Unit03-MOSFET P1+P2 Update basics explanation.pdf

Basic MOSFET 2

1. Unit 03 –MOSFET Basics
Ts. Amir Rasyadan
03

2. Learning Outcomes
•At the end of this lesson, students will be able to:
•Describe the physical structure and layers of a MOSFET.
•Explain how a MOSFET operates in different modes.
•Analyze the current-voltage characteristics of a MOSFET.
•Relate MOSFET behavior to its use in CMOS circuits.

3. Part 1: The Metal-Oxide Semiconductor Structure

4. Introduction to MOSFET
•MOSFET = Metal-Oxide-Semiconductor Field Effect
Transistor.
•It is the fundamental building block of CMOS
technology.
•Controls current flow using an electric field, not
current injection (unlike BJT).

5. Basic MOSFET Structure
•Four terminals: Gate (G),
Source (S), Drain (D), Body (B).
•Layers:
•Metal Gate
•Thin Oxide (SiO₂)
•Semiconductor Substrate
(p-type or n-type)
•Two main types:
•n-channel MOSFET (nMOS)
•p-channel MOSFET
(pMOS)

6. Cross-sectional View
•Source and Drain are n+
doped regions in a p-type
substrate.
•The gate is insulated from
the substrate by a thin
SiO₂ layer.
•The voltage at the gate
controls current between
source and drain.

7. Cross-sectional View
•Role of Oxide Layer
•SiO₂ layer acts as an insulator.
•Prevents direct current flow from gate to
substrate.
•Allows the gate voltage to control
channel formation via electric field.
•Very thin oxide layer → high gate
capacitance → sensitive control.
•Quiz

8. Principle of Operation
•MOSFET current depends on the
Gate-to-Source voltage (�
��).
•When �
�� exceeds a threshold
voltage (�
��), a conductive
channel forms.
•Drain current (�
??????) is determined
by �
�� , �
??????� , and device
dimensions (Width-to-Length
ratio; W/L).

9. Operating Regions of MOSFET
Region Condition Description
Cutoff VGS < VTH
No channel formed, ID
≈ 0
Linear
(Ohmic/
Triode)
VGS > VTH and VDS < (VGS – VTH)
Channel acts as a
resistor
SaturationVGS > VTH and VDS ≥ (VGS – VTH)
Channel pinches off,
current nearly
constant

10. Cutoff Region
•Condition:
•VGS < VTH (threshold voltage)
•Channel Status: No inversion
layer; channel does not form
•Current Flow: ID ≈ 0 (only
leakage current)
•Behavior: MOSFET is OFF
•Application Insight: Simple to
understand and rarely
confused; not used for active
control

11. Saturation Region
•Condition:
•VGS > VTH and VDS ≥ (VGS − VTH)
•Channel Status: Inversion layer
forms, but pinch-off occurs near the
drain
•Current Flow: ID saturates—limited
by gate voltage, not VDS
•Behavior: Acts like a current source
•Clarification: “Saturation” refers to
current saturation, not channel
saturation
•Application Insight: Most digital
switching applications use this
region

12. Linear (Ohmic/Triode) Region
•Condition:
•VGS > VTH and VDS < (VGS − VTH)
•Channel Status: Full inversion layer
connects source to drain
•Current Flow: ID ∝ VDS (linear
relationship)
•Behavior: Acts like a voltage-controlled
resistor
•Terminology:
•“Ohmic” and “Linear” → reflects resistor-
like behavior
•“Triode” → historical reference to vacuum
tube triodes
•Application Insight: Useful in analog
circuits like amplifiers or variable
resistors

13. pMOS Operation
•Channel formed when
VGS < VTH (negative
voltage).
•Source usually connected
to the positive supply
(VDD).
•Carries holes instead of
electrons.
•Complementary behavior
to nMOS.

14. Comparison: nMOS vs pMOS
Property nMOS pMOS
Channel Type n-type p-type
Majority Carriers Electrons
Holes (+ve
Charge)
Turn-on Condition VGS > VTH VGS < VTH
Carrier Mobility Higher Lower
Typical Use in CMOS Pull-down Pull-up

15. End of P1
•Quizz

16. Part 2: MOSFET Current–Voltage Characteristics

17. Drain Current Equation for nMOS
•Cutoff region:
•&#3627408444;&#3627408439; = 0
•Linear (Triode) region:
•&#3627408444;&#3627408439; = ??????&#3627408475; ∗ &#3627408438;&#3627408476;?????? ∗ (&#3627408458;/??????) ∗ [ (&#3627408457;&#3627408442;&#3627408454; – &#3627408457;&#3627408455;&#3627408443;) ∗ &#3627408457;&#3627408439;&#3627408454; – (&#3627408457;&#3627408439;&#3627408454;² / 2) ]
•Saturation region:
•&#3627408444;&#3627408439; = ½ ∗ ??????&#3627408475; ∗ &#3627408438;&#3627408476;?????? ∗ (&#3627408458;/??????) ∗ (&#3627408457;&#3627408442;&#3627408454; – &#3627408457;&#3627408455;&#3627408443;)²
•Where:
•μn = electron mobility
•Cox = oxide capacitance per unit area
•W/L = transistor geometry ratio

18. Calculation Exercise
Given:
•VGS = 4 V
•VTH = 1 V
•VDS = 2 V
•μnCox(W/L) = 200 μA/V²
•Determine:
•a) The region of operation.
•b) The drain current (ID).

19. •Given:
•VGS = 5 V
•VTH = 1 V
•μnCox(W/L) = 150 μA/V²
•Find the drain current (ID) when the MOSFET is in
saturation.
Calculation Exercise

20. LTSPice SImulation

21. Effect of Channel Geometry (W/L) on MOSFET
Performance
•W (Width): The width of the conducting channel.
•L (Length): The distance between source and drain.
•The ratio W/L defines the transistor’s drive strength.
•Drain current is proportional to the width-to-length ratio:
•ID ∝ (W/L)
•A larger W allows more current flow (stronger
transistor).
•A smaller L reduces resistance and increases speed.
Discussion:
•What happens to current if the channel is made twice
as wide?
•How does reducing L affect device performance?
•Why is W adjustable while L is fixed by fabrication
limits?
L

22. Discussion answers:
•What happens to current if the channel is made twice as
wide?
•Doubling the width (W) doubles the available path for electrons →
drain current (ID) approximately doubles.
Since ID ∝ (W/L), increasing W increases drive strength.
•How does reducing L affect device performance?
•Reducing the channel length (L) increases current (less resistance
and faster operation) but can cause short-channel effects,
leakage, and reduced control by the gate at very small scales.
•Why is W adjustable while L is fixed by fabrication limits?
•Channel length (L) is defined by the foundry’s manufacturing
process node technology (e.g., 5 nm, 3 nm, etc.) and cannot be
easily changed by designers.
Channel width (W) is a layout-level design parameter, so it’s
adjustable to tune current or balance circuit performance.

23. Transfer Characteristic (ID vs VGS)
•Shows how channel forms and current
rises as gate voltage increases.
•Below VTH → almost no current (cutoff).
•Above VTH → quadratic increase in current
(saturation region).
•The curve helps determine threshold
voltage.
•Simulate this curve in Ltspice.