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

Goals 2 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

MOS Field-Effect Transistors Primary component in high-density VLSI chips such as memories and microprocessors Symbolic View Drain I The control voltage determines the value of the current in the channel I Vcontrol Channel Gate How do we create the channel? I Source 3

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. vSB, vDS and vGS always positive during normal operation. vSB always < vDS and vGS to reverse bias pn junctions 4

Definitions Vt: Threshold Voltage for MOS Transistor in general. The gate voltage required to form the channel between the source and drain VTN: Threshold Voltage for N-Channel Transistor. VTP: Threshold Voltage for P-Channel Transistor. The body (or bulk) current (iB) is always zero The gate current (iG) is always zero 5

NMOS Transistor: Qualitative I-V Behavior VGS<<VTN & VGS<<0 : No Channel for current-conduction (2 back-back diodes), Only small leakage current flows, iD ≈ 0 . 0<VGS<VTN: Depletion region formed under gate merges with source and drain depletion regions. No current flows between source and drain. VGS>VTN: Channel formed between source and drain. If vDS>0,, finite iD flows from drain to source. OFF OFF ON 6

For vGS > VTN-- Triode Region Characteristics where, Kn= Kn’W/L Kn’=μnCox’’ (A/V2) Cox’’=εox/Tox εox=oxide permittivity (F/cm) Tox=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

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

MOSFET as Voltage-Controlled Resistor Example 1: Voltage-Controlled Attenuator If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then, If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then, To maintain triode region operation, or 9

NMOS Transistor: Saturation Region If vDS increases above triode region limit, channel region disappears, also said to be pinched-off. 10

VGS > VTN, and VGD < VTN VDS > VGS-VTN If vDS increases above triode region limit, channel region disappears ( pinched-off). Current saturates at constant value, independent of vDS. Saturation region operation mostly used for analog amplification. Saturation region VGS > VTN, and VGD < VTN VDS > VGS-VTN Channel pinches off  iD is independent on VDS 11

NMOS Transistor: Saturation Region (contd.) for is also called saturation or pinch-off voltage 12

Circuit Symbols for N-Channel MOSFET (Enhancement type) sedr42021_0410a.jpg 13

I-V Characteristics for N-Channel MOSFET (Enhancement type) sedr42021_0411a.jpg 14

iD-VGS characteristics Small vds: triode region Higher vds; saturation region iD VT Slope = KnVDSW/L vGS sedr42021_0412.jpg 15

Terminal Voltage Levels sedr42021_0412.jpg 16

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 vGS=0, negative vGS required to turn device off. 17

Transfer Characteristics of MOSFETS Plots drain current versus gate-source voltage for a fixed drain-source voltage 18

Enhancement-Mode PMOS Transistors: Structure P-type source and drain regions in n-type substrate. vGS<0 required to create p-type inversion layer in channel region For current flow, vGS< vTP To maintain reverse bias on source-substrate and drain-substrate junctions, vSB <0 and vDB <0 Positive bulk-source potential causes VTP to become more negative 19

Enhancement-Mode PMOS Transistors In figures (c) and (d) the body is connected to the source Direction of current is opposite to n-channel 20

Enhancement-Mode PMOS Transistors: Output Characteristics For , transistor is off. For more negative vGS, drain current increases in magnitude. PMOS is in triode region for small values of VDS and in saturation for larger values. 21

DC –Analysis of n-channel MOSFET Check VGS we start the analysis by assuming certain operating region VGS> Vt VGS< VTN VDS < VGS –VTN Triode region VDS > VGS –VTN Sat. region Cutoff region iD=0 22