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Lecture8 - ECE 124A VLSI Principles Lecture 8 LongLong-and...

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ECE 124A C LSI Principles VLSI Principles Lecture 8 Long Long-and Short and Short- Channel MOSFET Operation, Static and Channel MOSFET Operation, Static and Dynamic Characteristics Prof. Kaustav Banerjee lectrical and Computer Engineering Electrical and Computer Engineering E-mail: [email protected] Kaustav Banerjee Lecture 8, ECE 124A, VLSI Principles
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MOS Capacitor (Review) Kaustav Banerjee Lecture 8, ECE 124A, VLSI Principles
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OSFET Threshold Voltage (V 0 Concept MOSFET Threshold Voltage (V T0 ): Concept D S V S + + + G GS - n+ n+ p-substrate Depletion Region n-channel B Note: gate is insulated from the substrate…hence no dc current flows through Kaustav Banerjee Lecture 8, ECE 124A, VLSI Principles the oxide…channel is capacitively coupled to the gate through the electric-field in the oxide….that’s how it gets the name MOS-FET (field effect transistor)
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Recall…Inversion Definition of inversion Point at which density of electrons on oa c d e s y o e e c o s o surface = density of holes in bulk Surface potential is same as , but F different sign E C E p E i q F Remember: E V Fp q S = -q F q F = E F - E i Kaustav Banerjee Lecture 8, ECE 124A, VLSI Principles
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Physical Parameters that Affect V 0 y T0 hreshold voltage (V 0 voltage between Threshold voltage (V T0 ): voltage between gate and source required for inversion NMOS Transistor is “off” when V GS < V T0 Components: Work function difference between gate and channel (Flat-band voltage) Gate voltage to change surface potential ate voltage to offset depletion region charge Gate voltage to offset depletion region charge Gate voltage to offset fixed charges in gate oxide and in silicon-oxide interface Kaustav Banerjee Lecture 8, ECE 124A, VLSI Principles
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hreshold voltage (1) Threshold voltage (1) Work function difference C between gate and GC channel Represents built-in potential of MOS system For metal gate: GC = F (substrate) - M (gate) = ms For poly gate: GC = F (substrate) - F (gate) GC T V 0 Kaustav Banerjee Lecture 8, ECE 124A, VLSI Principles
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hreshold voltage (2) Threshold voltage (2) First component accounts for built-in voltage drop ow apply additional gate voltage to Now apply additional gate voltage to achieve inversion: change surface otential by - ote that negative for p pe potential by 2 F (note that F is negative for p-type substrate) C F GC T V 2 0 Kaustav Banerjee Lecture 8, ECE 124A, VLSI Principles
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hreshold voltage (3) Threshold voltage (3) ffset depletion region charge due to fixed acceptor ions Offset depletion region charge, due to fixed acceptor ions Calculate charge at inversion ( S =- F ) From before: So: F S Si A qN Q 2 F Si A B qN Q 2 2 0 Depletion charge is negative….why? (acceptor ions For non-zero substrate bias (V SB 0): after accepting electrons are –ve) Due to larger depletion region SB F Si A B V qN Q 2 2 Kaustav Banerjee
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