Department of Information Engineering256 Semiconductor Conduction is possible only if the electrons are free to move –But electrons are bound to their parent atoms To be freed, the electrons need energy –Conductor: need small energy –Insulator: need large energy –Semiconductor: need moderate energy

Department of Information Engineering257 Semiconductor Silicon, germanium –IV column in the periodic table –4 electrons at the outermost shell Most stable structure –each atom has 8 electrons at the outer most orbit (by sharing electrons with neighboring atoms) +4

Department of Information Engineering258 Semiconductor At low temperature –Not enough thermal energy –very few free electrons At higher temperature (room temperature) –electrons randomly receive thermal energy –electrons with high enough energy are freed create an electron-hole pair

Department of Information Engineering259 Electron-hole pair +4 free electron leaving a hole behind

Department of Information Engineering260 Hole as carrier Why holes can be used to conduct electricity? –region around the hole is more positively charged –attract neighboring electron to fill up the hole –The movement of the hole appears like a positively charged carrier +4 Movement of hole

Department of Information Engineering261 Recombination When a free electron fills up a hole, then the electron returns to its initial resting state –We lost two carriers a free electron and a hole +4 free electron recombination

Department of Information Engineering262 Doping Pure semiconductor –thermal excitation produces only a few free electrons and holes –Poor conductor Doping –to create more holes or free electrons by adding impurity –to create more holes, add Group III material –to create more free electrons, add Group V material

Department of Information Engineering263 P-type semiconductor doped with group III material –3 electrons at the outer-most shell Thermal energy creates electron-hole pairs –But the number of electrons is small Doping can create a large number of holes majority carrier - holes minority carrier - electrons (created by thermal energy) hole

Department of Information Engineering264 N-type semiconductor dope with group V material –5 electrons in the outer-most shell –majority carrier - electrons –minority carrier - holes (created by thermal energy) free electron

Department of Information Engineering265 p-n junction What happen if we join a p-type and a n-type material together? –p-type - lots of free holes –n-type - lots of free electrons P N - -

Department of Information Engineering266 Diffusion –holes and free electrons move randomly –statistically it is more likely that carriers will move from higher concentration to lower concentration –this process is called diffusion Direction of diffusion –Holes (from p-side to n-side) –electrons (from n-side to p-side)

Department of Information Engineering267 Diffusion current –movement of charges = current Direction of holes? –From, p to n, direction of current Direction of electrons? –From n to p, opposite to the direction of current Direction of diffusion current? –The current produced by holes and electrons are in the same direction –Total current = hole current + electron current

Department of Information Engineering268 PN junction Large number of holes move from P to N side Large number of electrons move from N to P side The holes and electrons meet at the junction between P and N –What happens when a hole meets an electron? –Recombination ! +3+5

Department of Information Engineering269 PN junction An electron from N side finds a hole in the P side –Recombination –P side is more –ve charged ! Similarly, a hole recombines with an electron in the N side –N side is more +ve charged ve +ve P N 0 0

Department of Information Engineering270 Some critical properties of the PN junction The build-up charges create a potential barrier P N potential barrier Junction capacitor !

Department of Information Engineering271 Some critical properties of the PN junction Potential barrier –Because of the potential barrier, the N side is more +ve charged –Repel holes coming in from P side back to P side –Similarly, electrons from N side is repelled back to N side Dynamic equilibrium – number of diffused charge = number of repelled charge –Net flow of charge = current = 0

Department of Information Engineering272 Some critical properties of the PN junction Why the number of diffused charge = number of repelled charge? If the potential barrier is too weak, so that the number of diffused charge > the number of repelled charge –More charges diffuse across the junction –Recombination –N side is more +ve charged (P side more –ve charged) –Barrier increases until the number of diffused charge is exactly the same as the number of repelled charge

Department of Information Engineering273 Some critical properties of the PN junction Depletion region –At the junction, electrons and holes are recombined, therefore this region has NO carriers –High resistance Depletion layer PN

Department of Information Engineering274 Diode Diode is simply a p-n junction P N

Department of Information Engineering275 Diode property of a diode –one-way street –current flows in one direction only v D > 0:short circuit (conducting) v D < 0:open circuit (non-conducting) vDvD iDiD

Department of Information Engineering276 Forward bias ( v D > 0 ) Most of the external voltage applies to the PN junction because it has the highest resistance Voltage is applied in a direction that reduces the potential barrier P N new potential barrier barrier at equilibrium vDvD vDvD vDvD

Department of Information Engineering277 Forward bias ( v D > 0 ) Forward bias lowered the potential barrier –The force of diffusion > potential barrier –More carriers can cross the barrier –Once crosses the barrier, the carriers are collected by the terminals Holes diffuse from p to n and are collected by the –ve terminal Electrons diffuse from n to p and are collected by the +ve terminal A small reduction in barrier leads to exponential increase in current

Department of Information Engineering278 Reverse bias ( v D < 0 ) Reverse bias makes the potential at p-side more -ve –Increases the potential barrier P N potential barrier barrier at equilibrium

Department of Information Engineering279 Reverse bias ( v D < 0 ) Increased potential barrier –A tiny current due to minority carriers still flow Minority carriers –Carriers created by thermal energy, –electrons on P side, holes on N side (minority) –Minority carriers can move across the barrier easily ! Electrons on P side attracted by the +ve potential at N side This is known as the reverse saturation current –A very small current carried by the minority carrier –The diode acts like a large resistor

Department of Information Engineering280 Diode characteristic Forward biased – exponential curve Backward biased – no current until diode breakdown Break down voltage Constant voltage over a very wide range of current. Perfect voltage source !!

Department of Information Engineering281 Diode current model An equation that approximates the diode current – –v D is the voltage across the diode –I 0 is the reverse saturation current –k is Boltzmann constant –T is temperature in unit of Kelvin –q is charge

Department of Information Engineering282 Simple model The diode conducts vigorously if V D > 0.6V Voltage drop across diode ~ 0.6V Half-wave rectifier

Department of Information Engineering283 Application - full-wave bridge rectifier D1 D3 D1 D3 + - e.g simplified circuit during positive cycle

Department of Information Engineering284 Zener diodes For ordinary diode, if the reverse-biased voltage is too large, the diode breaks down, conducts large current The diode will be burnt Zener diode –Break down at a very precise voltage, but will not be destroyed –Makes excellent voltage reference source

Department of Information Engineering285 Zener diodes