Explain the reasons for the following with reference to a transistor i) Emitter region is highly doped ii) Base region is lowly doped

Q.) Explain the reasons for the following with reference to a transistor i) Emitter region is highly doped ii) Base region is lowly doped

 Subject: Electronic Devices and Circuit

(i) Emitter region is highly doped:

The emitter region in a transistor is heavily doped with donor impurities to achieve several important objectives:

  1. High Injection Efficiency: The primary role of the emitter is to inject a large number of charge carriers (electrons or holes) into the base region. Heavy doping of the emitter ensures that there is a high concentration of majority carriers (electrons in an NPN transistor and holes in a PNP transistor) available for injection. This high doping level enhances the injection efficiency, allowing a significant flow of charge carriers from the emitter to the base.

  2. Minimized Emitter-Base Voltage Drop: Heavy doping of the emitter reduces the resistance of the emitter region. As a result, the voltage drop across the emitter-base junction is minimized. This is crucial because a high emitter-base voltage drop would hinder the injection of charge carriers into the base. By minimizing the voltage drop, the emitter can effectively inject charge carriers into the base even under low forward-bias conditions.

  3. Reduced Base-Width Modulation: The high doping level in the emitter helps reduce the effects of base-width modulation. Base-width modulation refers to the variation in the width of the base region due to the applied bias. In a heavily doped emitter, the high concentration of majority carriers screens the electric field extending from the collector-base junction. This screening effect suppresses base-width modulation, leading to more stable transistor characteristics.

(ii) Base region is lowly doped:

The base region in a transistor is lightly doped with acceptor or donor impurities (depending on the transistor type) for specific reasons:

  1. Maximize Minority Carrier Lifetime: The base region is designed to facilitate the diffusion of minority carriers (holes in an NPN transistor and electrons in a PNP transistor) from the emitter to the collector. A low doping level in the base minimizes the recombination rate of minority carriers. A high recombination rate would reduce the number of minority carriers reaching the collector, thereby decreasing the transistor's current gain.

  2. Control Transistor Gain: The doping level of the base region plays a critical role in determining the transistor's current gain (β). β is the ratio of the collector current to the base current. A lightly doped base results in a higher β because a larger proportion of minority carriers injected from the emitter can diffuse to the collector without recombining. Conversely, a heavily doped base leads to a lower β due to increased recombination.

  3. Prevent Punch-Through: Punch-through is a phenomenon that can occur when the collector-base junction voltage exceeds a certain threshold. In this case, the depletion regions of the collector and base merge, leading to a significant increase in the collector current. A lightly doped base helps prevent punch-through by widening the depletion region, which increases the voltage required for punch-through to occur.