Context: LEDs have also been used as a medium-quality voltage reference in electronic circuits. The forward voltage drop (e.g. about 1.7 V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available.

Question: What quality has LEDs been used as?
Answer: medium-quality

Question: What LED has the flattest I/V curve above the knee?
Answer: Red

Question: What LEDs have a very steep I/V curve?
Answer: Nitride-based LEDs

Question: LED forward voltage is more current-dependent than what diode?
Answer: Zener

Question: Zener diodes below what voltage are not widely available?
Answer: 3 V

Question: What quality hasn't LEDs been used as?
Answer: Unanswerable

Question:  What LED has the least flattest I/V curve above the knee?
Answer: Unanswerable

Question: What LEDs have a not very steep I/V curve?
Answer: Unanswerable

Question: What LEDs don't have a very steep I/V curve?
Answer: Unanswerable