Lect2UP050_(100430)

Lect2UP050_(100430) - Lecture 050 PN Junction and CMOS...

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Lecture 050 – PN Junction and CMOS Transistors (4/30/10) Page 050-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 050 - PN JUNCTIONS AND CMOS TRANSISTORS LECTURE ORGANIZATION Outline pn junctions • MOS transistors • Layout of MOS transistors • Parasitic bipolar transistors in CMOS technology • High voltage CMOS transistors • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 29-43 Lecture 050 – PN Junction and CMOS Transistors (4/30/10) Page 050-2 CMOS Analog Circuit Design © P.E. Allen - 2010 PN JUNCTIONS How are PN Junctions used in CMOS? PN junctions are used to electrically isolate one semiconductor region from another PN diodes • ESD protection • Creation of the thermal voltage for bandgap purposes • Depletion capacitors – voltage variable capacitors (varactors) Components of a pn junction: 1.) p -doped semiconductor – a semiconductor having atoms containing a lack of electrons (acceptors). The concentration of acceptors is N A in atoms per cubic centimeter. 2.) n -doped semiconductor – a semiconductor having atoms containing an excess of electrons (donors). The concentration of these atoms is N D in atoms per cubic centimeter.
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Lecture 050 – PN Junction and CMOS Transistors (4/30/10) Page 050-3 CMOS Analog Circuit Design © P.E. Allen - 2010 Abrupt PN Junction 060121-02 p + semiconductor n semiconductor Metal-semiconductor junction pn junction Metal-semiconductor junction p + semiconductor n semiconductor Depletion Region W x W 1 0 W 2 W 1 = Depletion width on p side W 2 = Depletion width on n side 1. Doped atoms near the metallurgical junction lose their free carriers by diffusion. 2. As these fixed atoms lose their free carriers, they build up an electric field, which opposes the diffusion mechanism. 3. Equilibrium conditions are reached when: Current due to diffusion = Current due to electric field Lecture 050 – PN Junction and CMOS Transistors (4/30/10) Page 050-4 CMOS Analog Circuit Design © P.E. Allen - 2010 Influence of Doping Level on the Depletion Regions Intuitively, one can see that the depletion regions are inversely proportional to the doping level. To achieve equilibrium, equal and opposite fixed charge on both sides of the junction are required. Therefore, the larger the doping the smaller the depletion region on that side of the junction. The equations that result are: W 1 = 2 ± ( ² o - v D ) qN A ³ ´ ´ µ · · ¸ 1 + N A N D ¹ 1 N A and W 2 = 2 ( o - v D ) qN D ³ ´ ´ µ · · ¸ 1 + N D N A ¹ 1 N D Assume that v D = 0, o = 0.637V and N D = 10 17 atoms/cm 3 . Find the p -side depletion region width if N A = 10 15 atoms/cm 3 and if N A = 10 19 atoms/cm 3 : For N A = 10 15 atoms/cm 3 the p -side depletion width is 0.90 μ m. For N A = 10 19 atoms/cm 3 the p -side depletion width is 0.9 nm.
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Lecture 050 – PN Junction and CMOS Transistors (4/30/10) Page 050-5 CMOS Analog Circuit Design © P.E. Allen - 2010 Graphical Characterization of the Abrupt PN Junction Assume the pn junction is open-circuited.
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Lect2UP050_(100430) - Lecture 050 PN Junction and CMOS...

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