Task: Text Categorization
Input (English): The Gunn type of these displays symmetric current/voltage behavior, and exhibit negative differential resistance after the threshold voltage is reached. The current passing through them varies exponentially with a forward bias and can be found with the Shockley equation, though that equation does not apply to Avalanche and Zener types of these. For 10 points, identify these circuit elements which only allow current to pass through in one direction, best known for their light emitting type.
Output (English): Science