because of the invention of transistors.The word transistor can be split into two basic words. The very first one ‘trans’ is referred to as transferring of the signals. The second part of the word is ‘istor’ this referred to as the property of the resistance that is offered at the respective junctions.
It possesses the characteristics of a switch. It has the capability of participating in the process of amplification as well as rectification of the signals either it may be voltage signals or current signals.
What is a Transistor?
A circuit that has low resistance participates in the transfer of the signals that are weak to the circuit with the high resistance. This type of circuit is defined as a transistor.
Construction of Transistor
A transistor is formed while two diodes of p-n junction can be connected in such a way that both back end connected together. In middle the connected area is very thin that is referred to as base.
One side is referred to as emitter that other is referred to as collector. In this way the transistors are constructed. Emitter is present on the right to the transistor whereas the collector presence can be observed on the left.
Types of Transistors
A basic transistor can be classified into two types based on the type of its construction. One is referred to as p-n-p and the other is referred to as n-p-n. The construction of this p-n-p and n-p-n is very simple.
The transistor with center as n-type and both the p-types results in the formation of p-n-p. The transistor formed with center as p-type and both the n-types on the either sides results in the formation of n-p-n.
There are the indications that are represented by using arrows which shows the conventional current flows in that particular direction. This can be the referred as the only difference present between n-p-n and p-n-p transistors. Each transistor has three basic terminals.
These three terminals are named as
- Base
- Emitter
- Collector
Basic Transistor Symbols Along with its Terminals
(1) Base
This lies at the center of the transistor. It has the interference with the two circuits one is referred to as input circuitry and the other is the output one. The input one is formed because of the emitter and base interaction and the output one is formed due to the collector and base.
The lower resistance can be seen at the input circuit that is emitter base interference side. The higher resistance is offered at the output circuit of base and collector. The doping concentration at the base is low. The size of the base is thin.
(2) Emitter
In order to supply the majority of the charge carriers always, the emitter base respective junction should be supplied with forward bias. It is doped with heavy consistency so that the majority of the carriers can be injected into the base. The size of the emitter will be moderate.
(3) Collector
As the name suggests it acts like a collector of the majority of the carriers. Hence it is considered for the collection of outputs this is the reason the interacted part of collector and base remain in reverse bias.
The doping consistency of the collector is moderate but the size of it is large compare to that of base and emitter.The above are the terminals present in the basic transistor.
Transistor Working Principle
The element named silicon is generally preferred for transistor construction. The silicon is less sensitive to the temperature. It has the capability of handling the high values of voltages and the greater ranges of currents.
As it is known that the emitter base junction must be in forward bias and the collector base junction remain in reverse bias. Because of the forward bias condition at the emitter base junction there is the majority of the carriers entered in to the base.
This is the reason for the constitution of the base current that tends to flow through the region of base. This current tends to flow towards the collector and in response the electron movement is observed in the collector region from base.
The base current is also responsible for creation of vacancy at the collector. But it has small magnitude. As we already know that the base present in the transistor has always lightly doped.
This is the reason there will be the lesser amount of charge carriers like electrons are less in amount in comparison with that of the emitter. These few amounts of electrons get interacted with respect to the holes at the base whereas the left over amount of electrons can be seen moving towards the collector.
This paved the way for the generation of collector current. Hence the variations at the base can constitute large amount of current at the collector.
Transistor Operating Modes
The condition that results in the various operating modes are decided due to the junctions formed at the emitter base and the collector base. The emitter base junction being forward biased and collector base junction being reverse biased results in the active region of that particular transistor in this way based on the further biasing conditions at the junction the various operating modes can be analyzed.
(1) FR
Considering the cases of emitter base and collector base junctions in this case the emitter base junction is forward biased whereas the collector base junction is at reverse bias. Hence these conditions results the transistor to operate in the active region. Once it is in active region the currents at collector are dependent on the current at the emitter.
(2) FF
At this condition the junction of emitter base and the collector base are at forward bias. This type of condition results the transistor to be in saturation region. This region is responsible for making the current at the collector independent of the current that is generated at the emitter.
(3) RR
Hence this case deals with the state where both the junctions of transistor operating at the reverse bias. As far as considered at reverse bias there is no conduction observed at the circuitry. This type of region is known as cut-off region.
The emitter at this stage is not able to supply the majority of the charge carriers and there is no collection of this carrier can be evident at the collector. This type of situation results the transistor to act like a closed switch.
(4) RF
The emitter base junction of the transistor is connected in reverse bias whereas the collector base junction is made forward bias in this condition. As collector is doped with light consistency it is not capable of supplying the majority charge carriers to the respective base of that transistor. Hence the transistor action is poor in this case.
In this way based on the type of bias at the junction applied the various types of operating regions is determined. Based on the essentials transistor biasing is done.
Please refer to this for How to Select a Transistor.
Please refer to this link to know more about MCQs on Transistors
Applications and the Uses of Transistor
In the modern world of electronics everything is somewhere or somehow dependent on the electronics. Either it can be an amplification circuitry or the switching circuitry there are various types of transistors can be utilized for the various purposes.
(1) The basic use of the transistor is as amplifier in various types of oscillators, modulators etc., further in the field of digital circuitry this transistors can be used for switching mechanism.
(2) In a case of transistor when the amount of light falls on it the generation of the current is noticed these are categorized as photo-transistors.
(3) Based on the requirement where the flow of large amount of current is needed from emitter to collector by keeping the base current at minimum amounts the transistor named BJT is required.
(4) In the devices where the control of voltages are required field effect transistors (FET) are utilized. This is because it consists of input impedance at higher value that results in the minimization’s of the current value.
(5) The requirements where the gain of the current must be high then a special type of transistors named Darlington pair of transistors are utilized. The main application of it is notices in creating sensitive touch buttons because these are capable of picking the value of currents from the human skin.
(6) In some cases there is the requirement for diverting the input currents that are high in order to prevent the transistor from entering in to the saturation region.
(7) If suppose the higher values of currents needed to be switched with in less duration of time avalanche transistors are useful for this purpose.
The above are the some of the applications and the uses of the transistors. In this way the basics of transistors are discussed. Do all the transistors have three terminals?