The phenomenon known as ‘Early effect’ in a bipolar transistor refers to a reduction of the effective base-width caused by

​Concept:

• A large collector base reverse bias is the reason behind the early effect by the BJTs.
• As reverse biasing of the collector to base junction increases, the depletion region penetrates more into the base, as the base is lightly doped.
• This reduces the effective base width and hence the concentration gradient in the base increases.
• This reduction in the effective base width causes less recombination of carriers in the base region which results in an increase in collector current. This is known as the Early effect.
• The decrease in base width causes ß to increase and hence collector current increases with collector voltage rather than staying constant.
• The slope introduced by the Early effect is almost linear with IC and the common-emitter characteristics extrapolate to an intersection with the voltage axis VA, called the Early voltage.

This is explained with the help of the following VCE (Reverse voltage) vs IC (Collector current) curve:

• The punch-through breakdown occurs if the collector-base reverse bias voltage is increased far enough to the extent that the depletion region fills the entire base.
• In this condition holes are swept directly from the emitter region to the collector and the normal transistor action is lost.
• Punch-through is undesirable and hence it is avoided in circuit design.