Lecture20-MOS+Transistors

Lecture20-MOS+Transistors - ECE 3040 Microelectronic...

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ECE 3040 - Dr. Ying Zhang Georgia Tech ECE 3040: Microelectronic Circuits Lecture 20 Reading: Pierret 16.2, 16.3, 17.1, 17.2
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ECE 3040 - Dr. Ying Zhang Georgia Tech Recapture MOS Capacitor Metal “Gate” Insulator Semiconductor “MOS” = Metal- Oxide- Semiconductor “MOS” actually refers to “Metal”– Silicon Dioxide – Silicon
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ECE 3040 - Dr. Ying Zhang 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) Recapture MOS Capacitor
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ECE 3040 - Dr. Ying Zhang Georgia Tech Recapture MOS Capacitor Since the insulator prevents any current from flowing, when we bring the materials together, the fermi-energy must be flat. Likewise, if no charges are stored on the “plates” (metal and semiconductor regions near the insulator) of the capacitor, the bands are not bent in the insulator nor semiconductor. Note the assumption of an equipotential surface in the metal simply states that a perfect conductor can not support and electric field (electrostatics).
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ECE 3040 - Dr. Ying Zhang Georgia Tech If V G = bias voltage applied to the gate (metal). For all V G the Fermi level in the each layer remains flat due to zero current through the structure. The applied bias separates the Fermi levels at the metal and semiconductor ends by qV G E F (metal) - E F (semiconductor) = -qV G In the oxide, bands energy the does so ith x, linearly w varies potential the Since ith x linearly w varies tan 0 Potential dx E V t Cons E dx dE oxide oxide oxide = = = Recapture MOS Capacitor under bias
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ECE 3040 - Dr. Ying Zhang Georgia Tech Recapture MOS Capacitor under bias For an n-type semiconductor. When V G > 0 the metal fermi-energy is lowered (E=-qV), the insulator has an electric field across it that terminates almost immediately in the near perfectly conducting metal, but terminates over a finite distance in the semiconductor of “finite resistivity”. The charge model indicates that negative charge must be created in the semiconductor near the interface. This charge is in the form of electrons. Since n = n i exp[(E F -E i )/ kT], the electron concentration in the semiconductor near interface increases. This is called accumulation
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ECE 3040 - Dr. Ying Zhang Georgia Tech Recapture MOS Capacitor under bias For an n-type semiconductor. When V
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This note was uploaded on 09/05/2009 for the course ECE 3040 taught by Professor Hamblen during the Spring '07 term at Georgia Tech.

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Lecture20-MOS+Transistors - ECE 3040 Microelectronic...

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