lecture2-3

lecture2-3 - Physical Operation of BJTs pn-junction review...

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Unformatted text preview: Physical Operation of BJTs pn-junction review Voltage vs. position in an npn structure cutoff active saturation Designing for high The Early effect Parasitic capacitance (C , C ) Periodic Table This abridged table contains elements with three to five valence electrons, with Si being the most important. Silicon Si has four valence electrons. Therefore, it can form covalent bonds with four of its neighbours When temperature goes up, electrons in the covalent bond can become free. Electron-Hole Pair Interaction With free electrons breaking off covalent bonds, holes are generated. Holes can be filled by absorbing other free electrons, so effectively there is a flow of charge carriers. The pn junction review p-type n-type anode cathode integrated circuit diode metal silicon oxide doped silicon wafer (chip) n~0, and donor ions are exposed Dopant distribution inside a pn junction excess electrons diffuse to the p-type region excess holes diffuse to the n-type region DEPLETION REGION: + p~0, and acceptor ions are exposed p>>n n>>p + + +---- Voltage in a pn junction p>>n n>>p + + +---- x charge, r (x) x x electric field, E(x) voltage, V(x) + ~0.7 volts (for Si) E x = 1 x x dx V x = x E x dx Zero Bias p>>n n>>p + + +--- x voltage, V(x) ~0.7 volts (for Si) At zero bias (v D =0), neither electrons nor holes can overcome this built-in voltage barrier of ~ 0.7 volts i D = 0 Forward Bias p>>n n>>p + + +--- x voltage, V(x) 0.65 volts v D 0.50 volts 0.0 volts As the bias (v D ), increases toward 0.7V, the electrons and holes can overcome the built-in voltage barrier . i D > 0 I D = I S e qV D kT 1 Reverse Bias p>>n n>>p + + +--- x voltage, V(x)-5 volts v D 0.0 volts 1 / 2 I s 1 / 2 I s I s As the bias (v...
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This note was uploaded on 03/27/2008 for the course EE 12 taught by Professor Rout during the Spring '08 term at Tufts.

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lecture2-3 - Physical Operation of BJTs pn-junction review...

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