Question

For an unbiased Silicon \(n\text{-}p\text{-}n\) transistor in thermal equilibrium, which one of the following electronic energy band diagrams is correct? (\(E_c\) = conduction band minimum, \(E_v\) = valence band maximum, \(E_F\) = Fermi level.) 

 

• A. (A)
• B. (B)
• C. (C)
• D. (D)

Hint:

At thermal equilibrium in any semiconductor junction, the Fermi level remains constant throughout the material.

Answer 1

The Correct Option is B

Step 1: Condition for thermal equilibrium.
In thermal equilibrium, no net current flows, and therefore the Fermi level \(E_F\) remains constant throughout the device. Any variation in conduction (\(E_c\)) and valence (\(E_v\)) bands must adjust to maintain this constant Fermi level.

Step 2: Behavior across the junctions.
In an \(n\text{-}p\text{-}n\) transistor, at equilibrium: - \(E_c\) is higher in the \(p\)-region and lower in the \(n\)-region. - \(E_v\) follows the same curvature but in the opposite sense. - \(E_F\) remains flat across the entire structure.

Step 3: Conclusion.
The correct band diagram is the one showing \(E_F\) constant and smooth bending of \(E_c\) and \(E_v\), corresponding to Option (A).