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.
What quality has LEDs been used as? What LED has the flattest I/V curve above the knee? What LEDs have a very steep I/V curve? LED forward voltage is more current-dependent than what diode? Zener diodes below what voltage are not widely available?