12-12-2012, 05:52 PM
LIGHT EMITTING DIODE – Materials Issues and Selection
LED2.ppt (Size: 1.75 MB / Downloads: 101)
At the end of this lecture you would be able to
Cite semiconductor materials suitable for LED of different colours (red, yellow, green, blue, white)
Describe the GaAsP system as an example of ternary compounds
Use the knowledge of band gap engineering to design LED material to emit suitable coloured lights
Discuss the current phenomenon in LED research activities
What is GaAs(1+x) Px?
GaAs(1+x) Px is a ternary compound based on GaAs and GaP
GaAs is a direct gap semiconductor and GaP is indirect semiconductor
When alloyed, there is cross over point where GaAs(1+x) Px will transformed from being direct gap material to indirect gap material
Red, yellow and orange coloured LED can be made with GaAs(1+x) Px
Typical Exam Question!!!!
The figure below shows the quantum efficiencies of GaAsP based LED as a function of alloy composition with and without nitrogen doping. Explain why the additional of nitrogen leads to such dramatic changes in the quantum efficiencies of the device. Why is this phenomenon important from a practical point of view? (100 marks +5 bonus)
IR & Red LED
GaAs direct band gap, p-n junctions are readily formed with high radiative efficiency. High radiative efficiency can be induced by doping GaAs with Zn or Si. Si doped GaAs is now the industry standard near IR LEDs.
GaAsP direct – in direct transition
GaInAsP Grown on InP substrate and band gap can be varied to get wavelength from 919nm to 1600nm. A true story of band gap engineering.
LED + band gap engineering
“LEDs are specialized semiconductor devices that can potentially convert electricity to light, without the wasteful creation of heat. The color emitted is controlled in large part by the energy gap of the semiconductor and in advanced structures by the “photonic band gap,” a range of wavelengths that cannot travel through that particular substance. By suppressing certain wavelengths and enhancing others, the band gap determines the color.”