Hello friends…We all heard about transistors from our school days .what is a transistor? Why we have to go for it???…. ya.. here its reason. The most important device in electronics is vaccum tube. As it was bulky, low efficient, requires higher power to operate; John Barden, Walter Brattain and William Shockley invented “Transistors” at Bell Labs in 1947 .The transistor is a semiconductor device. A semiconductor material has the electrical conductivity which lies between the conductors and the insulators. The main purpose of a transistor is, it can be operated as a switch and used as an amplifier. Due to the low power consumption, small, long life, transistors are mostly preferred than that of vaccum tubes.
There are two types of transistors.
*Bipolar Junction Transistors
*Field Effect Transistors
Bipolar Junction Transistors:
The Bipolar junction Transistor consists of two layers, P-layer and N-layer. The combination of either P or N layer with the opposite layer is called a Transistor. Based on the construction transistors are divided into 2 types.
*NPN transistor *PNP transistor
Same as like that of NPN, in PNP transistor the N-layer is sandwiched between the two P-layers.
There are three terminals in a transistor. Base, Emitter and Collector.
The Emitter emits the charge carriers to base. The size of the emitter is larger than that of the base. The Emitter is used as the input terminal and it is heavily doped.
The Base terminal is lightly doped. It is a thin layer than that of Emitter and Collector. It is at the middle of Emitter and Collector.
The collector collects the charge carriers. The size of the Collector is larger than that of Emitter and Base. Because the Collector has to handle the high power and it requires large area for heat dissipation. The Collector is moderately doped.
Next comes the working of the NPN &PNP.
Working of NPN transistor:
In NPN transistor, the emitter base junction is Forward Biased and the collector base junction is Reverse Biased. when N-terminal/P-terminal is connected to the negative /positive terminal of the battery respectively, then it is called as Forward Biased. Similarly, when P-terminal/N-terminal is connected to the negative/positive terminal of the battery, then it is called Reverse Biased. When F.B is applied to the N-type emitter, the electrons flows towards the base. As the Base is lightly doped, it consists of less numbers of holes. In base region, only less number of holes will combine with the less number of electrons and forms the base current IB .The remaining electrons are passed to the collector and it leads to the formation of collector current IC. This transistor is mostly used than that of PNP transistors .The sum of base and collector current is emitter current.
Working of PNP transistor:
Similar to that of the NPN transistor ,emitter base junction is forward biased while collector base is reverse biased. when F.B is applied to the P-type emitter ,the holes flow towards the base. As the Base is lightly doped ,only few numbers of holes combines with the electrons and forms emitter current IE. The remaining holes are passed to the collector and forms IC. The emitter current is equal to the sum of base and collector current.
Applying external supply to the transistor is biasing. The transistor operates in three different regions.
*Active region * saturation region *cut-off region
When the emitter base junction is forward biased and the collector base junction is reverse biased ,then the transistor operates in Active region. In this region the transistor acts as an amplifier.
When the emitter base junction and the collector base junction are F.B then, the transistor operates in saturation region. It acts as a closed switch.
When the emitter base junction and collector base junction are R.B then, the transistor operates in cut off region. There is no flow of current during this operation when the transistor acts as an open switch.
Stay tuned for the new updates in future posts……