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

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
• 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
• 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.
  
• 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 (V_GB > V_FB) 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.
  
• 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.
  
• 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.