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CMOS Devices PN junctions and diodes NMOS and PMOS transistors
Resistors Capacitors Bipolar transistors
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PN Junctions Diffusion causes depletion region
D.R. is insulator and establishes barrier This leads to 1-directional current flow Forms junction capacitor Capacitance highly voltage dependent Can be nuisance or benefits
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PN Junctions
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PN Junctions Initial impurity concentration
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PN Junctions
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PN Junctions Depletion region widths: Barrier potential:
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Note the large magnitude of the field
Example NA=10^15 atoms/cm^3, ND=10^16, vD=-10 Ni=2.25*10^20 Phi_o=26ln(10^15*10^16/2.25/10^20)=638mV xp= mm xn= 0.35 mm Max field = q*NA*xp/e = -5.4*10^4 V/cm Note the large magnitude of the field
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PN Junctions The depletion charge The junction capacitance
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Can be used as voltage controlled capacitor
Here m = 1/2 for the step change in impurity concentration. For gradual concentration change, m = 1/3.
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Impurity concentration profile for diffused pn junction
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Current density at boundary due to wholes:
Total: Diode current:
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The MOS Transistor
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TWO TYPES OF TRANSISTORS
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CMOS Transistors
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Capacitors Two conductor plates separated by insulator for a capacitor
Intentional capacitors vs parasitic capacitors Linear vs nonlinear capacitors Linear capacitors:
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Integrated Capacitors in CMOS
High density, good matching, but nonlinear
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High density, good matching, good linearity, but require
two-poly processes
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Other Caps Cgs of enhancement transistors metal to poly metal to metal
PN junction diffusion to substrate side wall Fringe caps from top layer metals
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Layering strategies
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Parasitic Capacitors
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Proper layout of capacitors
For achieving CA = 2CB, which one is better?
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Various Capacitor Errors
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Resistors in CMOS Diffusion resistor polysilicon resistor
well resistor metal layer resistor contact resistor
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Diffusion resistor in n-well
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Polysilicon resistor on FOX
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n-well resistor on p-substrate
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Bipolar in CMOS PNP
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CMOS Transistors
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Latch-up problem
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Preventing Latch-up
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Preventing Latch-up
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ESD protection A very serious problem Not enough theoretical study
Many trade secrets Learn from experienced designers
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reset CLK S1 S16 S2 S15 S3 S14 S4 S13 S5 S12 S6 S11 S7 S10 S8 S9 S R C
D Q reset CLK S1 S16 S2 S15 S3 S14 S4 S13 S5 S12 S6 S11 S7 S10 S8 S9
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