What is base-width modulation?

Answer 1

As the applied base-collector voltage (VBC) varies, the base-collector depletion region varies in size. This variation causes the gain of the device to change, since the gain is related to the width of the effective base region. This effect is often called the "Early Effect"

An NPN bipolar transistor can be considered as two diodes connected anode to anode. In normal operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased. In an npn-type transistor for example, electrons from the emitter wander (or "diffuse") into the base