Semiconductor Diode

Concept: A PN junction diode allows current mainly in one direction:

Forward bias → conducts easily (low resistance)
Reverse bias → blocks current (high resistance)
This property is used to test diodes using a multimeter.
Step 1: Forward bias behaviour. When the diode is forward biased:

Depletion region narrows
Current flows easily
Multimeter shows low resistance

Step 2: Reverse bias behaviour. When reverse biased:

Depletion region widens
Very small leakage current
Multimeter shows high resistance

Step 3: Conclusion. A good diode must:

Conduct in forward bias (low resistance)
Block in reverse bias (high resistance)