Transcript Class18.ppt
Putting Electrons to Work Doping and Semiconductor Devices N-type semiconductors • N-type semiconductor is doped with a material having extra valance electrons • Result is filled energy states in the band gap just below the conduction band • These electrons can easily gain energy to jump to the conduction band and move through the material P-type semiconductors • P-type semiconductor is doped with a material having fewer valance electrons • Result is “holes”, or empty energy states in the band gap just above the valance band • Since no single electron travels through the material, we describe the charge carrier as a positive hole moving the other way Doped Semiconductors Energy donor level acceptor level n-type p-type •Put them together? p-n junction Energy + + + + + + + + -- - - p-type n-type depleted region (electric field) - - - - P-n junction • As more electrons from the n-side combine with holes from the p-side, each additional combination adds to the potential difference across junction • This can be envisioned as shifting the energy bands, making it harder for electrons to travel across the barrier p-n junction Energy + + + + + + + Vo + -- - - p-type n-type depleted region (electric field) - - - - P-n junction • Originally both p and n sides are electrically neutral • Electrons in n side see holes in p side and combine Second electron needs add’l energy to get over charge barrier – can represent as rise in energy levels of p section Forward Biasing • Eventually, the potential difference is so large, electrons cannot travel across it without gaining energy • Applying a forward bias decreases the potential difference so current can flow Reverse Biasing • Applying a reverse bias will increase the barrier rather than decreasing it, so no current flows Light-emitting Diode • When an electron loses energy to recombine with a hole, it can emit that lost energy in the form of light. • This light always has roughly same E, so LEDs emit small range of wavelengths This light-emitting property of p-n junctions can be utilized to create a laser Be sure to come to class to hear Dr. Schowalter say . . . a Philips Lighting and Agilent Technologies joint venture that's changing the future of light. In the next century, LED-based lighting will quickly replace conventional lighting in a wealth of commercial, industrial and consumer applications. LumiLeds‘ LED-based solutions will bring irresistible value to lighting solutions of all kinds, earning us a leadership position in a fast-growing and lucrative marketplace. Our longlasting, energy-efficient products will also improve the planet, by reducing waste and power consumption. How does a semiconductor laser work? Absorption and Emission E photon in 1 n1 exp[ ( E1 E0 )] n0 1/ kBT photon out Eo Stimulated vs. Spontaneous Emission • We can now derive the ratio of the emission rate versus the absorption rate using the equilibrium concentrations of photons and excited atoms: wemis n( p, ) 1 . wabs n( p, ) Derived in 1917 by Einstein. Required stimulated emission. However, a “real” understanding of this was not achieved until the 1950’s. Laser needs a Population Inversion Biased junction Negative bias photon out p-type n-type depleted region (electric field) History of Lasers • First operating Laser in 1960 (Maser in 1958) – Simulated emission concept from Einstein in 1905 – Townes (1964) and Schawlow (1981) • First semiconductor injection Laser in 1962 – First was Robert Hall (GE) but many competing groups – Year before he had argued it was impossible Violet Laser Diode Currently costs about $2000 apiece! Nichia Laser Diode p-contact n-GaN blocking layers Active p-GaN/InGaN MQW p-GaN n-Al 0.15Ga0.85N SiO2 n-contact p-Al 0.15Ga0.85 N n-GaN sapphire substrate 10 mW CW 405 nm Epitaxial Lateral Overgrowth material 10,000 hours operation! Before the next class, . . . • Start Homework 20 • Read Chapters 4 Turton. • Do Reading Quiz