Led basic structure & light-emitting principle

The basic structure:

The traditional LED chips are fixed in the conductive, thermal conductivity with two lead metal bracket, a reflective cup is the negative lead wire, the other a positive lead wire . The chip seal to the epoxy resin, on the one hand it can protect the chip, on the other hand has focusing function like lens. If the LED two lead wire aren’t as long as, the longer is positive.

But if both are the same long, the positive lead wire is near the tube bulge. So we can know the LED lights mainly are composed of the bracket, silver plastic, gold wire and epoxy resin, etc. As the picture shown that the traditional DIP LED , high power COB LED and SMD LED.

DIP LED

dip-led

high power COB LED

cob-ledSMD LED

smd-led

The chip and packaging will develop the high-power direction, in the high current condition can produce 10-20 times luminous flux larger than 5mm  diameter LED, must adopt effective heat dissipation and non-degraded packaging materials to solve the light failure problem, therefore, the shell and the packaging technique is also key technology.

The thermal characteristic of power LED directly affect the LED working temperature, luminous efficiency, luminous wavelength, using lifespan, therefore, the packaging design and manufacturing technology is particularly important for the power LED chip.

The LED chip is the core of LED devices, The LED chip is a layered structure, the chip ends are metal electrodes; the bottom is a substrate material , the middle is a PN junction composed of a P-type layer and an N-type layer, the light-emitting layer is sandwiched between the P layer and N layer, which is the core area of light.

led-strcuction

The p-type layer, the n-type layer and the luminescent layer are made of a special epitaxial growth process on the substrate material. During the chip operation, the p-type layer and the n-type layer respectively provide holes and electrons required for light emission, and they are injected into the light-emitting layer to produce lights.

led-packaging

As shown in the figure is the LED packaging technology.However, the actual chip structure is much more complicated than that. LED chip production technology is one of the 21st century high-tech.

The basic light-emitting principle:

The LED is a kind of light-emitting device that will be directly injected current, it is internal stimulated electronic in semiconductor crystal return from high level to low level, and it’s normally called spontaneous emission transition during this process.

When the LED’s PN junction will be added positive bias voltage, the electrons are injected from N to P, and the holes are injected from P to N , the injected minority carrier and majority carrier can recombine and shine, as shown in figure 1-7. Assume that the light is occurred in the P area, then injected electrons and holes can directly composite led the light, or be captured by light center firstly, next combine with holes led the light.As picture,

basic-working-principle

Besides this light composite, some electrons are captured by non-luminescent center, and then recombine with holes, but each release of energy isn’t high so that cannot form a visible light. A great proportion of the light emitting compound than the non-composite, the quantum efficiency is higher. Because of the composite lights in the minority carrier diffusion area, so it merely produce light near the PN junction.

Proved by theory and practice, the light’s peak wavelength and light-emitting area are relatively to the semiconductor materials forbidden band width, then the visible light whose wavelength is 380(purple light)-780mm(red light). Semiconductor materials should be 3.26-1.63ev,It’s noteworthy that a large number of particles at high energy levels spontaneously emit a series of light waves, each of which has no fixed phase relationship, can have different polarization directions, and each particle emits light along all possible direction propagation, a process known as spontaneous emission.