Light Emitting Diode (LED)

A Light Emitting Diode (LED) is a semiconductor diode that emits light when an electric current is applied in forward direction of the device as in simple LED circuit. The effect is a form of electroluminescence where incoherent and narrow-spectrum light is emitted from the p-n junction.

For optical communication systems requiring bit rates less than approximately 100-200 Mb/s together with multimode fiber-coupled optical power in tens of microwatts, semiconductor light-emitting diodes (LEDs) are usually the best light source choice. LEDs require less complex drive circuitry than laser diodes since no thermal or optical stabilization circuits are needed and they can be fabricated less expensively with higher yields.

To be useful in fiber transmission applications and LED must have a high radiance output, a fast emission response time and high quantum efficiency. To achieve a high radiance and a high quantum efficiency, the LED structure must provide a means of confining  the charge carriers and the stimulated optical emission to the active region of the pn junction where radiative recombination takes place.

The two basic LED configurations being used for fiber optics are surface emitters and    edge emitters. 

Internal Quantum Efficiency

 The internal quantum efficiency η_int is an important parameter of an LED. It is defined as the fraction of the electron-hole pairs that recombine radiatively. If the radiative recombination rate is R_r and the non-radiative recombination rate isR_nr, then the internal quantum efficiency is the ratio is the ratio of the radaitive recombination rate to the total recombination rate. η_int is typically 50% in homojunction LEDs, but ranges from 60 to 80% in double-heterostructure LEDs.

Optical Power

If the current injected into the LED is I, then the total number of recombinations per second is I/q, where q is the electron charge. Total number of radaiative recombinations is equal to (η_int I/q). Since each photon has an energy hν, the optical power generated internally by the LED is: P_int = (η_int I/q)(hν).

External Quantum Efficiency

The external quantum efficiency (η_ext)of a LED is defined as the ratio of the photons emitted from the LED to the number of internally generated photons. Due to reflection effects at the surface of the LED typical values of η_out are < 10%.

LED Characteristics

Two important characteristics of a LED are its Light intensity vs. Current and Junction Voltage vs. Current characteristics. These are described briefly below.

 i)Light Intensity (Optical Power) vs. Current

This is a very important characteristic of an LED. It was shown earlier that the optical power generated by an LED is directly proportional to the injected current I (current through the LED). However, in practice the characteristic is generally non-linear, especially at higher currents. The near-linear light output characteristic of an LED is exploited in small length fiber optic analog communication links, such as fiber optic closed-circuit TV.

ii) Junction Voltage vs. Current

The junction voltage vs. current characteristic of an LED is similar to the V-I characteristics of diodes. However, there is one major difference. The knee voltage of a diode is related to the barrier potential of the material used in the device. Silicon diodes and bipolar junction transistors are very commonly used whose knee voltage or junction voltage is about 0.7 V. Very often it is wrongly assumed that other diodes also have the same junction voltage. In an LED, depending on the material used its junction voltage can be anywhere between 1.5 to 2.2 Volts.

Light Dependent Resistor (LDR)

 An electrical current consists of the movement of electrons within a material. Good conductors have a large number of free electrons that can drift in a given direction under the action of a potential difference. Insulators with a high resistance have very few free electrons, and therefore it is hard to make the them move and hence a current to flow.An LDR or photoresistor is made any semiconductor material with a high resistance. It has a high resistance because there are very few electrons that are free and able to move - the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance.As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice and some of their energy is transferred to the electrons. This gives some of them sufficient energy to break free from the crystal lattice so that they can then conduct electricity. This results in a lowering of the resistance of the semiconductor and hence the overall LDR resistance. The process is progressive, and as more light shines on the LDR semiconductor, so more electrons are released to conduct electricity and the resistance falls further.

I-V Characteristics of LDR