Transistor and Types of Transistor || BJT and FET || Basic Electronics
History
Bipolar junction transistor (BJT) was invented by William Shockley and John Bardeen. While the first transistor was invented 70 years ago but till now it changed the world from mysterious big computers to small smartphones. The invention of transistor changed the concept of electrical circuits to integrated circuits (IC). Nowadays, the use of BJT is decreasing because CMOS technology took place in the design of digital ICs.
What is BJT – Bipolar Junction Transistor?
Bipolar junction transistor (BJT) is a bidirectional device that uses both electrons and holes as charge carriers. While Unipolar transistor i.e. Field effect transistor uses only one type of charge carrier. BJT is a current controlled device. The current flows from emitter to collector or from collector to emitter depending on the type of connection. This main current is controlled by a very small current at the base terminal.
Construction
Bipolar junction transistor is formed by the combination of two back-to-back doped semiconductor materials. In other words, BJT is formed by the “sandwich” of back to back extrinsic semiconductor materials. These extrinsic semiconductors are PN junction diodes. Two PN junctions’ diodes are sandwiched together to form a three-terminal device known as BJT transistor. BJT is a three-terminal device having two junctions.
After doping an intrinsic semiconductor with Trivalent or Pentavalent impurities a P-type semiconductor or N-type semiconductor respectively is made. If the number of electrons are greater than the number of holes (positive carriers) then that is known as N-type semiconductor material. While in P-type semiconductor, the number of holes is greater than the number of electrons. When P-type and N-type material are connected together then it becomes a PN-junction diode. BJT transistors are formed after connecting two PN junctions back to back. These transistors are known as PNP or NPN bipolar junction transistors depending on whether P or N-type is sandwiched.
Basically, transistors have three parts and two junctions. These three portions are called Emitter, Collector, and Base. The emitter and collector sandwich the base in between them. The middle portion (base) forms two junctions with the emitter & collector. The junction of the base with the emitter is known as the Emitter-Base junction while the junction of the base with the collector is known as the Collector-Base junction.
Terminals of BJT
There are three terminals of BJT. These terminals are known as collector, emitter and base. These terminals are briefly discussed here.
Emitter
The emitter is the portion on one side of the transistor which emits electrons or holes to the other two portions. The rule is always reverse bias with respect to emitter so that it can emit a large number of majority carriers. It is the most heavily doped region of the BJT. The emitter-base junction should always be forward bias in both PNP and NPN transistors. Emitter supplies electrons to the emitter-base junction in NPN while it supplies holes into the same junction in PNP transistor.
Collector
The portion on the opposite side of the Emitter that collects the emitted charge carriers (i.e. electrons or holes) is known as collector. The collector is heavily doped but the doping level of the collector is in between the lightly doped level of base and heavily doped level of emitter. Collector-base junction should always be reversed biased in both PNP and NPN transistors. The reason for reverse biasing is to remove charge carriers (electrons or holes) from the collector-base junction. The collector of NPN transistor collects electrons emitted by emitter. While in PNP transistor, it collects holes emitted by emitter.
Base
The base is the middle portion between collector and emitter & it forms two PN junctions between them. The base is the most lightly doped portion of the BJT. Being the middle portion of the BJT allows it to control the flow of charge carriers between emitter and collector. The base-collector junction shows high resistance because this junction is reversed bias.
Type of BJT
This three-layer device formed by back to back connection has specific names. It can be weather PNP or NPN. Both connections are disused here briefly.
PNP Construction
In PNP bipolar transistor, the N-type semiconductor is sandwiched between two P-type semiconductors. PNP transistors can be formed by connecting cathodes of two diodes. The cathodes of the diodes are connected together at a common point known as base. While the anodes of the diodes that are on the opposite sides are known as the collector and the emitter.
The emitter-base junction is forward bias while collector-base junction is reverse bias. So, in PNP type current flows from emitter to collector. The emitter, in this case, is at high potential to both collector and base.
Related Post: What is PNP Transistor? tion, Working & Applications
NPN Construction
NPN type is exactly opposite to PNP type. In NPN bipolar transistor, the P-type semiconductor is sandwiched between two N-type semiconductors. When the anodes of two diodes are connected together it forms an NPN transistor. The current will flow from the collector to emitter because the collector terminal is more positive than emitter in NPN connection.
The difference between PNP and NPN symbol is the arrow mark at the emitter which shows the direction of flow of current. The current will either flow from emitter to collector or from collector to emitter. The arrow mark in PNP transistor is inward, which shows the flow of current from emitter to collector. In case of NPN collector, the arrow mark is outward, which shows the flow of current from collector to emitter.
Working of BJT
The word “transistor” is the combination of two words, “Trans” (Transform) and “istor” (Varistor). So, it means the transistor can transform its resistance. The resistance varies in such a way that it can either act as an insulator or conductor by applying small signal voltage. This changing ability makes it able to perform both as an “Amplifier” or a “Switch”. It can be used either as a switch or an amplifier at a single time. Therefore, BJT can operate in three different regions to perform the said operation.
