MOS TRANSISTOR
So far,
we have treated MOS transistor as ideal switches
§ An ON transistor
passes a finite amount of current
§ Depends
on terminal voltages
§ Derive
current-voltage (I-V) relationships
§ Transistor
gate, source, drain all have capacitance
§ I = C
(ΔV/Δt) àΔt = (C/I) ΔV
§ Capacitance
and current determine speed
1. MOS Terminal Voltage
and Modes of operation
§ Mode of
operation depends on Vg, Vd, Vs
§ Vgs
= Vg – Vs
§ Vgd
= Vg – Vd
§ Vds
= Vd – Vs = Vgs - Vgd
§ Source
and drain are symmetric diffusion terminals
§ By
convention, source is terminal at lower voltage
§ Hence Vds³ 0
§ nMOS body
is grounded. First assume source is 0
too.
§ Three
regions of operation
Cutoff
Linear
Saturation
2. Application
Digital
integrated circuits such as microprocessors and memory devices contain
thousands to millions of integrated MOSFET transistors on each device,
providing the basic switching functions required implementing logic gates and
data storage.
Discrete
devices are widely used in applications such as switch mode power supplies,
variable frequency drives and other power electronics applications where each
device may be switching hundreds or thousands of watts.
Radio-frequency
amplifiers up to the UHF spectrum use MOSFET transistors as analog signal and
power amplifiers.
Radio
systems also use MOSFETs as oscillators, or mixers to convert frequencies.
MOSFET devices are also applied in audio-frequency power amplifiers for public
address systems, sound reinforcement and home and automobile sound systems.
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