The GATE Grind

GATE 2024 EC – Question 52

Electronic Devices · P-N Junction and Semiconductor Devices · 2 marks · Multiple select

Which of the following statements is/are true for a BJT with respect to its DC current gain $\beta$?

  1. Under high-level injection condition in forward active mode, $\beta$ will decrease with increase in the magnitude of collector current.
  2. Under low-level injection condition in forward active mode, where the current at the emitter-base junction is dominated by recombination-generation process, $\beta$ will decrease with increase in the magnitude of collector current.
  3. $\beta$ will be lower when the BJT is in saturation region compared to when it is in active region.
  4. A higher value of $\beta$ will lead to a lower value of the collector-to-emitter breakdown voltage.

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (A) Under high-level injection condition in forward active mode, $\beta$ will decrease with increase in the magnitude of collector current.; (C) $\beta$ will be lower when the BJT is in saturation region compared to when it is in active region.; (D) A higher value of $\beta$ will lead to a lower value of the collector-to-emitter breakdown voltage.

Explanation

(A) True: at high-level injection the base is flooded with carriers, emitter efficiency drops (Webster effect, Kirk effect) and $\beta$ falls as $I_C$ rises. (B) False: at low current the recombination current in the emitter-base depletion region dominates the base current, so $\beta$ is small and it increases with $I_C$. (C) True: in saturation the collector junction is forward biased and the forced $\beta$ ($I_C/I_B$) is smaller than in the active region. (D) True: $BV_{CEO}\approx BV_{CBO}/\beta^{1/n}$, so higher $\beta$ gives lower $BV_{CE}$.