STRUCTURE AND FUNCTION
A white LED basically consists of six components: an LED chip, in which the light emission takes place, a bonding layer, or bonding wire connecting the LED chip to the anode, the electrical connections, that are the anode and cathode, a phosphor layer and finally a silicon lens, which surrounds the assembly.
The entire construction is only a few millimeters. The bases of the LEDs shown here are for example just 3x5mm and 3.45x3.45mm tall. It is important to note, however, that a plurality of individual LED chips needs to be used to produce a sufficient luminous flux for illumination purposes. These are then combined typically in LED modules or LED arrays. The transport costs of LEDs benefit from the small size, too.
Functionality of the LED
LEDs are electroluminescence radiators meaning they produce light not by the annealing of a body, but the body is through the application of an electrical voltage excited to emit electromagnetic radiation in the form of light.
To achieve this effect, the principle of the compound semiconductor is used in the LED. The light is generated by the recombination of charge carrier pairs. The light colour or wavelength is dependent of the semiconductor materials and their doping.
1) Passive state
The operation of semiconductors can be explained with a band model. Thus, a semiconductor can be divided into three layers: the valence band, the conduction band and the band gap. In this model the valence band represents the electrons bound to the atoms. The conduction band reflects the free electrons in the material, these are responsible for the conductivity. Between the valence and the conduction band is the band gap. This describes the resistance which the electrons must overcome to enter from the valence band into the conduction band
2) Electron absorbs thermal energy
The thermal energy of the ambient temperature is already sufficient to dissipate a few elec-trons from the valence band. In this case, the electron absorbs the thermal energy, and moves freely in the conduction band.
3) Excited state
In this state, there is an excess of electrons in the conduction band, while a hole is present in the valence band. There is talk of an excited state.
4) Recombination (light emission)
Light from an LED is caused by the recombination of a free movable electron and a hole in the valence band. In this case, the previously absorbed energy of the electron gets released in the band gap in the form of a photon.
Doping indicates a targeted modification of the material by foreign atoms. By an n-type doping additional electrons are introduced into the material. P-doping decreases the number of electrons on the other hand, holes are created.
By doping can the efficiency be improved, as well as a slight variation of the light colour be achieved.
Dimmability of LEDs
LEDs can be very precisely and over a very large range dimmed. They can be continuously regulated from their initial value to 0.1% of their light output. Also the colour of an RGB LED can be adjusted continuously and precisely.
Basically, there are two dimming methods for LEDs. On the one hand can be dimmed by controlling the current, on the other the method of pulse width modulation is used. A combination of both methods is possible, in this case first the current change and from a certain percentage the pulse width modulation is used.
Regardless of the dimming method the control of the dimming can be achieved by the common types of 1-10V or DALI method.
Dimming by current change
Since the light output of an LED is linearly dependent in a particular area on the current flowing through, the amount of light can be regulated with appropriate control devices through a change of the current strength.
This method is not suitable to achieve very low brightnesses because the voltage characteristics of LEDs differ slightly even within the same type and it may happen that some LEDs go out already, while others remain on.
Basically, the amount of light can be reduced down to about 20% by changing the current strength. Below this range at the latest, pulse width modulation needs to be used for dimming.
Dimming by pulse width modulation
With pulse width modulation, the LED is operated at rated current. To achieve the dimming effect, the light is cycled on and off at a high frequency, depending on the desired brightness. While the human eye can’t detect these short pulses individually, instead, it notices only a lower total flux.
Also the colour of an RGB LED can be controlled by pulse width modulation. Thereunto the various RGB diodes are controlled individually and as with the dimming according to the colour of your choice turned on or off. This method allows for subtle colour schemes.