LIGHT EMITTING DIODE – Design Principles EBB424 USM SMMRE LIGHT EMITTING DIODE – Design Principles Dr Zainovia Lockman L2

1907 Publication report on Curious Phenomenon On applying a potential to a crystal of carborundum (SiC), the material gave out a yellowish light H.J. Round, Electrical World, 49, 309, 1907

3 Lectures on LED OBJECTIVES: To learn the basic design principles of LED To relate properties of semiconductor material to the principle of LED To be able select appropriate materials for different types of LED To be able to apply knowledge of band gap engineering to design appropriate materials for a particular LED To acknowledge other materials that can and have been used in LED

4 Main Issues The device configuration Materials requirements Materials selection Material issues

By the end of this lecture you must be able to … Draw a typical construction of an LED. Explain your drawing. State all the issues regarding the materials selection of an LED. State all of the possible answers regarding your materials issues. Explain band gap engineering Explain the isoelectronic doping in GaAsP system State examples of materials that emit, UV, Vis, IR lights

Semiconductors bring quality to light! What is LED? Semiconductors bring quality to light! LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The qaunta of light energy released is approximately proportional to the band gap of the semiconductor.

Applications of LEDs

Your fancy telephone, i-pod, palm pilot and digital camera

Getting to know LED Advantages of Light Emitting Diodes (LEDs) Longevity: The light emitting element in a diode is a small conductor chip rather than a filament which greatly extends the diode’s life in comparison to an incandescent bulb (10 000 hours life time compared to ~1000 hours for incandescence light bulb) Efficiency: Diodes emit almost no heat and run at very low amperes. Greater Light Intensity: Since each diode emits its own light Cost: Not too bad Robustness: Solid state component, not as fragile as incandescence light bulb

LED chip is the part that we shall deal with in this course

Luminescence is the process behind light emission Luminescence is a term used to describe the emission of radiation from a solid when the solid is supplied with some form of energy. Electroluminescence  excitation results from the application of an electric field In a p-n junction diode injection electroluminescence occurs resulting in light emission when the junction is forward biased

Excitation E Electron (excited by the biased forward voltage) is in the conduction band k Normally the recombination takes place between transition of electrons between the bottom of the conduction band and the top of the valance band (band exterma). The emission of light is therefore; hc/ = Ec-Ev = Eg(only direct band gap allows radiative transition) Hole is in valance band

How does it work? Recombination produces light!! P-n junction Electrical Contacts A typical LED needs a p-n junction There are a lot of electrons and holes at the junction due to excitations Electrons from n need to be injected to p to promote recombination Recombination produces light!! Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen

Injection Luminescence in LED Under forward bias – majority carriers from both sides of the junction can cross the depletion region and entering the material at the other side. Upon entering, the majority carriers become minority carriers For example, electrons in n-type (majority carriers) enter the p-type to become minority carriers The minority carriers will be larger  minority carrier injection Minority carriers will diffuse and recombine with the majority carrier. For example, the electrons as minority carriers in the p-region will recombine with the holes. Holes are the majority carrier in the p-region. The recombination causes light to be emitted Such process is termed radiative recombination.

Recombination and Efficiency eVo Eg p n+ h =Eg (a) (b) Electrons in CB Holes in VB EC EF EV Ideal LED will have all injection electrons to take part in the recombination process In real device not all electron will recombine with holes to radiate light Sometimes recombination occurs but no light is being emitted (non-radiative) Efficiency of the device therefore can be described Efficiency is the rate of photon emission over the rate of supply electrons

Emission wavelength, g The number of radiative recombination is proportional to the carrier injection rate Carrier injection rate is related to the current flowing in the junction If the transition take place between states (conduction and valance bands) the emission wavelength, g = hc/(EC-EV) EC-EV = Eg g = hc/Eg

Calculate If GaAs has Eg = 1.43ev What is the wavelength, g it emits? What colour corresponds to the wavelength?

Construction of Typical LED Light output SiO2 p n Electrical contacts Substrate

LED Construction Efficient light emitter is also an efficient absorbers of radiation therefore, a shallow p-n junction required. Active materials (n and p) will be grown on a lattice matched substrate. The p-n junction will be forward biased with contacts made by metallisation to the upper and lower surfaces. Ought to leave the upper part ‘clear’ so photon can escape. The silica provides passivation/device isolation and carrier confinement

Efficient LED Need a p-n junction (preferably the same semiconductor material only different dopants) Recombination must occur  Radiative transmission to give out the ‘right coloured LED’ ‘Right coloured LED’  hc/ = Ec-Ev = Eg  so choose material with the right Eg Direct band gap semiconductors to allow efficient recombination All photons created must be able to leave the semiconductor Little or no reabsorption of photons

Materials Requirements EBB424 USM SMMRE Correct band gap Direct band gap Materials Requirements Efficient radiative pathways must exist Material can be made p and n-type Dr Zainovia Lockman L2

Direct band gap materials  UV-ED  ~0.5-400nm Eg > 3.25eV  LED -  ~450-650nm Eg = 3.1eV to 1.6eV  IR-ED-  ~750nm- 1nm Eg = 1.65eV Direct band gap materials e.g. GaAs not Si Candidate Materials Materials with refractive index that could allow light to ‘get out’ Readily doped n or p-types

Typical Exam Question Describe the principles of operation of an LED and state the material’s requirements criteria to produce an efficient LED. (50 marks)

Visible LED Violet ~ 3.17eV Blue ~ 2.73eV Green ~ 2.52eV Definition: LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = 390- 770nm. This coincides with the energy value of 3.18eV- 1.61eV which corresponds to colours as stated below: The band gap, Eg that the semiconductor must posses to emit each light Violet ~ 3.17eV Blue ~ 2.73eV Green ~ 2.52eV Yellow ~ 2.15eV Orange ~ 2.08eV Red ~ 1.62eV Colour of an LED should emits

Electromagnetic Spectrum The appearance of the visible light will be the results of the overlap integral between the eye response curve and the spectral power of the device  the peak of the luminous curve will not in general be the same as the peak of the spectral power curve V ~ 3.17eV B ~ 2.73eV G ~ 2.52eV Y ~ 2.15eV O ~ 2.08eV R ~ 1.62eV Visible lights

Candidate Materials for LED’s

Question 1 Indicate the binary compounds that can be selected for red, yellow, green and blue LED.

Candidate Materials Group III-V & Group II-VI Group IV Group V iv iii v N P As ii Al Ga In Periodic Table to show group III-V and II-V binaries

Group III-V (1950) The era of III–V compound semiconductors started in the early 1950s when this class of materials was postulated and demonstrated by Welker (1952, 1953). The class of III–V compounds had been an unknown substance prior to the 1950s that does not occur naturally. The novel man-made III–V compounds proved to be optically very active and thus instrumental to modern LED technology.

Group III-V LED materials Ga In N P As AlN, AlP,AlAs GaN, GaP, GaAs InN, InP, InAs Binary compounds GaP GaAl GaAsP GaAsAl Ternary compounds GaAs Questions to ask when choosing the right material: Can it be doped or not? What wavelength it can emit? Would the material able to allow radiative recombiation? Direct or indirect semiconductor?