Consider an n-p-n transistor with its base emitter junction...

Consider an $n-p-n$ transistor with its base emitter junction forward biased and collector base junction reverse biased. Which of the following statements are true? $\quad$ [NCERT Exemplar] (a) Electrons crossover from emitter to collector (b) Holes move from based to collector (c) Electrons move from emitter to base (d) Electrons from emitter move out of base without of base without of base without going to the collector

Consider an $n-p-n$ transistor with its base emitter junction forward biased and collector base junction reverse biased. Which of the following statements are true? $\quad$ [NCERT Exemplar]
(a) Electrons crossover from emitter to collector
(b) Holes move from based to collector
(c) Electrons move from emitter to base
(d) Electrons from emitter move out of base without of base without of base without going to the collector

Key Concepts

Carrier Injection and Diffusion

Once electrons are injected from the emitter into the base, they become minority carriers in the p-type base. Due to the forward bias at the emitter-base junction and the diffusion process, these electrons travel across the thin base region. Their mobility allows a large fraction of them to reach the reverse-biased collector-base junction, which is fundamental for the transistor's amplification action.

Recombination in the Base Region

As electrons diffuse through the base, a portion of them may recombine with the holes that are majority carriers in the p-type base. However, because the base is designed to be very thin and lightly doped, recombination is minimized, ensuring that most electrons continue their journey to the collector. This concept is key to the high efficiency and current gain observed in well-designed transistors.

Emitter-Base Junction Forward Bias

In an n-p-n transistor, forward biasing the emitter-base junction reduces the barrier for charge carriers and allows electrons in the emitter (which are majority carriers) to be injected easily into the base region. This is the first step in the transistor’s operation where carrier injection is essential for initiating current flow within the device.

Collector-Base Junction Reverse Bias

Reverse biasing the collector-base junction establishes a strong electric field across this junction. This field is crucial for attracting the electrons that have diffused through the base from the emitter, thus sweeping them into the collector efficiently and contributing to the overall current gain of the transistor.

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