MOS Basics
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Transcript of MOS Basics
[Type text] [Type text] Dr. D. V. Kamath
MOS Basics
Characteristics BJT MOS
Current conduction
Bipolar
Unipolar
Noise MOSFETs are less noisy than BJTs. Hence, MOSFETs are more
suitable for signal processing applications
Source Type
Current Controlled Current Source
(CCCS)
Voltage Controlled Current
Source (VCCS)
Input Impedance
Low
High
Characteristic
parameter
β (ℎ𝐹𝐸)
𝑔𝑚
Current
𝐼𝑐 = 𝐼𝑠 𝑒𝑉𝐵𝐸 𝑉𝑇⁄
𝐼𝑑𝑠 =µ𝑛𝐶𝑜𝑥
2
𝑊
𝐿(𝑉𝑔𝑠 − 𝑉𝑡ℎ)
2
Transconductance
𝑔𝑚 = 𝐼𝑐 𝜂𝑉𝑇⁄
𝑔𝑚 = √2𝛽𝐼𝑑𝑠
Current driving
capability
High Low
Package density Low High
Bandwidth
MOSFETS have higher bandwidth compared to BJTs. It is relatively easy to control device geometries and scaling in MOS
devices when compared with BJT circuits.
***
[Type text] [Type text] Dr. D. V. Kamath
NMOS Device
LD (lateral diffusion) - Level 2 SPICE model parameter
DL (delta length) - BSIM model parameter ***
MOS parameters
Parameter Description Equation Unit
µ𝑛 Surface mobility of carrier - cm2 Vsec⁄
𝑊 𝐿⁄ Channel aspect ratio - -
𝐶𝑔 Gate-to-channel capacitance 𝐶𝑔 =
ɛ𝑖𝑛𝑠ɛ𝑜𝑊𝐿
𝐷
pF
𝐶𝑜𝑥 Gate capacitance per unit area 𝐶𝑜𝑥 =𝐶𝑔
𝑊𝐿=
ɛ𝑖𝑛𝑠ɛ𝑜
𝐷
𝑝𝐹 µ𝑚2⁄
𝐾𝑛 Process transconductance
parameter
𝐾𝑛 = µ𝑛𝐶𝑜𝑥
= µ𝑛
ɛ𝑖𝑛𝑠ɛ𝑜
𝐷
𝐴 𝑉2⁄
β Beta 𝛽𝑛 = 𝐾𝑛
𝑊
𝐿
𝐴 𝑉2⁄
𝛥𝑉 Overdrive potential 𝛥𝑉 = 𝑉𝑔𝑠 − 𝑉𝑡h
V
[Type text] [Type text] Dr. D. V. Kamath
Enhancement-type nMOS transistor in non-conducting condition
***
Enhancement-type nMOS transistor with 𝑽𝑮𝑺 > 𝑽𝑻𝑯 and 𝑽𝑫𝑺 = 0
***
Enhancement-type nMOS transistor with 𝑽𝑮𝑺 > 𝑽𝑻𝑯 and a small 𝑽𝑫𝑺
***
[Type text] [Type text] Dr. D. V. Kamath
Increasing VDS
causes the channel to acquire a tapered shape
***
MOS drain characteristics
Drain characteristics of depletion type NMOS
[Type text] [Type text] Dr. D. V. Kamath
