Ch 1 Bipolar Junction Transistor for student use

BJT and its characteristics

1. Bipolar Junction Transistor A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector, and the emitter. A signal of a small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process .

2. Construction of Bipolar Junction Transistor BJT is a semiconductor device that is constructed with 3 doped semiconductor Regions i.e. Base, Collector & Emitter separated by 2 p-n Junctions. Bipolar transistors are manufactured in two types,  PNP  and  NPN , and are available as separate components, usually in large quantities. The prime use or function of this type of transistor is to amplify current. This makes them useful as switches or amplifiers. They have a wide application in electronic devices like mobile phones, televisions, radio transmitters, and industrial control.

3. Operation of Bipolar Junction Transistor There are three operating regions of a bipolar junction transistor : Active region: The region in which the transistors operate as an amplifier . Saturation region: The region in which the transistor is fully on and operates as a switch such that collector current is equal to the saturation current . Cut-off region: The region in which the transistor is fully off and collector current is equal to zero.

4. Transistor as a Switch When used as an AC signal amplifier, the transistors Base biasing voltage is applied in such a way that it always operates within its “active” region, that is the linear part of the output characteristics curves are used. However, both the NPN & PNP type bipolar transistors can be made to operate as “ON/OFF” type solid state switch by biasing the transistors Base terminal differently operating the transistor as a switch. Solid state switches are one of the main applications for the use of transistor to switch a DC output “ON” or “OFF”. Some output devices, such as LED’s only require a few milliamps at logic level DC voltages and can therefore be driven directly by the output of a logic gate. However, high power devices such as motors, solenoids or lamps, often require more power than that supplied by an ordinary logic gate so transistor switches are used .

5. If the circuit uses the  Bipolar Transistor as a Switch , then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the ” I-V ” characteristics curves we have seen previously . The areas of operation for a transistor switch are known as the  Saturation Region  and the  Cut-off Region . This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below.

6.   Cut-off Region Here the operating conditions of the transistor are zero input base current ( I B  ), zero output collector current ( I C  ) and maximum collector voltage ( V CE  ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

7. Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch as being, both junctions reverse biased, V B  < 0.7v and I C  = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.

8. Saturation Region Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched “Fully-ON”.