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.

Figure 5: Diode

When 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

$$\begin{eqnarray*} i = I_0 \left( e^{qv/kT} -1 \right) \end{eqnarray*}$$

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.

Figure 6: Diode's IV characteristic

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.

Figure 7: Light Emitting Diode