Leds And Lasers

LEDs and LASERs March 9, 2009

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History 

The first practical LED was invented by Nick Holonyak, Jr., in 1962 while he was at General Electric Company.

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The first LEDs became commercially available in late 1960s, and were red. They were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area.

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Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, and LEDs became bright enough to be used for illumination.

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Introduction

Introduction

A Light-Emitting Diode (LED) in essence is a P-N junction solidstate semiconductor diode that emits light when a current is applied though the device. By scientific definition, it is a solid-state device that controls current without the deficiency of having heated filaments.

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Principle 

The essential portion of the Light Emitting Diode is the semiconductor chip. Semiconductors can be either intrinsic or extrinsic.

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Intrinsic semiconductors are those in which the electrical behavior is based on the electronic structure inherent to the pure material.

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Extrinsic semiconductor are those in which the electrical characteristics are dictated by impurity atoms. 4

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This chip is further divided into two parts or regions which are separated by a boundary called a junction. The pregion is dominated by positive electric charges (holes) and the n-region is dominated by negative electric charges (electrons). The junction serves as a barrier to the flow of the electrons between the p and the n-regions. This is somewhat similar to the role of the band-gap because it determines how much voltage is needed to be applied to the semiconductor chip before the current can flow and the electrons pass the junction into the p-region.

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Different Led Materials

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LED Device Structure

LED Device Structure

One way to construct a LED is to deposit three semiconductor layers on a substrate. Between p-type and n-type semiconductor layers, an active region emits light when an electron and hole recombine.

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Symbol

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cross section of traditional indicator led How does a LED work? How does a LED work?

The positive power is connected to one side of the LED semiconductor through the anode and a whisker and the other side of the semiconductor is attached to the top of the anvil or the negative power lead (cathode). 9

Mechanism 

Band-gaps determine how much energy is needed for the electron to jump from the valence band to the conduction band. As an electron in the conduction band recombines with a hole in the valence band, the electron makes a transition to a lower-lying energy state and releases energy in an amount equal to the band-gap energy.

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This energy is released in photons. Normally the energy heats the material. In a LED this energy goes into emitted infrared or visible light.

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The bandgap energy, Eg is approximately equal to the emitted photon’s energy.

where h is the Planck’s constant  h = 6.626 x 10-34 Js =4.135 x 10-15 eVs 

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If a large enough electric potential difference (voltage) is absent, across the anode and cathode, the junction serves as an electric potential barrier to the flow of electrons. When sufficient voltage is applied across the chip of the LED, the electron has enough driving force to move in one direction over the junction that separates the p-region and the n-region. 11

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The electrons from the n-region basically flow across the junction into the p-region. In the p-region, the electrons are attracted to the positive charges due the mutual Coulombic forces of attraction between opposite charges of same magnitude. Thus “recombination” occurs.

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After every successful recombination, electric potential energy is transformed into electromagnetic energy. This releases a quantum electromagnetic energy that is emitted in the form of a photon of light with frequencies characteristic of the semiconductor that was used in the process.

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LED Radiation Patterns LED Radiation Patterns

A LED is a directional light source, with the maximum emitted power in the direction perpendicular to the emitting surface. The typical radiation pattern shows that most of the energy is emitted within 20° of the direction of maximum light. Some packages for LEDs include plastic lenses to spread the light for a greater angle of visibility. 13

LED Types 

There are two basic types of LED structures: edge emitters and surface emitters.

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Edge emitters are more complex and expensive devices, but offer high output power levels and high speed performance. The output power is high because the emitting spot is very small, typically 30-50 µm, allowing good coupling efficiency to similarly sized optical fibers. Edge emitters also have relatively narrow emission spectra.

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Surface emitters have a comparatively simple structure, are relatively inexpensive, offer low-to-moderate output power levels, and are capable of low-to-moderate operating speeds. The total LED chip optical output power is as high or higher than the edge-emitting LED, but the emitting area is large, causing poor coupling efficiency to the optical fiber. 15

LED Characteristics

LED Characteristics

When a LED is forward biased to the threshold of conduction, its current increases rapidly and must be controlled to prevent destruction of the device. The light output is quite linearly proportional to the current within its active region, so the light output can be precisely modulated to send an undistorted signal through a fiber optic cable.

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Led Characteristics 

It does not have any moving parts, which makes the device extremely resistant to damage due to vibration and shocks.

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These characteristics make it ideal for purposes that demand reliability and strength.

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LEDs are highly monochromatic, only emitting a single pure color in a narrow frequency range. The color emitted from an LED is identified by peak wavelength (lpk) which is measured in nanometers (nm). The peak wavelength is a function of the material that is used in the manufacturing of the semiconductor. 17

The relative response time versus different wavelengths of light

(The lower the response time the better. Currently, most LEDs are made with higher wavelengths (i.e. longer response time) because they are cheaper to manufacture.)

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Light Emitting Diodes I-V Characteristics

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