Question

In an unbiased p-n junction, at equilibrium, which of the following statements is true ?

• A. Diffusion current is zero but drift current exists.
• B. Diffusion current exists but drift current is zero.
• C. Diffusion and drift currents are equal and opposite.
• D. Both the diffusion and drift currents exist but are unequal.

Hint:

For an unbiased p-n junction at equilibrium, \[ I_{\text{diffusion}} = I_{\text{drift}} \] in magnitude, but they flow in opposite directions. Therefore, \[ I_{\text{net}}=0. \] Remember: \[ \boxed{\text{Equilibrium } \Rightarrow \text{ Net current } = 0} \] because diffusion current is exactly balanced by drift current.

Answer 1

The Correct Option is C

Concept: A p-n junction is formed by joining a p-type semiconductor and an n-type semiconductor. Immediately after the formation of the junction, there exists a large concentration difference of charge carriers on the two sides of the junction:
• The p-region contains a large number of holes and a very small number of electrons.
• The n-region contains a large number of electrons and a very small number of holes. Due to this concentration difference, majority charge carriers begin to move across the junction. Electrons diffuse from the n-side to the p-side, and holes diffuse from the p-side to the n-side. This movement of charge carriers due to concentration gradient produces the diffusion current. As these charge carriers cross the junction, they leave behind immobile ions near the junction, creating a region depleted of free charge carriers called the depletion region. The charged ions in this region establish an electric field directed from the n-side to the p-side. This electric field opposes the further diffusion of charge carriers and produces another current called the drift current. At thermal equilibrium and in the absence of any external bias, the junction reaches a stable condition where the two currents exactly balance each other.

Step 1: Understand the origin of diffusion current.
Because the concentration of electrons is much higher in the n-region than in the p-region, electrons diffuse from the n-side to the p-side. Similarly, because the concentration of holes is much higher in the p-region than in the n-region, holes diffuse from the p-side to the n-side. This movement of majority charge carriers gives rise to the diffusion current. Therefore, \[ I_{\text{diffusion}} \neq 0. \]

Step 2: Understand the origin of drift current.
The diffusion of charge carriers creates uncovered positive and negative ions near the junction, resulting in the formation of an electric field. This electric field causes minority charge carriers to move across the junction, producing the drift current. Hence, \[ I_{\text{drift}} \neq 0. \]

Step 3: Apply the equilibrium condition of an unbiased p-n junction.
At equilibrium, there is no net flow of charge through the junction. Therefore, the total current through the junction must be zero. Hence, \[ I_{\text{net}} = I_{\text{diffusion}} + I_{\text{drift}} = 0. \] This implies, \[ I_{\text{diffusion}} = - I_{\text{drift}}. \] Therefore, the magnitudes of the two currents are equal, but their directions are opposite. \[ |I_{\text{diffusion}}| = |I_{\text{drift}}|. \] Thus, \[ \boxed{\text{Diffusion current and drift current are equal and opposite.}} \] Hence, the correct option is \[ \boxed{\text{(C)}} \]