integrated circuit devices professor ali javey summer 2009 mosfets reading: chapters 17 & 18
TRANSCRIPT
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Integrated Circuit Devices
Professor Ali Javey
Summer 2009
MOSFETsReading: 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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Today’s MOSFET Technology
Gate oxides as thin as 1.2 nm can be manufactured reproducibly.Large tunneling current through the oxide limits oxide-thicknessreduction.
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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)
When Vg = Vdd , the NFET is on and the PFET is off. When Vg = 0, the PFET is on and the NFET is off.
NFET PFET
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Qualitative discussion: n-MOSFETVG > VT ; VDS 0ID increases with VDS
VG > VT; VDS small, > 0ID increases with VDS , but rate of increase decreases.
VG > VT; VDS pinch-offID reaches a saturation value, ID,sat The VDS value is called VDS,sat
VG > VT; VDS > VDS,sat
ID does not increase further, saturation region.
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Threshold voltage for NMOS and PMOSWhen VG = VT, s = 2 F; we get expression for VT.
FSi
A
ox
SioxFT 2
22
Nq
xVIdeal n-channel(p-silicon) deviceboth terms positive
|2|2
2 FSi
D
ox
SioxFT
NqxV
Ideal p-channel(n-silicon) deviceboth terms negative
Si / ox = = 11.9 / 3.9 3
](bulk)[1
FiF EEq
F > 0 means p-type F < 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.
tgsdsnstgsoxedsat VVVVVCL
WI )(
Vds = 10mV
Ids
Vgs
Vt
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Quantitative ID-VDS Relationships “Square Law”
2
2DS
DSTGoxn
DV
VVVCL
ZI satDS,DS0 VV TG VV ;
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.
2TGox
satD, 2VV
L
CZI
satDS,D VV TG VV ;
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 << L) MOSFET (n-channel), for various values of VG