Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1

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Transcript of Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1

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Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1 Slide 2 Goals Describe operation of MOSFETs. Define MOSFET characteristics in operation regions of 1. cutoff, 2. triode and 3. saturation. Develop mathematical models for i-v characteristics of MOSFETs. Introduce graphical representations for output and transfer characteristic descriptions of electron devices. Define and contrast characteristics of enhancement-mode and depletion- mode FETs. Define symbols to represent MOSFETs in circuit schematics. 2 Slide 3 MOS Field-Effect Transistors Primary component in high-density VLSI chips such as memories and microprocessors Channel I I V control The control voltage determines the value of the current in the channel I Drain Source Gate Symbolic View How do we create the channel? 3 Slide 4 Enhancement Type NMOS Transistor: Structure 4 device terminals: Gate(G), Drain(D), Source(S) and Body(B). Source and drain regions form pn junctions with substrate. v SB, v DS and v GS always positive during normal operation. v SB always < v DS and v GS to reverse bias pn junctions 4 Slide 5 Definitions V t : Threshold Voltage for MOS Transistor in general. The gate voltage required to form the channel between the source and drain V TN : Threshold Voltage for N-Channel Transistor. V TP : Threshold Voltage for P-Channel Transistor. The body (or bulk) current (i B ) is always zero The gate current (i G ) is always zero 5 Slide 6 NMOS Transistor: Qualitative I-V Behavior V GS For v GS > V TN -- Triode Region Characteristics where, K n = K n W/L K n = n C ox (A/V 2 ) C ox = ox /T ox ox =oxide permittivity (F/cm) T ox= oxide thickness (cm) for A channel is induced between the source and drain, and current will flow Electrons flow from the source to drain Current flows from drain to source 7 Slide 8 NMOS Transistor: Triode Region Characteristics (contd.) Output characteristics appear to be linear. FET behaves like a (gate-source) voltage- controlled resistor between source and drain with 8 Slide 9 MOSFET as Voltage-Controlled Resistor Example 1: Voltage-Controlled Attenuator If K n =500A/V 2, V TN =1V, R=2k and V GG =1.5V, then, To maintain triode region operation, or If K n =500A/V 2, V TN =1V, R=2k and V GG =1.5V, then, 9 Slide 10 NMOS Transistor: Saturation Region If v DS increases above triode region limit, channel region disappears, also said to be pinched-off. 10 Slide 11 V GS > V TN, and V GD < V TN V DS > V GS -V TN Channel pinches off i D is independent on V DS Saturation region If v DS increases above triode region limit, channel region disappears ( pinched- off). Current saturates at constant value, independent of v DS. Saturation region operation mostly used for analog amplification. 11 Slide 12 NMOS Transistor: Saturation Region (contd.) for is also called saturation or pinch-off voltage 12 Slide 13 Circuit Symbols for N-Channel MOSFET (Enhancement type) 13 Slide 14 I-V Characteristics for N-Channel MOSFET (Enhancement type) 14 Slide 15 iDiD VTVT Slope = K n V DS W/L v GS Small v ds : triode regionHigher v ds ; saturation region i D -V GS characteristics 15 Slide 16 Terminal Voltage Levels 16 Slide 17 Depletion-Mode MOSFETS NMOS transistors with Ion implantation process used to form a built-in n-type channel in device to connect source and drain by a resistive channel Non-zero drain current for v GS =0, negative v GS required to turn device off. 17 Slide 18 Transfer Characteristics of MOSFETS Plots drain current versus gate-source voltage for a fixed drain-source voltage 18 Slide 19 Enhancement-Mode PMOS Transistors: Structure P-type source and drain regions in n-type substrate. v GS DC Analysis of n-channel MOSFET Check V GS V GS < V TN Cutoff region i D =0 V GS > V t V DS < V GS V TN Triode region V DS > V GS V TN Sat. region we start the analysis by assuming certain operating region 22