Construction and Operation

The npn transistor consists of a thin, lightly-doped p-type layer sandwiched between two n-type regions, Figure 93.

**Figure 93:** *npn* transistor construction showing junctions and barrier potential V_barrier.
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In normal operation under normal bias conditions, the emitter-base junction is forward biased ( v_BE > 0), and the collector-base junction is reverse biased ( v_CB > 0), Figure 94 and Figure 95.

**Figure 94:** *npn* transistor under normal bias conditions.
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The voltages V_BB and V_CC indicate the voltages applied to achieve the forward EB and reverse BC biases. The EB depletion layer is narrowed, and the BC depletion layer is widened.

**Figure 95:** Main electron and hole flows in an *npn* transistor under normal bias conditions.
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The n-type emitter region is heavily doped, and has a great many conduction band electrons which easily diffuse into the p-type base region. In the emitter the electrons are majority carriers while in the base they are minority carriers. The base region is thin and lightly doped, and so only a small fraction of the electrons injected into the base get recombined. The base current is small as a result. Most of the electrons get pulled into the n-type collector region by the reverse bias voltage, and form the collector current.

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