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EARLY EFFECT AND BASE-WIDTH MODULATION IN TRANSISTORS (BJT)


A BJT Transistor consisting of three terminals, collector, base, and Emitter and two Junctions, Emitter-Base Junction and Collector-Base Junction.
Normally a bipolar Junction Transistor operates on three regions

1. ACTIVE REGION
2. SATURATION REGION
3. CUT-OFF REGION

It is well explained in my previous article about Different Operating Modes and Biasing of Transistors,i hope you read it.

Now back to our topic, EARLY EFFECT AND BASE-WIDTH MODULATION.
In active region, the Emitter-Base Junction is forward biased and Collector-Base Junction is reverse biased, so the barrier width at Emitter-Base Junction is negligible in comparison with the space charge width at Collector-Base Junction.

The transition or space-charge region at a Junction is the region of uncovered charges on both sides of the Junction.
As the Voltage applied across the Junction increases, the transition region penetrates deeper into the Collector-Base Junction. Because neutrality of the charge is to be maintained, the number of uncovered charges on each side remains equal. Since the doping in the base is substantially smaller than that of Collector, the penetration of the transition region into the base is much larger than that into the Collector region. Hence the Collector depletion region is neglected and all the immobile charges is indicated in the base region.


The decrease in effective base width Wb with increasing reverse bias has three consequences.



1. There is less chance for recombination within the base region.

2. The concentration gradient of minority carriers is increased within the base, and consequently, the current of minority carriers injected across the Emitter Junction increases, with increasing reverse Collector Voltage.

3. For extremely large Voltages, Wb may be reduced to Zero causing Voltage breakdown.
The lowering of Emitter Junction Voltage causes an excessively large Emitter current, thus placing an upper limit on the magnitude of the Collector Voltage this phenomenon is known as PUNCH THROUGH.


The PUNCH THROUGH phenomenon is defined as the mechanism by which a Transistor usefulness may be terminated by increasing the Collector Voltage.


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