Transistor and Types of Transistor || BJT and FET || Basic Electronics
History
Bipolar junction transistor (BJT) was invented by William Shockley and John Bardeen. While the first transistor was invented 70 years ago but till now it changed the world from mysterious big computers to small smartphones. The invention of transistor changed the concept of electrical circuits to integrated circuits (IC). Nowadays, the use of BJT is decreasing because CMOS technology took place in the design of digital ICs.
What is BJT – Bipolar Junction Transistor?
Bipolar junction transistor (BJT) is a bidirectional device that uses both electrons and holes as charge carriers. While Unipolar transistor i.e. Field effect transistor uses only one type of charge carrier. BJT is a current controlled device. The current flows from emitter to collector or from collector to emitter depending on the type of connection. This main current is controlled by a very small current at the base terminal.
Construction
Bipolar junction transistor is formed by the combination of two back-to-back doped semiconductor materials. In other words, BJT is formed by the “sandwich” of back to back extrinsic semiconductor materials. These extrinsic semiconductors are PN junction diodes. Two PN junctions’ diodes are sandwiched together to form a three-terminal device known as BJT transistor. BJT is a three-terminal device having two junctions.
After doping an intrinsic semiconductor with Trivalent or Pentavalent impurities a P-type semiconductor or N-type semiconductor respectively is made. If the number of electrons are greater than the number of holes (positive carriers) then that is known as N-type semiconductor material. While in P-type semiconductor, the number of holes is greater than the number of electrons. When P-type and N-type material are connected together then it becomes a PN-junction diode. BJT transistors are formed after connecting two PN junctions back to back. These transistors are known as PNP or NPN bipolar junction transistors depending on whether P or N-type is sandwiched.
Basically, transistors have three parts and two junctions. These three portions are called Emitter, Collector, and Base. The emitter and collector sandwich the base in between them. The middle portion (base) forms two junctions with the emitter & collector. The junction of the base with the emitter is known as the Emitter-Base junction while the junction of the base with the collector is known as the Collector-Base junction.
Terminals of BJT
There are three terminals of BJT. These terminals are known as collector, emitter and base. These terminals are briefly discussed here.
Emitter
The emitter is the portion on one side of the transistor which emits electrons or holes to the other two portions. The rule is always reverse bias with respect to emitter so that it can emit a large number of majority carriers. It is the most heavily doped region of the BJT. The emitter-base junction should always be forward bias in both PNP and NPN transistors. Emitter supplies electrons to the emitter-base junction in NPN while it supplies holes into the same junction in PNP transistor.
Collector
The portion on the opposite side of the Emitter that collects the emitted charge carriers (i.e. electrons or holes) is known as collector. The collector is heavily doped but the doping level of the collector is in between the lightly doped level of base and heavily doped level of emitter. Collector-base junction should always be reversed biased in both PNP and NPN transistors. The reason for reverse biasing is to remove charge carriers (electrons or holes) from the collector-base junction. The collector of NPN transistor collects electrons emitted by emitter. While in PNP transistor, it collects holes emitted by emitter.
Base
The base is the middle portion between collector and emitter & it forms two PN junctions between them. The base is the most lightly doped portion of the BJT. Being the middle portion of the BJT allows it to control the flow of charge carriers between emitter and collector. The base-collector junction shows high resistance because this junction is reversed bias.
Type of BJT
This three-layer device formed by back to back connection has specific names. It can be weather PNP or NPN. Both connections are disused here briefly.
PNP Construction
In PNP bipolar transistor, the N-type semiconductor is sandwiched between two P-type semiconductors. PNP transistors can be formed by connecting cathodes of two diodes. The cathodes of the diodes are connected together at a common point known as base. While the anodes of the diodes that are on the opposite sides are known as the collector and the emitter.
The emitter-base junction is forward bias while collector-base junction is reverse bias. So, in PNP type current flows from emitter to collector. The emitter, in this case, is at high potential to both collector and base.
Related Post: What is PNP Transistor? tion, Working & Applications
NPN Construction
NPN type is exactly opposite to PNP type. In NPN bipolar transistor, the P-type semiconductor is sandwiched between two N-type semiconductors. When the anodes of two diodes are connected together it forms an NPN transistor. The current will flow from the collector to emitter because the collector terminal is more positive than emitter in NPN connection.
The difference between PNP and NPN symbol is the arrow mark at the emitter which shows the direction of flow of current. The current will either flow from emitter to collector or from collector to emitter. The arrow mark in PNP transistor is inward, which shows the flow of current from emitter to collector. In case of NPN collector, the arrow mark is outward, which shows the flow of current from collector to emitter.
Working of BJT
The word “transistor” is the combination of two words, “Trans” (Transform) and “istor” (Varistor). So, it means the transistor can transform its resistance. The resistance varies in such a way that it can either act as an insulator or conductor by applying small signal voltage. This changing ability makes it able to perform both as an “Amplifier” or a “Switch”. It can be used either as a switch or an amplifier at a single time. Therefore, BJT can operate in three different regions to perform the said operation.
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