LED

Light Emitting Diode.

These work in principle by adding an undoped “Intrinsic” layer between the P and N layers (which are doped). Holes from P and electrons from N recombine in the middle I layer and produces photons.

The brightness (number of photons coming out) is set by the forward current, $I_F$.

$V_F$ in LEDs is poorly controlled and dependent on LED color, so usually we ensure that $V_{DD}>V_F$ by a few volts.

Closed-loop feedback can be used provide a precise current independent of $V_F$.