A diode has very high resistance in reverse bias (blocking current) and very low resistance in forward bias above a threshold voltage (allowing current).
For a silicon diode, the threshold voltage is approximately 0.6 V. Below this voltage, effectively no current flows even in forward bias.
The I-V characteristic graph shows almost no current in reverse, then a sharp rise above the threshold voltage in forward bias. LEDs are a type of diode that emit light when forward-biased.
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