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Question
Why are the emitter, the base, and the collector of a BJT doped differently?
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Solution
A BJT is a bipolar device, both electrons and holes participate in the conduction process. Under the forward-biased condition, the majority of carriers injected from the emitter into the base constitute the largest current component in a BJT. For these carriers to diffuse across the base region with negligible recombination and reach the collector junction, they must overwhelm the majority of carriers of the opposite charge in the base. The total emitter current has two components, due to majority carriers in the emitter and due to minority carriers diffusing from the base into the emitter. The ratio of the current component due to the injected majority carriers from the emitter to the total emitter current is a measure of the emitter efficiency. To improve the emitter efficiency and the common-base current gain (α), it can be shown that the emitter should be much more heavily doped than the base.
Also, the base width is a function of the base-collector voltage. A low doping level of the collector increases the size of the depletion region. This increases the maximum collector-base voltage and reduces the base width. Further, the large depletion region at the collector-base junction-extending mainly into the collector-corresponds to a smaller electric field and avoids avalanche breakdown of the reverse-biased collector-base junction.
[Note: Effective dopant concentrations of (a) npn transistor (b) pnp transistor are shown below.]
The base doping is less than the emitter doping but greater than the collector doping.
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