Question

Which of the following p-n junction diodes is forward biased?

• A. $V_p = +5\ \text{V},\ V_n = +10\ \text{V}$
• B. $V_p = -1.0\ \text{V},\ V_n = -1.5\ \text{V}$
• C. $V_p = 0\ \text{V},\ V_n = 1\ \text{V}$
• D. $V_p = -2\ \text{V},\ V_n = 0\ \text{V}$

Hint:

A diode is forward biased when its p-side is at a higher potential than its n-side.

Answer 1

The Correct Option is B

Step 1: Work with the potential difference across each diode:
Define the voltage across the diode as $V_D = V_p - V_n$. A positive $V_D$ means forward bias, and a negative $V_D$ means reverse bias.

Step 2: Compute $V_D$ for all four cases:
(A) $V_D = 5 - 10 = -5$ V.
(B) $V_D = -1.0 - (-1.5) = +0.5$ V.
(C) $V_D = 0 - 1 = -1$ V.
(D) $V_D = -2 - 0 = -2$ V.

Step 3: Read the signs:
Only (B) has a positive $V_D$. The current in a diode flows from p to n, and it flows only when the p-side is more positive.

Step 4: Note on the negative values:
A negative terminal voltage does not by itself mean reverse bias. What matters is the difference. In (B) both terminals are negative, yet the p-side is less negative, so the diode conducts.

Final Answer:
The forward biased diode is the one in option (B). \[ \boxed{\text{Option (B)}} \]