Junction Transistors (BJT) Questions and Answers

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Q1
What is the main doping structure of a Bipolar Junction Transistor (BJT)?
  • A Three alternating p and n layers (p-n-p or n-p-n)
  • B A single uniform p-type slab
  • C A MOS capacitor structure
  • D A Schottky barrier diode structure
Answer: Option A
Explanation: A BJT is formed by three alternating layers of p-type and n-type semiconductor material (either NPN or PNP).
Q2
In an NPN transistor, which region is the majority charge carrier emitter?
  • A Holes in p-type material
  • B Electrons in n-type material
  • C Ions in intrinsic region
  • D Majority holes in n-type
Answer: Option B
Explanation: In an NPN transistor the emitter is heavily doped n-type so the majority carriers are electrons.
Q3
Which terminal of a BJT controls the base current to modulate collector current?
  • A Emitter
  • B Collector
  • C Base
  • D Substrate
Answer: Option C
Explanation: The base terminal controls transistor action; small base current controls larger collector current in BJT.
Q4
What is the typical relationship between collector current (IC) and base current (IB) in active region?
  • A IC is proportional to IB via β
  • B IC equals IB squared
  • C IC is independent of IB
  • D IC is inverse of IB
Answer: Option A
Explanation: In active region IC ≈ β * IB where β is current gain (hFE).
Q5
Which region must the base-emitter junction be biased to operate a BJT in active mode?
  • A Base-emitter forward biased and base-collector reverse biased
  • B Both junctions reverse biased
  • C Both junctions forward biased
  • D Base-emitter reverse biased and base-collector forward biased
Answer: Option A
Explanation: For active mode the base-emitter junction is forward biased and base-collector junction is reverse biased.
Q6
What is meant by "saturation" in a BJT?
  • A Both base-emitter and base-collector are forward biased
  • B Both junctions reverse biased
  • C Base-emitter forward, base-collector reverse
  • D Emitter-collector shorted internally
Answer: Option A
Explanation: Saturation means both base-emitter and base-collector junctions are forward biased; transistor fully on and VCE is minimal.
Q7
What is "cut-off" condition of a BJT?
  • A Transistor fully on
  • B No current flows (transistor off)
  • C Only base current flows
  • D Collector-emitter short-circuit only
Answer: Option B
Explanation: Cut-off means base-emitter junction is not forward biased enough; essentially no collector current flows (transistor off).
Q8
Which parameter represents the small-signal current gain of a BJT?
  • A h_fe (or β)
  • B gm
  • C r_o
  • D Cje
Answer: Option A
Explanation: Small-signal current gain is h_fe (or β in many contexts).
Q9
Which of these increases when the emitter doping is made heavier while keeping other regions same?
  • A Emitter injection efficiency increases
  • B Base resistance increases
  • C Collector leakage increases
  • D Transistor becomes PNP
Answer: Option A
Explanation: Heavier emitter doping increases emitter injection efficiency, raising current gain.
Q10
What is "early effect" in BJTs?
  • A Base width modulation by VCB (Early effect)
  • B Avalanche breakdown of base
  • C Thermal runaway effect
  • D Punch-through only
Answer: Option A
Explanation: Early effect is base-width modulation by collector-base voltage causing change in collector current with VCE; modeled by Early voltage.

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