unit 1.2 introduction1
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MOS TRANSISTORTHEORY
UNIT -1
Chapter -2
(Neil weste p:- 41- 91)
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Introduction
A MOS transistor is a majority-carrier device,in which the current in a conducting channel
between the source and the drain is
modulated by a voltage applied to the gate.
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NMOS (n-type MOS transistor)
1) Majority carrier = electrons
2) A positive voltage applied on the gate with respect tothe substrate enhances the number of electrons in the
channel and hence increases the conductivity of the
channel.
3) If gate voltage is less than a threshold voltage Vt , thechannel is cut-off (very low current between source &
drain).
PMOS (p-type MOS transistor)
1) Majority carrier = holes2) Applied voltage is negative
with respect to substrate.
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Relationship between Vgs andIds, for a fixed Vds:
n-channel enhancement
n-channel depletionp-channel enhancement
n-channel depletion
Ids
Ids
Ids Ids
VgsVgs
VgsVgs
+ Vt
+ Vt
- Vt
- Vt
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Devices that are normally cut-off with zero
gate bias are classified as
"enhancementmode"devices.
Devices that conduct with zero gate bias are
called "depletion-mode"devices.
Enhancement-mode devices are more
popular in practical use.
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nMOS EnhancementTransistor
At Vds=+V, Vgs=0V, no current flows
from source to drain because they are
insulated by two reverse biased pn
junction
1. Accumulation mode (Vgs Vt)
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The factors that influence the level of drain current
Ids (b/w S and D)
Distance b/w S and D
Channel width Threshold voltage Vt
Thickness of SiO2
Dielectric constant of insulator
Carrier mobility
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Threshold voltage (Vt):
The voltage at which an MOS device begins to
conduct ("turn on"). The threshold voltage is a
function of
(1) Gate conductor material
(2) Gate insulator material
(3) Gate insulator thickness
(4) Impurity at the silicon-insulator interface
(5) Voltage between the source and the substrate Vsb
(6) Temperature
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Threshold voltage (Vt) Equations
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Threshold voltage (Vt)equation
Vt ,can be expressed as
Where Vt-mos is the ideal thresholdvoltage of an ideal transistor and Vfb istermed as flat-band voltage.
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Body Effect
Effect due to series connection oftransistors
Increases if source voltage increasesbecause source is connected to thechannel
Increase in Vt with Vs is called the bodyeffect
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( )2
cutoff
linear
saturatio
0
2
2n
gs t
dsds gs t ds ds dsat
gs t ds dsat
V V
VI V V V V V
V V V V
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