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A semiconductor diode is a two-terminal electronic device made up of a p-type and an n-type semiconductor material joined together. It has two types of biases: forward bias and reverse bias. In forward bias, the voltage is applied such that the current flows primarily in one direction, making the diode a unidirectional device. In reverse bias, the voltage is applied in such a way that it tries to impede the flow of current.

Zener breakdown is a phenomenon that occurs in semiconductor diodes when the reverse voltage applied across the diode becomes high enough to cause electrons to tunnel through the depletion region. This leads to a large increase in reverse current flow, and the diode starts conducting in the reverse direction.

Now that we know the Zener breakdown occurs when the reverse voltage applied across the diode becomes high enough, we can look at the options given in the question: (A) Forward current exceeds certain value - Incorrect, as this is related to forward bias and not reverse bias (B) Reverse bias exceeds certain value - Correct, as Zener breakdown occurs when the reverse bias (or voltage) exceeds a certain value (C) Forward bias exceeds certain value - Incorrect, as this is related to forward bias and not reverse bias (D) Potential barrier is reduced to zero - Incorrect, as Zener breakdown is related to the increase in reverse bias, not the reduction of potential barrier. Therefore, the correct answer is: (B) Reverse bias exceeds certain value.