DIGITAL ELECTRONICS
COME223
MOSFET
BAU
Introduction
2
Metal-Oxide-Semiconductor-Field-Effect-Transistor
(MOSFET)
 Device used to amplify or switch electronic signals
 Used in the fabrication of logic gates and amplifiers.

 Most
widely used in complex digital circuits such as
memories and microprocessors
Can be made very small and their operation requires
little power
 Invented in 1960 at Bell Labs by Kahng and Mohamed
Atalla (Egyptian – American Engineer)

Introduction
3
All FET transistors: JFET, MOSFET, CMOS, FinFET, and CNFET
are Unipolar Transistor.
 Unipolar Transistor conduct either electrons (-) or holes (+), not
both unlike BJT (Bipolar Junction Transistor).
 FET are Voltage Controlled unlike BJT, which is Current
Controlled.

MOSFET vs BJT
4
Feature
BJT
MOSFET
Control type
Current-controlled (base current Voltage-controlled (gate voltage drives
drives operation)
operation, very little input current)
Amplification (gain,
linearity)
High gain, better linearity →
good for analog (audio, RF)
Lower gain, less linear → widely used
in digital switching
Switching speed
Faster at high current levels
Faster at low/medium currents,
dominates in digital ICs
Current handling
Higher current density in small
area
Limited current density but scalable
Power consumption
Consumes more power (base
current required)
Very low power (nearly zero gate
current)
Integration (ICs)
Harder to integrate densely
Scales extremely well → billions of
devices per chip
Cost & manufacturing
More complex for IC
fabrication
Simpler, cheaper, dominant in VLSI and
mixed-signal design
Device Structure
5
Described
as:
Voltage
controlled
current source
 Two
types
of
transistors:

 n-channel
or NMOS
due
to
(Current
electrons)
 p-channel or PMOS
(Current
due
to
holes)

Four terminal device.
MOSFET – Symbol
6

In all the cases we will study, the body will be connected to the
source
N-MOS
7
With drain positive and source, gate, body grounded, the drain–
body PN junction is reverse biased.
 The diode is OFF, so no current flows from drain to source or body
(ID ≈ 0).
 The gate is insulated by thin oxide layer  no current can
flow through the gate  IG ≈ 0

N-MOS
8
High e⁻ concentration in n⁺
drain and source → a
positive
gate
voltage
attracts electrons under the
gate.
 When gate voltage reaches
a threshold, the p-type
channel is inverted to n-type.
 At this point, a conductive
channel forms between drain
and source, allowing current
flow.

N-MOS
9




N-channel MOSFET: Formed in a p-substrate by inverting the
surface → channel = inversion layer.
Gate voltage (VGS): Higher VGS → lower channel resistivity
→ higher drain current (ID) for same VDS.
Voltage control: MOSFET current is controlled by gate voltage,
not gate current.
Threshold voltage (Vt): Minimum VGS required to create the
channel and turn MOSFET ON.
Operation Mode: Cutoff
10
Cutoff Region (vGS < Vt )
 Positive voltage is applied
between the Drain and the
Source, vDS
 Remains in Cutoff until vGS
reaches a particular value =
“Threshold voltage” Vt ( 0.5 ~
1.5V)

iD  0 for vGS  Vt
Operation Mode: linear Region
11

Linear Region (vGS > Vt and vDS < [vGS -Vt])
For small values of (vDS ≈ 50mV)  The transistor is
turned ON
 N-channel created under the gate allowing current flow
between the Drain and the Source
 Transistor acts like a resistor between source and drain
controlled by gate voltage (resistance decreases as vGS
increases)
2

vDS
W
iD   nCox vGS  Vt vDS  

Physical Parameters
L
2 
2

vDS
W
iD  k
 vGS  Vt vDS  
L
2 
'
n


Operation Mode: Saturation
12
Saturation (Active) Region (vGS > Vt and vDS> [vGS -Vt])
 As VDS increases, the gate-to-drain voltage (VGD) decreases.
 When VGD = Vt, the channel at the drain end vanishes →
pinch-off region forms.
 In this region, ID saturates (independent of VDS) and depends
only on VGS.
 The excess voltage above threshold (VGS − Vt) is called the
overdrive voltage (VOV).

Operation Mode: Saturation
13

In saturation mode:

Pinch-off region

2
1 'W
iD  k n
vGS  Vt
2 L
𝑊𝑖𝑡ℎ 𝑘𝑛′
𝑊
= 𝐾𝑛
𝐿
Summary: operation modes
14
Linear or
Summary: operation modes
15
Linear
Example 1
16
Solution: (a)
Example 1: Solution
17
Solution: (b)
Example 2
18

For the following circuit assume that:
Example 2: Solution
19
Solution :
 As given, we have Kn = 1mA/V2 & Vt = 1.5 V
 From the circuit : VG = 4.7 x IG
as IG = 0  VG = 0
 ID = IS = 0.5 mA (Always ID = IS As IG = 0)
 Apply Ohm’s law across 15 KΩ  10 – VD = 15 ID
 𝑽𝑫 = 10 − 15
× 0.5 = 𝟐. 𝟓V

𝑉𝐷 > 𝑉𝐺 → 𝑉𝐷 − 𝑉𝑆 > 𝑉𝐺 − 𝑉𝑆 → 𝑉𝐷𝑆 > 𝑉𝐺𝑆 → 𝑉𝐷𝑆 > 𝑉𝐺𝑆 − 𝑉𝑡

2
Saturation region  𝐼𝐷 = 𝐾𝑛 𝑉𝑂𝑉
→
1
2
𝑉𝑂𝑉 =
2 𝐼𝐷
= ±1 𝑉
𝐾𝑛
To have current through the MOSFET, the last one should be ON  VOV > 0
 VOV = 1V
 VOV = VGS – Vt  VGS = 1 + 1.5 = 2.5 V
 VGS = VG – VS  VS = VG - VGS = 0 – 2.5 = -2.5 V

Example 2: Solution
20