1. Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction1

2. 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

3. 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

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

5. 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

6. 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

7. 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

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

9. 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

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

11. 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

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

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

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

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

16. Terminal Voltage Levels
sedr42021_0412.jpg 16

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

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

19. 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

20. 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

21. 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

22. 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