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UNIT I
MOS TRANSISTOR THEORY
AND
PROCESS TECHNOLOGY
TOPICS
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NMOS and PMOS transistors
Threshold voltage
Body effect
Design equations
Second order effects
MOS models
Small signal AC characteristics
Basic CMOS technology
MOS TRANSISTOR
• MOS  Metal Oxide Semiconductor
• MOS transistor is a majority carrier device,
current is the conducting channel b/w source &
drain.
• 2 types of MOS transistor:
n-MOS transistor
p-MOS transistor
• Various symbol representation for n-MOS &
p-MOS
n-MOS
n-MOS
• Majority carriers  electrons
• When +ive voltage is applied on gate, no. of
electrons will be increased.
• So, conductivity of channel is increased.
• If Vg < Vt ,Then the channel is cutoff.
• Threshold voltage is the voltage at which MOS
device starts to conduct.
p-MOS
• Majority carriers  holes
• 2 types of modes in n-MOS & p-MOS:
– Enhancement mode
– Depletion mode.
• n-MOS Enhancement mode:
– Device will be cut off when Vgs= 0
• n-MOS Depletion mode:
– Device will conduct even if Vgs= 0
• p-MOS Enhancement mode:
– Above Vtp, device will start to conduct.
• p-MOS Depletion mode:
– Device will be in conducting state even if Vgs= 0
Conduction characteristics
n-MOS ENHANCEMENT TRANSISTOR
• It has Moderately doped
p - type silicon substrate
• In that p-substrate,
Heavily doped n + source
and drain.
• Channel – a thin
insulating layer made up
of Silicon dioxide (SiO 2)
• Gate –polycrystaline
silicon (polysilicon)
Working principle
• When Vgs= 0, Vds is applied.
• There is no current flow b/w source & drain.
• When positive voltage is applied to gate,
electric field is produced across p-substrate.
• It attracts electrons towards the channel.
• It is continued when gate voltage is increased
further, the region below gate will be
converted from p-type to n-type.
• So the channel becomes n-type(n-channel).
• Three types of modes of MOS transistor:
– Accumulation mode
– Depletion mode
– Inversion mode
Accumulation mode
• In this mode Vgs < Vt
• Initially p-substrate is
having holes only.
Depletion mode
• In this mode Vgs =Vt
• Depletion region is
created in this mode .
• Vgs is increased and
reach Vt
• So holes are repelled
from the channel.
• Because of this,
depletion region is
created.
Inversion mode
• In this mode Vgs > Vt
• Voltage increased
further, so electrons are
attracted towards the
region below gate.
• So, the layers of
electrons will be formed
below the gate.
• Bcoz of this layer, this
mode is known as
inversion mode.
Operation regions of MOS transistor
• Cut-off mode
• Non-saturated mode(linear or resistive or
unsaturated mode)
• Saturated mode
Cut-off mode
• When Vgs > Vt and Vds = 0.
• Depletion layer is created.
• So the region is completely cut-off.
Non-saturated mode
• When Vds < Vgs - Vt
• Deep channel is created in this mode.
• Inversion region is weak in this region.
Saturated mode
• Here, Vds > Vgs - Vt and
Vgd < Vt
• The channel becomes
pinched off.
• Inversion is strong.
• Channel current is
controlled by gate voltage
& it is independent of
drain voltage
• Ids depends following:
– Distance b/w source & drain
– Channel width
– Threshold voltage
– Thickness of oxide layer
– Dielectric constant of gate insulator
– Carrier mobility(μ)
Conclusion
• Three conduction regions are available in
nMOS enhancement transistor.
Cut-off region (no current flow)
Non-saturated (Id depends Vg and Vd)
Saturated mode (Id independent of Vds )
p-MOS ENHANCEMENT TRANSISTOR