Lecture23-MOS Capacitors

Lecture23-MOS Capacitors - Lecture 23 Metal Oxide...

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ECE 3040 - Dr. Alan Doolittle Georgia Tech Lecture 23 Metal Oxide Capacitors Reading: Pierret 16.2-16.3 Notes
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ECE 3040 - Dr. Alan Doolittle Georgia Tech MOS Capacitor Metal “Gate” Insulator Semiconductor “MOS” = Metal- Oxide- Semiconductor “MOS” actually refers to “Metal”– Silicon Dioxide – Silicon Other material systems have similar “MIS” structures formed by Metal – Insulator – Semiconductor The capacitor itself forms the basis of digital logic circuits, and DRAM storage units (storing charge) or can simply supply a capacitance for an analog integrated circuit. It will also be the building block for the most common transistor produced – the MOS transistor. The substrate is normally taken to be grounded and the “Gate” electrode can be biased with a voltage, V G
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ECE 3040 - Dr. Alan Doolittle Georgia Tech Key assumptions: 1) Metal is an equipotential region. 2) Oxide is a perfect insulator with zero current flow. 3) Neither oxide nor oxide-semiconductor interface have charge centers. 4) Semiconductor is uniformly doped. 5) An ohmic contact has been established on the back side of the wafer. 6) Analysis will be one-dimensional. 7) The semiconductor is thick enough to have a quasi-neutral region (where electric field is zero and all energy bands are flat). 8) Certain energy relationships exist: Φ M = Φ S = χ + (E C -E F ) FB (terms defined in next few slides) MOS Capacitor
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ECE 3040 - Dr. Alan Doolittle Georgia Tech MOS Capacitor E O = Vacuum Energy Level. The minimum energy an electron must have to free itself from the material. Φ M = “Work function” of the metal. This is the energy difference from the fermi energy (average energy) of an electron in the metal to the vacuum energy level. Φ S = “Work function” of the semiconductor. This is the energy difference from the fermi energy (average energy) of an electron in the semiconductor to the vacuum energy level. Note that this energy depends on doping since E F depends on doping χ = Electron Affinity of the semiconductor. This is the energy difference from the conduction band minimum in the semiconductor to the vacuum energy level. Note that this energy does NOT depend on doping (E C -E F ) FB = Φ S χ in the quasi-neutral region where the bands are not bent or are in “flat band”
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Georgia Tech The insulator is simply a very wide bandgap, intrinsically doped semiconductor characterized by an electron affinity, χ i . The semiconductor can have an electric field near the insulator that forces the energy bands to bend near the insulator-semiconductor interface. MOS Capacitor
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This note was uploaded on 08/23/2011 for the course ECE 3040 taught by Professor Doolittle during the Spring '11 term at University of Florida.

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Lecture23-MOS Capacitors - Lecture 23 Metal Oxide...

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