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Published byGerard Stanley Modified over 9 years ago
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Introduction to MOS Transistors Section 6.1-6.4
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Outline Similarity Between BJT & MOS Introductory Device Physics Small Signal Model
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BJT & MOS Transistor [Chapter 4,5] [Chapter 6,7]
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Analogous Devices Terminals – “C”↔”D” – “E” ↔”S” – “B” ↔”G” Analogous Devices – NPN ↔NMOS – PNP ↔PMOS
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Similarity in the Small Signal Equivalent Circuit
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Terminal Resistance
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NPN to NMOS
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Introductory Device Physics
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A Crude Metal Oxide Semiconductor (MOS) Device P-Type Silicon is slightly conductive. Positive charge attract negative charges to the interface between insulator and silicon. A conductive path is created If the density of electrons is sufficiently high. Q=CV. V2 causes movement of negative charges, thus current. V1 can control the resistivity of the channel. The gate draws no current!
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NMOS in 3D n+ diffusion allows electrons move through silicon. (provide electrons)(drain electrons) W
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Typical Dimensions of MOSFETs These diode must be reversed biased. tox is made really thin to increase C, therefore, create a strong control of Q by V.
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A Closer Look at the Channel Formulation Need to tie substrate to GND to avoid current through PN diode. Positive charges repel the holes creating a depletion region, a region free of holes. Free electrons appear at V G =V TH. V TH =300mV to 500 mV (OFF)(ON)
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Change Drain Voltage Resistance is determined by V G.
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Change Gate Voltage A higher V G leads to a lower channel resistance, therefore a larger slope.
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Length Dependence The resistance of a conductor is proportional to the length. fixed V D fixed V G
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Dependence on Oxide Thickness Q=CV C is inversely proportional to 1/t ox. Lower Q implies higher channel resistance. fixed V D fixed V G
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Width Dependence The resistance of a conductor is inversely proportional to the cross section area. A larger device also has a larger capacitance!
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Channel Pinch Off Q=CV – V=VG-V OXIDE-Silicon V OXIDE-Silicon can change along the channel! Low V OXIDE-Silicon implies less Q.
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VG-VD is sufficiently large to produce a channel VG-VD is NOT sufficiently large to produce a channel No channel Electrons are swept by E to drain. Drain can no longer affect the drain current!
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Regions No channel (No Dependence on VDS) Assumption:
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