Integrated Circuit Devices

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Integrated Circuit Devices

• Professor Ali Javey
• Summer 2009

MOSFETs Reading Chapters 17 18
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The First Transistor
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1956 Physics Nobel Prize
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Invention of the Field-Effect Transistor
In 1935, a British patent was issued to Oskar
Heil. A working MOSFET was not demonstrated
until 1955.
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Todays MOSFET Technology
Gate oxides as thin as 1.2 nm can be manufactured
reproducibly. Large tunneling current through the
oxide limits oxide-thickness reduction.
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Introduction to the MOSFET
Basic MOSFET structure and IV characteristics
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Introduction to the MOSFET
Two ways of representing a MOSFET
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Complementary MOSFETs (CMOS)
NFET PFET
When Vg Vdd , the NFET is on and the PFET is
off. When Vg 0, the PFET is on and the NFET is
off.
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Qualitative discussion n-MOSFET
VG gt VT VDS ? 0 ID increases with VDS
VG gt VT VDS small, gt 0 ID increases with VDS ,
but rate of increase decreases.
VG gt VT VDS ? pinch-off ID reaches a saturation
value, ID,sat The VDS value is called VDS,sat
VG gt VT VDS gt VDS,sat ID does not increase
further, saturation region.
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Threshold voltage for NMOS and PMOS
When VG VT, ?s 2 ?F we get expression for VT.
Ideal n-channel (p-silicon) device both terms
positive
Ideal p-channel (n-silicon) device both terms
negative
?Si / ?ox 11.9 / 3.9 ? 3
?F gt 0 means p-type ?F lt
0 means n-type
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How to Measure the VT of a MOSFET
Vt is measured by extrapolating the Ids versus
Vgs (at low Vds) curve to Ids 0.
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Quantitative ID-VDS Relationships Square Law

ID will increase as VDS is increased, but when VG
VDS VT, pinch-off occurs, and current
saturates when VDS is increased further. This
value of VDS is called VDS,sat. i.e., VDS,sat
VG VT and the current when VDS VDS,sat is
called IDS,sat.

Here, Cox is the oxide capacitance per unit area,
Cox ?ox / xox
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ID-VDS characteristics expected from a long
channel (?L ltlt L) MOSFET (n-channel), for
various values of VG