What is Inside an LED?
LED's are special diodes that emit light when connected in a circuit. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not. A clear (or often colored) epoxy case enclosed the heart of an LED, the semi-conductor chip.
1) by the flat side of the bulb, and
2) 2) by the shorter of the two wires extending from the LED.
The negative lead should be connected to the
negative terminal of a battery. LED's operate at relative low voltages
between about 1 and 4 volts, and draw
currents between about 10 and 40 milliamperes. Voltages and
currents substantially above these values can melt a LED chip.
The most important part of a light emitting diode (LED) is the semi-conductor
chip located in the center of the bulb as shown at the right. The chip has two regions separated by a junction. The p region is dominated by positive electric charges, and the n region is dominated by negative
electric charges. The junction
acts as a barrier to the flow of electrons between the p and the n regions. Only when
sufficient voltage is applied to the semi-conductor chip, can the current flow,
and the electrons cross the junction into the p region.
In the absence of a large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to the flow of electrons.
What Causes the LED to Emit Light and What Determines the Color of the Light?
When sufficient voltage is applied to the chip across
the leads of the LED, electrons can move easily in only one direction
across the junction between the p and n regions. In the p region
there are many more positive than negative charges. In the n region the electrons are more numerous than the positive electric
charges. When a voltage is applied and the current starts to flow, electrons in
the n region have sufficient energy
to move across the junction into the p
region. Once in the p region the
electrons are immediately attracted to the positive charges due to the mutual
Coulomb forces of attraction between opposite electric charges. When an
electron moves sufficiently close to a positive charge in the p region, the two charges
How Much Energy Does an LED Emit?
The electric energy is proportional to the voltage needed to cause electrons to flow across the p-n junction. The different colored LED's emit predominantly light of a single color. The energy (E) of the light emitted by an LED is related to the electric charge (q) of an electron and the voltage (V) required to light the LED by the expression: E = qV Joules. This expression simply says that the voltage is proportional to the electric energy, and is a general statement which applies to any circuit, as well as to LED's. The constant q is the electric charge of a single electron, -1.6 x 10-19 Coulomb.
Finding the Energy from the Voltage
Suppose you measured the voltage across the leads of an LED, and you wished to find the corresponding energy required to light the LED. Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts. So the Energy required to light the LED is E = qV or E = -1.6 x 10-19 (1.71) Joule, since a Coulomb-Volt is a Joule. Multiplication of these numbers then gives E = 2.74 x 10-19 Joule.
Finding the Frequency from the Wavelength of Light
The frequency of light is related
to the wavelength of light in a very simple way. The spectrometer can be used
to examine the light from the LED, and to estimate the peak wavelength of the
light emitted by the LED. But we prefer to have the frequency of the peak
intensity of the light emitted by the LED. The wavelength is related to the
frequency of light by , where c is the speed of light (3 x 108 m/s)