Presentation on theme: "Integrated Circuit Devices"— Presentation transcript:
1 Integrated Circuit Devices Professor Ali JaveySummer 2009PN JunctionsReading: Chapter 5
2 PN Junctions A PN junction is present in every semiconductor device. DonorsN-typeP-type–IVINPVReverse biasForward biasdiodesymbolA PN junction is present in every semiconductor device.
3 Energy Band Diagram and Depletion Layer of a PN Junction N-regionP-regionEf(a)EcEcEf(b)EvEvEcEf(c)A depletion layer exists at the PN junction. n 0 and p 0 in the depletion layer.EvNeutralDepletionNeutralN-regionlayerP-regionEcEf(d)Ev
5 Qualitative Electrostatics Band diagramBuilt in-potentialFrom e=-dV/dx
6 Formation of pn junctions When the junction is formed, electrons from the n-side and holes from the p-side will diffuse leaving behind charged dopant atoms. Remember that the dopant atoms cannot move! Electrons will leave behind positively charged donor atoms and holes will leave behind negatively charged acceptor atoms.The net result is the build up of an electric field from the positively charged atoms to the negatively charged atoms, i.e., from the n-side to p-side. When steady state condition is reached after the formation of junction (how long this takes?) the net electric field (or the built in potential) will prevent further diffusion of electrons and holes. In other words, there will be drift and diffusion currents such that net electron and hole currents will be zero.
7 Equilibrium Conditions Under equilibrium conditions, the net electron current and hole current will be zero.E-fieldN-typeP-typeNA = 1017 cm3ND = 1016 cm3hole diffusion currentnet current = 0hole drift current
9 The Depletion Approximation We assume that the free carrier concentration inside the depletion region is zero.We assume that the charge density outside the depletion region is zero and q(Nd-Na) inside the depletion.
10 Field in the Depletion Layer On the P-side of the depletion layer, = –qNadEqN=-adxesqN(x)=Ea(x-x)epsEOn the N-side, = qNdqN=+E(x)d(xx)ens
11 Field in the Depletion Layer The electric field is continuous at x = 0.Naxp = NdxnA one-sided junction is called a N+P junction or P+N junction
21 The PN Junction as a Temperature Sensor What causes the IV curves to shift to lower V at higher T ?
22 Other PN Junction Devices–From Solar Cells to Laser Diodes Also known as photovoltaic cells, solar cells can convert sunlight to electricity with 15-30% energy efficiency
23 Solar Cells short circuit I Dark IV light N P I Eq.(4.9.4) - 0.7 V V E sc-0.7 VVESolar CellcIVEq.(4.12.1)MaximumE–Ipower-outputvsc+(a)(b)
24 p-i-n PhotodiodesOnly electron-hole pairs generated in depletion region (or near depletion region) contribute to currentOnly light absorbed in depletion region contributes to generationStretch depletion regionCan also operate near avalanche to amplify signal
25 Light Emitting Diodes (LEDs) LEDs are typically made of compound semiconductorsWhy not Si