Gentle Intro to Semiconductors ENGN/PHYS 207 The ubiquitous LED Collection of diodes Diode = electronic valve Forward bias: Current flows! Reverse bias: No current flow! The Silicon Valley Warm-up question(s) What does it mean for a material to be able to “conduct” electricity? Pure silicon lattice (intrinsic semiconductor) 2D lattice model showing covalent bonds in Si crystal Band Theory of Solids Quantum Mechanics: Energy levels occupied by electrons are discrete. Amphitheatre analogy Conduction Band Band gap Valence Band Aspendos Theatre, Turkey Energy Level Diagrams Electron-Hole Pairs (EHPs) Wait—what’s a hole? Vacancy in the valence band Acts like mobile positive charge An electron can fall into a hole (Recombination) Important Fact Thermal energy to move and shake around a lattice at (T = 300K) kT = 1/40 eV = 0.025 eV Why pure silicon isn’t so hot Large energy barrier! Let’s dope up (?) Group Number III (extra hole) IV V (extra e-) That’s dope: N and P type N-type: Extra electrons P-type: Extra holes Dopant electrons and holes are mobile Nuclei locked in the lattice, they are NOT mobile. Why doping is actually good for us N-type: easily donates electrons into conduction band P-type: easily accepts electrons into valence band The PN junction Let there be Light… I VB The Diode Equation Silicon Diodes as Rectifiers “Electronic Valve” action Current can essentially flow in one direction only 0.7 V Reverse bias Forward bias Rainbow of Fruit Flavors Idealized LED turn-on voltages I (mA) 1.8V 3.5V 3.6V VF (volts) Images: http://49chevy.blogs.com/fusor/2008/10/fun-with-electr.html http://wwwinst.eecs.berkeley.edu/~ee143/sp06/lectures/Semiconductor_tutorial.pdf http://www.allaboutcircuits.com/vol_3/chpt_2/3.html http://free-zg.t-com.hr/Julijan-Sribar/preview/vol1.html http://www.launc.tased.edu.au/online/sciences/Physics/photonics/bias.htm http://www.mtmscientific.com/diodes.html http://www.mpoweruk.com/semiconductors.htm http://free-zg.t-com.hr/Julijan-Sribar/preview/vol1.html http://electrons.wikidot.com/methods-for-the-determination-of-ground-statewavefunction http://www.electronics-tutorials.ws/diode/diode_8.html Animations: http://pvcdrom.pveducation.org/SEMICON/PN.HTM http://www.acsu.buffalo.edu/~wie/applet/students/jiawang/pn.html http://www.youtube.com/watch?v=O3x7NdUuu0Q&feature=related http://www.youtube.com/watch?v=DbjR-2knrpo Sources