Metal Oxide Field Effect
Transistor (MOSFET)
Lecture-5
Outline
• Field Effect Transistor (FET)
o Trends of Gate Dimension
o Difference between FET and BJT
o FET’s family
o FET’s Types
o FETs Channel
• Metal Oxide Field Effect Transistor (MOSFET)
Basic MOSFET
Inversion Charge (Channel Creation)
Current – Voltage (I-V) Characteristics
MOSFET Types
MOSFET Comparison
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Trend of FET Gate Dimensions
Minimum gate dimension
in commercial integrated
circuit
Fig. 1 Chapter – 6, Page # 294
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Difference b/w FET & BJT
Fig. 2 Chapter – 6, Page # 295
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FETs Family
Fig. 3, Chapter – 6, Page # 295
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FET’s Channel
Buried Channel Surface Inversion Channel
MESFETs and JFETs are buried-channel MODFETs, MOSFETs are surface-
devices channel devices
• Buried channels are based on bulk • The surface inversion channel is a
conduction and, thus, are free of two-dimensional charge sheet of
surface effects such as scattering and thickness in the order of few nm
surface defects, resulting in better while buried layer is thicker
carrier mobility.
• for depletion-mode device, it is
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common to use buried channel
FET’s Types
• Junction FET (JFET)
• Metal-Oxide-Semiconductor FET (MOSFET)
• Metal-Semiconductor FET (MESFET)
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FET Types
• Junction FET (JFET)
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FET Types
MESFET
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FET Types
• Metal-Oxide-Semiconductor FET (MOSFET)
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MOSFET
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MOSFET Structure
Fig. 5,6 Chapter – 6, Page # 297-8
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MOSFET Device
Transistor (Bell Labs)
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Inversion Charge in Channel
Fig. 7 Chapter – 6, Page #
2D-band diagram of n-channel MOSFET.
299
(a) Device configuration Physics of Semiconductor device, rd
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(b) Flat-band zero-bias equilibrium Edition S . M. Sze
Inversion Charge in Channel
2D-band diagram of n-channel MOSFET
(c) Equilibrium condition (VD = 0) under gate bias(Vg > 0) Fig. 7 Chapter – 6, Page #
d) Non-equilibrium condition under both gate and drain 299
biases Physics of Semiconductor device,
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Inversion Charge in Channel
Comparison of charge
distribution and energy-
band variation of an
inverted p-region
(a) equilibrium and
(b) Non-equilibrium at drain end
Fig. 8 Chapter – 6, Page # 300
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Inversion Charge in Channel
Total semiconductor surface charge is then obtained from Gauss' law
Debye length is Surface potential w. r. t bulk
Inversion charge per unit area Qn, (after strong inversion)
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Current-Voltage (I-V) Characteristics
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I-V Characteristics
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Ideal Output Characteristics
Fig. 9 Chapter – 6, Page # 303
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I-V Characteristics
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Ideal Transfer Characteristics
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Field-Dependent Mobility
(Two-Piece Linear Approximation)
The Critical Electric
field (Ec)
Ec = vsat / µ
Vsat - saturation velocity
µ - Low field mobility
Fig. 11 Chapter – 6, Page # 307
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Field-Dependent Mobility:
(Two-Piece Linear Approximation)
Surface potential
M is a function of doping concentration and oxide thickness
Dielectric constant
Fermi level (intrinsic Fermi level)
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Threshold Voltage
When a substrate bias is applied
Back-substrate voltage
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Sub-Threshold
When the gate bias is below the threshold and the semiconductor
surface is in weak inversion or depletion, the corresponding drain
current is called the sub-threshold current.
The subthreshold region tells how sharply the current drops with
gate bias
• important for low-voltage (MOSFET used as a switch in digital logic and
memory applications)
In weak inversion and depletion,
The electron charge is small, thus, the drift current is low.
Hence, the drain current is dominated by diffusion current only.
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Sub-Threshold Region
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MOSFET Type
Versions of MOSFETs based on their output and transfer characteristics
Fig. 4, Chapter – 6, Page # 297
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MOSFET Types
Versions of MOSFETs; their output and transfer
characteristics
Fig. 4, Chapter – 6, Page # 297
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MOSFET Symbols
Enhancement Mode Enhancement Mode
Depletion Mode Depletion Mode
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MOSFET Comparison
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Thank you!
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