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. 

(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". Incorrect

(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. Correct

(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). Correct

(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. Correct

The statement that is "not correct" about the *direct effect* of reverse biasing the CBJ is (a).

\[ \boxed{\text{It decreases the depletion region across the collector base junction}} \]