Introduction to LED’s

Introduction

A Light Emitting Diode or LED’s, are among the most widely used of all the different types of semiconductor diodes available today. LED’s are capable of emitting a fairly narrow bandwidth of visible or invisible light when its internal diode junction attains a forward electric current or voltage. The visible lights that an LED emits are usually orange, red, yellow, or green. The invisible light includes the infrared light.

A Light Emitting Diode (LED), is basically just a specialized type of PN junction diode, made from a very thin layer of fairly heavily doped semiconductor material.

When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single color) of light. Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a colored light output. This energy is emitted in the form of heat and light. Then we can say that when operated in a forward biased direction Light Emitting Diodes are semiconductor devices that convert electrical energy into light energy.

 Construction of a Light Emitting Diode

The construction of a Light Emitting Diode is very different from that of a normal signal diode. The positive power is applied to one side of the LED semiconductor through a lead (1 anode) and a whisker (4). The other side of the semiconductor is attached to the top of the anvil (7) that is the negative power lead (2 cathode). It is the chemical makeup of the LED semiconductor (6) that determines the color of the light the LED produces. The epoxy resin enclosure (3 and 5) has three functions. It is designed to allow the most light to escape from the semiconductor, it focuses the light (view angle), and it protects the LED semiconductor from the elements. As you can see, the entire unit is totally embedded in epoxy. This is what make LEDs virtually indestructible. There are no loose or moving parts within the solid epoxy enclosure.

Light Emitting Diode Colours

So how does a light emitting diode get its color. Unlike normal signal diodes which are made for detection or power rectification, and which are made from either Germanium or Silicon semiconductor materials, Light Emitting Diodes are made from exotic semiconductor compounds such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP), Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all mixed together at different ratios to produce a distinct wavelength of colour.

Different LED compounds emit light in specific regions of the visible light spectrum and therefore produce different intensity levels. The color emitted from an LED is identified by peak wavelength (lpk) and measured in nanometers (nm). Peak wavelength is a function of the LED chip material. Although process variations are ±10 NM, the 565 to 600 NM wavelength spectral region is where the sensitivity level of the human eye is highest. Therefore, it is easier to perceive color variations in yellow and amber LEDs than other colors.

Light Emitting Diode Colour

Thus, the actual colour of a light emitting diode is determined by the wavelength of the light emitted, which in turn is determined by the actual semiconductor compound used in forming the PN junction during manufacture. Therefore the colour of the light emitted by an LED is NOT determined by the colouring of the LED’s plastic body although these are slightly coloured to both enhance the light output and to indicate its colour when its not being illuminated by an electrical supply.

By mixing together a variety of semiconductor, metal and gas compounds the following list of LEDs can be produced.

•  Gallium Arsenide (GaAs) – infra-red
•  Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange
•  Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red, orange-red, orange, and yellow
•  Gallium Phosphide (GaP) – red, yellow and green
•  Aluminium Gallium Phosphide (AlGaP) – green
•  Gallium Nitride (GaN) – green, emerald green
•  Gallium Indium Nitride (GaInN) – near ultraviolet, bluish-green and blue
•  Silicon Carbide (SiC) – blue as a substrate
•  Zinc Selenide (ZnSe) – blue
•  Aluminium Gallium Nitride (AlGaN) – ultraviolet

Connecting an LED

A LED must be connected around the correct way in a circuit and it must have a resistor to limit the current. The LED in the first diagram does not illuminate because a red LED requires 1.7v and the cell only supplies 1.5v. The LED in the second diagram is damaged because it requires 1.7v and the two cells supply 3v. A resistor is needed to limit the current to about 25mA and also the voltage to 1.7v, as shown in the third diagram.  The fourth diagram is the circuit for layout #3 showing the symbol for the LED, resistor and battery and how the three are connected. The LED in the fifth diagram does not work because it is around the wrong way.

So it’s good practice to use a series resistor R_series connected to the LED. Once the forward bias of the device exceeds, the current will increase at a greater rate in accordance to a small increase in voltage.

This shows that the forward resistance of the device is very low and the importance of using an external series current limiting resistor.

Series resistance is determined by the following equation.

R_series = (V_supply – V)/I

Where

V_supply – Supply Voltage

V – LED forward bias voltage

I – Current

The commercially used LED’s have a typical voltage drop between 1.5 Volt to 2.5 Volt or current between 10 to 50 milliamperes. The exact voltage drop depends on the LED current, colour, tolerance, and so on.

Advantages of LED’s

• Very low voltage and current are enough to drive the LED.
• Voltage range – 1 to 2 volts.
• Current – 5 to 20 milliamperes.
• Total power output will be less than 150 milliwatts.
• The response time is very less – only about 10 nanoseconds.
• The device does not need any heating and warm up time.
• Miniature in size and hence light weight.
• Have a rugged construction and hence can withstand shock and vibrations.
• An LED has a life span of more than 20 years.

Disadvantages

• A slight excess in voltage or current can damage the device.
• The device is known to have a much wider bandwidth compared to the laser.
• The temperature depends on the radiant output power and wavelength.