For LEDs, the threshold voltages are much higher and slightly different for different colours.
The reverse breakdown voltages of LEDs are low generally around 5 V. It is due to this reason, the care is taken that high reverse voltages do not appear across LEDs. (1) There is very little resistance to limit the current in LED. Therefore, a resistor must be used in series with the LED to avoid any damage due to the high current to it. (1) The semiconductor used for fabrication of visible LEDs must adeast have a band gap of 1.8 eV (spectral range of visible light is from about 0.4 |lm to 0.7 |Xm, i.e. from about 3 eV to 1.8 eV).