Introduction to Bipolar Transistor

           Introduction to Bipolar Transistor 

• A bipolar transistor is an active semiconductor device formed by two P­N junctions which amplify electric current 
• Structure of Bipolar transistor

1. A Bipolar transistor consists of three differently doped regions called the Emitter, the Base and the Collector

• NPN 
  

1. It consists of p­doped semiconductors between two n layers 

• PNP 

1. It consists of n­doped semiconductor between two layers of p­doped material.  

• Hetero junction transistor 

1. This type is similar to Bipolar transistor and can handle very high frequencies. 

• Regions of operation: Bipolar transistors have five distinct regions of operation. They are

1. Forward Active 
2. Reverse Active 
3. Saturation 
4. Cut off 
5. Avalanche Breakdown 

• Forward Active: The base emitter junction is forward biased and the base collector junction is reverse biased. 

• Reverse Active: The base emitter junction is reverse biased and the base collector junction is forward biased. 
• Saturation: When both junctions are forward biased, it is in saturation. 
• Cut off: When both junctions are reverse biased, it is in cut off. 
• Avalanche Breakdown region: Due to the electric field created, there is current flowing through good insulators. It is avalanche breakdown. 
• Functions of Bipolar transistor: The functions of a bipolar transistor are to amplify current and to regulate current. 
• Specifications: Voltage between Collector­ Emitter, Voltage between Collector­Base should be above 12V and Voltage between Emitter­Base should be between 5­6V. 
• Applications of Bipolar Transistors: BPT is used an amplifier to amplify input waves in a microphone. It is also used as a switch. It also is a part of Integrated Circuit (IC). 
• Testing of Diodes: Diodes can be tested using multimeter

1. Set the multimeter to diode function. 
2. Anode to positive, Cathode to negative. 
3. Note down the voltage drop. 

• Transistors can be tested using multimeter. 

1. Black probe – Base, Red probe Emitter, Collector. 
2. Red Probe –Base, Black Probe­ Emitter , Collector. 

• If the readings don’t change with the change of probe at base then the transistor is bad.