Question

Which of the following is not correct, when the collector base junction is reverse biased?

• A. It decreases the depletion region across the collector base junction
• B. Early effect occurs
• C. Forms depletion region across the collector junction
• D. Large current flow in collector

Hint:

Reverse bias widens the depletion region of a p-n junction.
Forward bias narrows the depletion region.
Early effect in BJTs is related to the modulation of base width by the CBJ depletion region width.

Answer 1

The Correct Option is A

When a p-n junction (like the Collector-Base Junction, CBJ) is reverse biased:
The width of the depletion region increases (widens).
A small reverse saturation current flows. Analyzing the options: (a) "It decreases the depletion region across the collector base junction": This is FALSE. Reverse bias increases the depletion width. So this statement is "not correct". (b) "Early effect occurs": The Early effect (base-width modulation) is the change in the effective base width due to the change in the CBJ depletion width as \(V_{CB}\) (reverse bias) changes. This phenomenon is associated with a reverse-biased CBJ. TRUE. (c) "Forms depletion region across the collector junction": A depletion region is inherently formed at any p-n junction due to initial diffusion of carriers, regardless of bias (though bias modifies it). TRUE. (d) "Large current flow in collector": In the active mode of a BJT, the CBJ is reverse biased, and a large collector current (controlled by base-emitter junction) flows. So, a large collector current can indeed flow while the CBJ is reverse biased. TRUE in context of transistor operation. The statement that is "not correct" about the direct effect of reverse biasing the CBJ is (a). \[ \boxed{\text{It decreases the depletion region across the collector base junction}} \]