Question

A good diode checked by a multimeter should indicate:

• A. high resistance in reverse bias and a low resistance in forward bias
• B. high resistance in both forward bias and reverse bias
• C. low resistance in both reverse bias and forward bias
• D. high resistance in forward bias and low resistance in reverse bias

Hint:

While checking a diode with a multimeter: \[ \text{Forward Bias} \Rightarrow \text{Low Resistance} \] \[ \text{Reverse Bias} \Rightarrow \text{High Resistance} \] If both readings are low, the diode is shorted. If both readings are high, the diode is open or damaged.

Answer 1

The Correct Option is A

Concept: A \(p\)-\(n\) junction diode is a semiconductor device that allows electric current to flow easily in one direction while offering a very large opposition to current in the opposite direction. This property is called unidirectional conduction and is the fundamental operating principle of a diode. When a diode is tested using a multimeter, the resistance measured depends upon whether the diode is forward biased or reverse biased.
• In forward bias, the depletion layer becomes thin and current flows easily.
• In reverse bias, the depletion layer widens and current flow becomes extremely small. Therefore, a healthy diode should show different resistance values in the two biasing conditions.

Step 1: Understand the behaviour of a diode in forward bias. In forward bias:
• The positive terminal of the battery is connected to the \(p\)-side.
• The negative terminal is connected to the \(n\)-side.
• The potential barrier decreases.
• Majority charge carriers cross the junction easily. As a result, current flows readily through the diode. Therefore, the resistance offered by the diode is very small. Hence, \[ \boxed{\text{Forward bias } \Rightarrow \text{Low resistance}} \]

Step 2: Understand the behaviour of a diode in reverse bias. In reverse bias:
• The positive terminal is connected to the \(n\)-side.
• The negative terminal is connected to the \(p\)-side.
• The depletion region width increases.
• Majority carriers are pulled away from the junction. Consequently, almost no current flows through the diode. Therefore, the diode offers a very large resistance. Hence, \[ \boxed{\text{Reverse bias } \Rightarrow \text{High resistance}} \]

Step 3: Examine each option carefully. Option (A): \[ \text{High resistance in reverse bias} \] and \[ \text{Low resistance in forward bias} \] This exactly matches the behaviour of a good diode. Therefore, Option (A) is correct. Option (B): \[ \text{High resistance in both directions} \] This indicates an open or defective diode. Hence incorrect. Option (C): \[ \text{Low resistance in both directions} \] This indicates a short-circuited diode. Hence incorrect. Option (D): \[ \text{High resistance in forward bias} \] and \[ \text{Low resistance in reverse bias} \] This is opposite to the actual diode behaviour. Hence incorrect.

Step 4: Write the final conclusion. A good diode must show \[ \boxed{\text{Low resistance in forward bias}} \] and \[ \boxed{\text{High resistance in reverse bias}}. \] Therefore, the correct answer is \[ \boxed{\text{(A)}}. \]