The Variation in the effective width of a base in a Bipolar Junction Transistor is due to a variation in the applied ______.

Effective Width of Base in a Bipolar Junction Transistor (BJT):

• The effective width of the base in a Bipolar Junction Transistor (BJT) can change due to a phenomenon known as the "Early effect" or "base-width modulation." This effect occurs when the base-collector voltage (V_BC) is varied, causing the width of the depletion region at the base-collector junction to change.
• When the base-collector voltage increases, the depletion region at the base-collector junction widens, effectively reducing the width of the base. This reduction in base width increases the gradient of the carrier concentration in the base, which in turn affects the current flow through the transistor.
   

Explanation of Options:

1. Emitter to collector voltage: - This is not the direct cause of the variation in the effective width of the base. The emitter-collector voltage (V_EC) primarily affects the overall voltage across the transistor but not specifically the base width.

2. Base to collector voltage: - This is correct. The variation in the base-collector voltage (V_BC) directly causes the change in the effective width of the base due to the Early effect.

3. Collector current: - While the collector current (I_C) can be influenced by the base width, it is not the cause of the base width variation. It is rather a result of the applied voltages and resulting base width modulation.

4. Emitter current: - Similar to the collector current, the emitter current (I_E) is not the cause of the base width variation but is influenced by it.

Conclusion:

• The variation in the effective width of a base in a Bipolar Junction Transistor is due to a variation in the applied: base to collector voltage