Figure 1: a) examples of LEDs; b) circuit symbol An inorganic Light-Emitting Diode (LED; Figure 1) is a diode which is forward-biased (switched on). By doing so electrons are able to recombine with holes within the vicinity of the junction (depletion region) thereby releasing energy in the form of light (photons) as shown in Figure 2 a). One basic requirement for the used semiconductor is to be a direct band gap semiconductor. This means that the maximum of the valence-band and the minimum of the conduction band meet at the same k-value. Figure 2 b) and c) show the 2 types of semiconductors. Since Si is an indirect band-gap semiconductor no photons are emitted by Si-based p-n-junctions. Figure 2: a) forward biased p-n-junction b) indirect band-gap semiconductor; c)direct Figure 3: a) band diagram in a heterojunction LED; b) schematic display of a double heterostructure LED Figure 3 a) shows the band diagram of an improved LED where a heterojuncion is used. The figure shows that the central material (where the light is generated) is surrounded by a region with a higher energy gap. This allows confining the injected charge carriers inside the un-doped central region with a lower band-gap thereby increasing the emissive radiation rate. Figure 3 b) shows the configuration of a double heterorstructure LED. This type of confinement is also utilized for semiconductor lasers. The usage of a heterojunction is further advantageous to obtain a higher output of light out of the LED since the generated photons don’t have enough energy to generate an electron-hole pair within the semiconductor with wider band-gap. Figure 4: examples of types of semiconductors used for LED fabrication & spectral sensitivity of the human eye below LEDs are usually produced on a sapphire substrate from III-V semiconductors where often a gallium-based compound is used (see Figure 4). A characteristic feature of LEDs is that they emit light of only one certain wavelength (with a typical width of 5nm – 20nm) which is determined by the width of the band gap. In order to obtain white LEDs (e.g. used as car headlights of recently released cars) one has to modify LEDs which happens in two ways: 1. Mix LEDS with different colors to make up white light (e.g. large screens) 2. use a phosphorous material to convert monochromatic light from a blue or UV LED to broaden the spectrum to white light (similar how a fluorescent light bulb works) After the generation of light a more severe problem is the out-coupling of the light out of the semiconductor. 3 difficulties hinder the out-coupling of photons (which are generated with random direction inside the semiconductor): 1. Absorption within the LED: as already mentioned the usage of heterojunctions reduces this problem 2. Reflection: usage of a reflective mirror on the backside of the LED-chip (see Figure 5) 3. Total internal reflection: the usage of anti-reflection coatings (with a refractive index closer to the one of air) can reduce this effect (see also: Kim et. al.; Advanced Functional Materials: doi: 10.1002/adma.200701015) Figure 5: LED with a reflective contact on the backside and total internal reflection Usually LEDs are encapsulated (see Figure 1 a & Figure 6) to protect them from ambient conditions. The encapsulation is also utilized (by using mirrors & lenses) to shape the spatial light emission and to provide a heat-sink. Figure 6: packaged LED with plastic lens, heat-sink and connection wires This is very important since LEDs are diodes which have an exponential increase of current with increasing temperature (which can lead to thermal destruction of the device). The electrical supply to be done by a constant current power-source to avoid this. Sources: Figure 1: (both): Wikipedia/light emitting diode) Figure 2: a) & b) lecture notes of Prof. Hadley (2011); c) Physics of semiconductor devices by Sze (3rd edition; Wiley 2006) Figure 3: a) Physics of semiconductor devices by Sze (3rd edition; Wiley 2006); b) www.LightEmittingDiodes.org Figure4: Physics of semiconductor devices by Sze (3rd edition; Wiley 2006) Figure 5: lecture notes of Prof. Hadley (2011) Figure 6: www.LightEmittingDiodes.org .