Bipolar Junction Transistor | Electronic Devices - Electronics and Communication Engineering (ECE) PDF Download

Bipolar Junction Transistor (BJT)

  • A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions.
  • In an NPN transistor, a thin and lightly doped P-type base is sandwiched between a heavily doped N-type emitter and another N-type collector
  • In a PNP transistor, a thin and lightly doped N-type base is sandwiched between a heavily doped P-type emitter and another P-type collector.
    NPN transistor and PNP transistor
    NPN transistor and PNP transistor
  • Terminals of BJT: BJT has three terminals. They are Emitter, Base, and Collector. An emitter is heavily doped, a collector is moderately doped and a base is lightly doped. A transistor is an active device which has the ability to control the electron flow through it.
    NPN transistor and PNP transistor
    NPN transistor and PNP transistor
  • The direction of arrow signifies the direction of emitter current when base emitter junction is forward biased.
  • An NPN transistor can be considered as two diodes with a shared anode region. In typical operation, the emitter–base junction is forward biased and the base–collector junction is reverse biased.

Configurations of BJT

  • BJT can be operated in three configurations. They are Common Base, Common Emitter, and Common collector configurations.
  • Due to the presence of two junctions each junction can be operated in forward or reverse bias leading to three different regions of operation.
  • The transistor as a Switch: The transistor can be used as a switch in logic gates will be operated in extreme regions of input output characteristics in which both regions will be forward biased (On state) or reverse biased (Off state) which are called saturation and cut off regions of operation simultaneously.
  • Transistor as an amplifier: Transistor, when used as an amplifier, is operated in the active region in which input junction will be forward biased and output junction will be reverse biased. There are three types of operating modes of amplifier i.e. Common Base (CB) amplifier, Common Emitter (CE) amplifier and Common Emitter (CE) amplifier. Another region inverse active region is of less importance in practice.

MOS capacitor 

  • An MOS capacitor is made of a semiconductor body or substrate, an insulator film, such as SiO2, and a metal electrode called a gate.
  • A capacitor is formed when two conducting layers are separated by a dielectric layer.
  • The value of MOS capacitor depends on the region of operation.
    Bipolar Junction Transistor | Electronic Devices - Electronics and Communication Engineering (ECE)
  • The flat band voltage is an important term related to the MOS capacitor. It is defined as the voltage at which there is no charge on the capacitor plates and hence there is no static electric field across the oxide.
  • Accumulation: An applied positive gate voltage larger than the flat band voltage (VGB > VFB) then a positive charge is induced on the metal (poly silicon) gate and negative charge in the semiconductor. The only negative charged electrons are available as negative charges and they accumulate at the surface. This is known as surface accumulation.
    Bipolar Junction Transistor | Electronic Devices - Electronics and Communication Engineering (ECE)
  • Depletion: If the applied gate voltage is lower than the flat band voltage (VGB < VFB) then a negative charge is induced at the interface between the polysilicon gate and the oxide and positive charge in the semiconductor. This is only possible by pushing the negatively charged electrons away from the surface exposing the fixed positive charges from donors. This is known as surface depletion.
    Bipolar Junction Transistor | Electronic Devices - Electronics and Communication Engineering (ECE)
  • Inversion: As the potential across the semiconductor increases beyond twice the bulk potential, another type of negative charge emerges at the oxide-semiconductor interface: this charge is due to minority carriers, which form a so-called inversion layer.
The document Bipolar Junction Transistor | Electronic Devices - Electronics and Communication Engineering (ECE) is a part of the Electronics and Communication Engineering (ECE) Course Electronic Devices.
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FAQs on Bipolar Junction Transistor - Electronic Devices - Electronics and Communication Engineering (ECE)

1. What is a bipolar junction transistor?
Ans. A bipolar junction transistor (BJT) is a three-layer semiconductor device that can amplify or switch electronic signals and electrical power. It consists of three regions, namely the emitter, base, and collector, which are made of either N-type or P-type semiconductor material.
2. What are the different types of bipolar junction transistors?
Ans. There are two main types of bipolar junction transistors: NPN (negative-positive-negative) and PNP (positive-negative-positive). In NPN transistors, the majority charge carriers are electrons, while in PNP transistors, the majority charge carriers are holes.
3. How does a bipolar junction transistor work?
Ans. A bipolar junction transistor works based on the principles of both PNP and NPN junction diodes. When a small current is applied to the base-emitter junction (forward bias), it allows a larger current to flow from the collector to the emitter. This current amplification is the basis of transistor operation.
4. What are the applications of bipolar junction transistors?
Ans. Bipolar junction transistors have numerous applications in electronics and communication engineering. They are used in amplifiers, oscillators, switches, voltage regulators, logic gates, and various other electronic circuits. They are also crucial components in radio and television receivers, computers, and telecommunications systems.
5. What are the advantages and disadvantages of bipolar junction transistors?
Ans. The advantages of bipolar junction transistors include their high current gain, fast switching speeds, and low saturation voltage. They also have good linearity and can operate in a wide range of frequencies. However, they have some disadvantages, such as higher power consumption, larger size compared to other transistor types, and sensitivity to temperature variations.
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