Early effect

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Early effect voltage as seen in the output-characteristic plot of a BJT

The Early effect is the variation in the width of the base in a BJT due to a variation in the applied collector voltage.

As the applied base–collector voltage ( $V B C$ ) varies, the base–collector depletion region varies in size. This change is often called the "Early effect" after its discoverer James M. Early.

The Early effect implies a variation in the width of the base region of the BJT. An increase in the collector voltage, for example, causes a greater reverse bias across the collector–base junction, increasing the collector–base width, decreasing the width of the base. This has two consequences:

Graded hetero-junction bipolar transistor. Top=closed, bottom=open, grey=band gap, blue=electrons, white=holes, y-axis=energy. Left=emitter, middle=base, right=collector

There is a lesser chance for recombination within the "smaller" base region.
The charge gradient is increased across the base, and consequently, the current of minority carries injected across the emitter junction increases.

Both these factors mean an increase in the collector or "output" current of the transistor due to an increase in the collector voltage.

In the forward active region the Early Effect modifies the collector current ( $i C$ ) and the forward common emitter current gain ( $β F$ ) to the following equations.

$i_\mathrm{C} = I_\mathrm{S} e^{\frac{V_\mathrm{BE}}{V_\mathrm{T}}} \left(1 + \frac{V_\mathrm{CE}}{V_\mathrm{A}}\right)$