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CSE- Module-3: SEMICONDUCTOR LIGHT EMITTING DIODE :LED (5Lectures)

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CSE- Module-3: SEMICONDUCTOR LIGHT EMITTING DIODE :LED (5Lectures) CSE-Module- 3:LED PHYSICS CSE- Module-3: SEMICONDUCTOR LIGHT EMITTING DIODE :LED (5Lectures) Light Emitting Diodes (LEDs) Light Emitting Diodes (LEDs) are semiconductors p-n junction operating under proper forward biased conditions and are capable of emitting external spontaneous radiations in the visible range (370 nm to 770 nm) or the nearby ultraviolet and infrared regions of the electromagnetic spectrum General Structure LEDs are special diodes that emit light when connected in a circuit. They are frequently used as “pilot light” in electronic appliances in to indicate whether the circuit is closed or not. The structure and circuit symbol is shown in Fig.1. The two wires extending below the LED epoxy enclose or the “bulb” indicate how the LED should be connected into a circuit or not. The negative side of the LED is indicated in two ways (1) by the flat side of the bulb and (2) by the shorter of the two wires extending from the LED. The negative lead should be connected to the negative terminal of a battery. LEDs operate at relative low voltage between 1 and 4 volts, and draw current between 10 and 40 milliamperes. Voltages and Fig.-1 : Structure of LED current substantially above these values can melt a LED chip. The most important part of a light emitting diode (LED) is the semiconductor chip located in the centre of the bulb and is attached to the 1 CSE-Module- 3:LED top of the anvil. The chip has two regions separated by a junction. The p- region is dominated by positive electric charges, and the n-region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and n-regions. Only when sufficient voltage is applied to the semi-conductor chip, can the current flow and the electrons cross the junction into the p-region. In the absence of large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to flow of electrons. LED: p-n Junction When an n-type semiconductor is brought into the physical contact with a p-type semiconductors, a p-n junction is created. At the boundary of the junction, electrons from the n-side diffuse to the p-side and recombine with holes and at the same time, holes from the p-side diffuse to the n-side and recombine with electrons. Thus, a finite region called the depletion or space charge region forms. Here there are no mobile electrons or holes. Since positive ions at the n-side and negative ions at the p-side within the depletion region are left without electrons or holes, these ions create an internal electric field called a contact potential. This potential is noted by VD in Fig. 2(a). Fig.-2 : Light radiation by the p-n junction of a semiconductor (a) Depletion region and Depletion voltage VD (b) Light radiation as the result of electron-hole recombinations. 2 CSE-Module- 3:LED To radiate the quantum of light-photon it is necessary to sustain electron- hole recombinations. But the depletion voltage prevents electrons and holes from penetrating into a voltage is called as forward biasing voltage, V as shown in Fig. 2 (b). Emission of Light When sufficient voltage is applied to the semiconductor chip across the leads of the LED, electrons can move easily in one direction across the junction between the p and n-regions. In the p-region there are many more positive than negative charges. In the n-region the electrons are more numerous than the positive electric charges. When a voltage is applied and the current starts to flow, electrons in the n- region have sufficient energy to move across the junction into the p-region. Once in the p-region the electrons are immediately attracted to the positive charges due to the mutual coulomb forces of attraction between opposite electric charges. When an electron moves sufficiently close to a positive charge in the p- region, the two charge “re-combine”. Each time an electron recombines with hole, electric potential energy is converted into electromagnetic energy. For each electron-hole recombination, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semiconductor material (usually a combination of the chemical elements gallium, arsenic and phosphorus). Only photons in a very narrow frequency range can be emitted by any material. LEDs that emit different colours are made of different semiconductor materials and require different energies to light them, as given below : GaAs infra red radiation (invisible) GaP red or green light GaAsP red or yellow light 3 CSE-Module- 3:LED Gallium Arsenide has much probabilities of radiative recombination than other semiconductor such as Ge or Si. Energy of Emission The electric energy is proportional to the voltage needed to cause electrons to flow across the p-n junction. The different coloured LEDs emit predominantly light of a single colour. The energy of the light emitted by an LED is given by E = electric charge ×voltage required to light the LED E = qV Joules In one knows the voltage across the leads of an LED, one can find the corresponding energy required to light the LED. Let us say that one has a red LED, and the voltage measured between the leads is 1.65 volts. So the energy required to light the LED is E = qV = 1.6×10-19 (1.65) Joules = 2.64×10-19 joule Difference between an LED and a Regular diode The difference between an LED and a regular diode is that in a regular diode the electron-hole recombinations release energy in the thermal rather than the visible portion of the spectrum. Due to this reason, the electronic devices warm up when you turn on. In an LED, however, these recombinations result in the release of radiation in the visible, or light, part of the spectrum. The first type of recombination is called nonradiative while the second type is called radiative recombination. In a diode both types of recombinations take place, in case of LED the majority of recombinations are radiative. 4 CSE-Module- 3:LED Radiated Power and Input-Output Characteristic When forward biased voltage is applied to the leads of LED, the forward current injects electrons into the depletion region, where they recombine with holes in radiative and nonradiative ways. The nonradiative recombinations take excited electrons from radiative recombinations, this results in the decrease in the efficiency of the process which is characterized by the internal quantum efficiency, int, (a) (b) Fig.-3 : (a) Electronic circuit of an LED under operation and (b) input-output characteristic. Quantum Efficiency (int) and LED Power The internal quantum efficiency in the active region is fraction of the electron-hole pairs that recombine radiatively. If the radiative recombination rate is Rr and the nonradiative recombination rate is Rnr, then the internal quantum efficiency int is the ratio of the radiative recombination rate of the total recombination rate Rr int (1) Rr Rnr 5 CSE-Module- 3:LED For exponential decay of excess carriers, the radiative recombination n lifetime is r and the nonradiative recombination lifetime is Rr n nr . Rnr Thus the internal quantum efficiency can be expressed as n r int n r n nr 1 r int 1 r 1 nr int (2) r where is the bulk recombination life time, given by the expression : 1 1 1 r nr The power of radiated light as a function of forward current can be plotted in the form of a graph known as input-output characteristic of an LED, shown in Fig.-3(b). From figure, it is clear that the greater the forward current, the greater the number of electrons that will be excited at the conduction band and the grater the number of photons (light) that will be emitted. Hence radiated light power of an LED is directly proportional to the forward current. However, for higher values of forward current one expects the saturation effect as shown by dotted line in Fig.-3(b). This takes at the point where all the available mobile electrons will be involved in radiation and further increasing the current value, will not produce additional photons hence the emitted light. Light Power, P (energy per second), is the number of photons (np) times the energy of an individual photon, Ep 6 CSE-Module- 3:LED n p E p P (3) t where, np Nint where, N is the number of excited (injected) electrons. Nint E p P (4) t Ne Also current I (5) t Putting the value of N from equation (5) in (4), we get. Hence the radiated light power is It int E p P e t int E p P I (6) e If E p is measured in electron volts (eV) and I in milliamperes (mA), the PmW intEp eV ImA (7) To find the emitted power, one needs to consider the external quantum efficiency (ext ). This is defined as the ratio of the photons emitted from the LED to the number of internally generated photons. This can be expressed as 1 c T 2 sin d ext (8) 4 0 7 CSE-Module- 3:LED where c is the critical angle, in incidence angle, T() is the Fresnel co- efficient or Fresnel transmissivity. The critical angle is given as n 1 2 c sin (9) n1 where n1 is the refractive index of the semiconductor material and n2 is the refractive index of the outside material.
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