While LEDs are semiconductors and need a minimum voltage to operate, they are still diodes and need to be operated in a current mode. There are two main ways to operate LEDs in DC mode. The easiest and most common is using a current limiting resistor.  The disadvantage to this method is the high heat and power dissipation in the resistor. In order for the current to be stable over temperature changes and from device-to-device, the supply voltage should be much greater than the forward voltage of the LED. In applications where the operating temperature range is narrow (less than 30°C) or the output of the LED is not critical, a simple circuit utilizing a current limiting resistor may be used as shown here.