THE LIGHT EMITTING DIODE[发光二极管](PPT-51).ppt

GaAs GaP 1.90 eV GaAs GaP 2.00 eV GaAs GaP 2.26 eV _ + Energy Momentum Addition of a nitrogen recombination center to indirect GaAsP . Both As and P are group V elements. (Hence the nomenclature of the materials as III-V compound semiconductors.) We can replace some of the As with P in GaAs and make a mixed compound semiconductor GaAs1-xPx. When the mole fraction of phosphorous is less than about 0.45 the band gap is direct, and so we can engineer the desired color of LED that we want by simply growing a crystal with the proper phosphorus concentration! X CB Minimum Γ VB Maximum N Level Γ CB Minimum (a) Direct-gap GaAs N Level (b) Crossover GaAs0.50P0.50 N Level (c) Indirect-gap GaP Schematic band structure of GaAs, GaAsP, and GaP. Also shown is the nitrogen level. At a P mole fraction of about 45-50 %, the direct-indirect crossover occurs. Materials for visible wavelength LEDs We see them almost everyday, either on calculator displays or indicator panels. Red LED use as “ power on” indicator Yellow, green and amber LEDs are also widely available but very few of you will have seen a blue LED. Red LEDs can be made in the GaAsP (gallium arsenide phosphide). GaAs1-xPx for 0x0.45 has direct-gap for x0.45 the gap goes indirect and for x=0.45 the band gap energy is 1.98 eV. Hence it is useful for red LEDs. N-GaAs substrate N-GaAsP P = 40 % p-GaAsP region Ohmic Contacts Dielectric (oxide or nitride) Fig. GaAsP RED LED on a GaAs sub. Isoelectronic Centre Isoelectronic means that the centre being introduced has the same number of valance electrons as the element it is replacing. For example, nitrogen can replace some of the phosphorus in GaP. It is isoelectronic with phosphorus, but behaves quite differently allowing reasonably efficient green emission. How isoelectronic centres work? For our isoelectronic centre the position is very well-defined, hence there is a considerable spread in its momentum state. Isoelectronic centre has the same valance configuration as