 What type of bias is required for an LED to emit light? Before a light emitting diode can “emit” any form of light it needs a current to flow through it, as it is a current dependent device with their light output intensity being directly proportional to the forward current flowing through the LED.

What is a light-emitting diode?
A diode is a two-terminal semiconductor device. It can be thought of as an electronic valve that only allows current to flow in one direction. The symbol for the diode is shown in the lefthand picture of figure 5. The symbol is shaped like an arrow that indicates the direction in which current may flow.

The terminal marked with a positive sign is called the anode and the terminal marked with a negative sign is called the cathode. The righthand picture depicts the physical device. It looks similar to a resistor except that it has a single band on one end. In a forward biased diode, the current will flow from the end without a band to the end of the cylinder with the band.

hen the voltage $v$ is positive and greater than a minimum threshold voltage $V_t$, then the diode is said to be forward biased. A forward biased diode will conduct current $i$, in the direction shown in the figure. If a diode is not forward biased, then we say it is reverse biased. A reverse biased diode will also conduct a current that has the opposite sense of that shown in figure 5.

This reverse current, however, will be extremely small so that the forward biased diode is seen as conducting, whereas the reverse biased diode is seen as not conducting.

As with the resistor, the diode is completely characterized once we know the relationship between the voltage and current. The diode’s IV characteristic satisfies the following equation

where $q$ is the charge of an electron, $k$ is Boltzmann’s constant ( $1.381 \times 10^{-23}$ J/K), and $T$ is the material’s temperature (Kelvin). The reference current $I_0$ is usually very small, on the order of $10^{-9}$ or $10^{-15}$ amperes. Plotting this function leads to the IV characteristic shown in the lefthand graph of figure 6. Note that this graph is actually the V-I curve since it shows how current varies as a function of voltage.

The lefthand plot in figure 6 has three distinct operating regions. The forward bias region corresponds to those positive voltages that are above a specified threshold level.

The threshold voltage, $V_T$, is a function of the physical properties of the semi-conductor material. Common values for this threshold voltage lie between $0.6$ and $1.4$ volts. For voltages that lie below this threshold, the diode essentially stops conducting. There is a small leakage current that is on the order of $I_0$. But as noted earlier this current is extremely small. If we further decrease the voltage, then we enter another region of operation known as the breakdown region.

We generally operate a diode in either its forward or reverse biased modes. In particular, we usually idealize this behavior so we can think of the diode as a valve that is open when $v$ is greater than the threshold voltage $V_T$ and is closed otherwise. These considerations lead to the simplified I-V characteristic that is shown in the righthand graph of figure 6. In this simplified plot, we see that the reverse bias region is idealized so that zero current is passed in this region if $v < V_t$. If the diode is forward biased, then the current is potentially unbounded, which means that the diode behaves like a short circuit. In other words, a forward biased diode behaves like a short circuit and a reverse biased diode acts like an open circuit.

An LED is a light emitting diode. The LED emits light when it is forward biased and it emits no light when it is reverse biased. The intensity of light is proportional to the square of the current flowing through the device. Figure 7 shows a picture of an LED. Note that LEDs have two leads.

One lead is longer than the other. These leads are used to indicate which end of the diode is positive (anode) and which is negative (cathode). In many cases the longer lead is the anode, but you can easily test this by connecting the LED to a battery and seeing which orientation causes the LED to light up.

Why does LED not operate under reverse bias?

LED is the short form of light emitting diode. A diode is a piece of semiconductor, which is divided into two parts; these two parts are doped with two different chemicals, which makes one part positive and the second part negative. This is called a P-N junction and in our case, a diode. Now, current can only flow through this positive-negative “junction” /diode when:

When you collect the positive terminal of a battery to the positively charged “p” part of the P-N junction, and the negative terminal to the negatively charged “n” part of the P-N junction. It makes a path for the electrons to flow when you do that. Vice-verca and the diode will “block” the current, basicly – open the circuit. “It is like a valve which lets electricity flow in only one way” – ElectroBoom

(Continued) You use a Zener Diode. But let’s not complicate matters. You don’t know what that is. Maybe watch a video or two about diodes – Simply Electronics is a great channel for learning these basics.

So yeah, an LED is just a diode that gives out light when + voltage is applied at its anode and a – voltage at its cathode. Using the rules of a diode which are mentioned above, you can see that reverse biasing an LED won’t light that LED up. And, please don’t apply 5 volts from an Arduino to a 3 volt LED. Some may handle it but some just blow up in a milli-second.