Early Effect

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

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

As the applied base-collector voltage (V_BC) varies, the base-collector depletion region varies in size. This is often called the "Early Effect" after its discover James M. Early.

This effectively means 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 :

• 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.

Where

• V_CE is the collector-emitter voltage
• V_A is the Early voltage (15 V to 150 V)
• β_F0 is forward common emitter current gain at zero bias