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lecture2-3 - Physical Operation of BJTs pnjunction review...

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    Physical Operation of BJTs pn-junction review Voltage vs. position in an npn structure cutoff active saturation Designing for high  β The Early effect Parasitic capacitance (C π , C μ )
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    Periodic Table This abridged table contains elements with three to five valence electrons, with Si being the most important.
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    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.
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    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.
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    The pn junction review p-type n-type anode cathode integrated circuit diode metal silicon oxide doped silicon wafer (chip)
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    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 + + + - - - -
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    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 ε 0 x ρ x dx V x =− 0 x E x dx
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    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
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    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
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    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 D ) becomes negative, the barrier becomes larger.   Only electrons and holes due to broken bonds in the depletion  region contribute to the diode current.  i D  = -I s
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