Drain characteristics of enhancement type NMOS ***
Ids versus Vds relationship
Charge induced below the thinox layer between drain and source is dependent on Vgs
Drain current Ids is
thus
dependent on both Vds and Vgs
[Type text] [Type text] Dr. D. V. Kamath
Electron transit time computation
Ids versus Vds relationship for MOS in non-saturation region
Voltage along the channel varies linearly with distance X from the source due to the IR
drop in the channel, the average value is Vds/2
Effective gate voltage is Vgs -
Vth
[Type text] [Type text] Dr. D. V. Kamath
Alternate expressions of Ids for MOS in non-saturation region
[Type text] [Type text] Dr. D. V. Kamath
Ids expression for MOS in saturation region
Substituting limiting condition Vds = Vgs –Vth
in (1), we get Ids expression for saturation region
Drain current equations for NMOS
Region of operation Equation Condition
Cut-off
𝐼𝑑𝑠𝑛 = 0
𝑉𝑔𝑠𝑛 < 𝑉𝑡ℎ𝑛
Ohmic
𝐼𝑑𝑠𝑛 =𝛽𝑛
2𝑉𝑑𝑠{2(𝑉𝑔𝑠𝑛 − 𝑉𝑡ℎ𝑛) − 𝑉𝑑𝑠𝑛}
𝑉𝑔𝑠𝑛 > 𝑉𝑡ℎ𝑛
𝑉𝑑𝑠𝑛 < 𝑉𝑔𝑠𝑛 − 𝑉𝑡ℎ𝑛
Saturation
𝐼𝑑𝑠𝑛 =𝛽𝑛
2(𝑉𝑔𝑠𝑛 − 𝑉𝑡ℎ𝑛)
2
𝑉𝑔𝑠𝑛 > 𝑉𝑡ℎ𝑛
𝑉𝑑𝑠𝑛 ≥ 𝑉𝑔𝑠𝑛 − 𝑉𝑡ℎ𝑛
Drain current equations for PMOS
Region of
operation
Equation Condition
Cut-off
𝐼𝑑𝑠𝑝 = 0
𝑉𝑠𝑔𝑝 < |𝑉𝑡ℎ𝑝| or
|𝑉𝑔𝑠𝑝| < |𝑉𝑡ℎ𝑝|
Ohmic 𝐼𝑠𝑑𝑝 =
𝛽𝑝
2𝑉𝑠𝑑{2(𝑉𝑠𝑔𝑝 − |𝑉𝑡ℎ𝑝|) − 𝑉𝑠𝑑𝑝}
or
−𝐼𝑑𝑠𝑝 = 𝐼𝑠𝑑𝑝 =𝛽𝑝
2𝑉𝑑𝑠𝑝{2(𝑉𝑔𝑠𝑝 − 𝑉𝑡ℎ𝑝)
− 𝑉𝑑𝑠𝑝}
|𝑉𝑔𝑠𝑝| > |𝑉𝑡ℎ𝑝|
|𝑉𝑑𝑠𝑝| < |𝑉𝑔𝑠𝑝| − |𝑉𝑡ℎ𝑝|
[Type text] [Type text] Dr. D. V. Kamath
Saturation 𝐼𝑠𝑑𝑝 =
𝛽𝑝
2(𝑉𝑠𝑔𝑝 − |𝑉𝑡ℎ𝑝|)
2
or
−𝐼𝑑𝑠𝑝 = 𝐼𝑠𝑑𝑝 =𝛽𝑝
2(𝑉𝑔𝑠𝑝 − 𝑉𝑡ℎ𝑝)
2
|𝑉𝑔𝑠𝑝| > |𝑉𝑡ℎ𝑝|
|𝑉𝑑𝑠𝑝| > |𝑉𝑔𝑠𝑝| − |𝑉𝑡ℎ𝑝|
***
Deep Ohmic region
Region of
operation
Description Equation
Deep ohmic
𝐼𝑑𝑠 = 𝛽(𝑉𝑔𝑠 − 𝑉𝑡ℎ)𝑉𝑑𝑠
𝑅𝑜𝑛 =1
𝛽(𝑉𝑔𝑠−𝑉𝑡ℎ)
𝑉𝑔𝑠 > 𝑉𝑡ℎ
𝑉𝐷𝑆 ≪ 2(𝑉𝐺𝑆 − 𝑉𝑇𝐻)
***
MOS transconductance
𝑔𝑚 =𝜕𝐼𝐷
𝜕𝑉𝐺𝑆|𝑉𝑑𝑠,𝑐𝑜𝑛𝑠𝑡
𝐼𝑑𝑠 =𝛽
2(𝑉𝑔𝑠 − 𝑉𝑡ℎ)
2 𝑓𝑜𝑟 𝑀𝑂𝑆 𝑖𝑛 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑖𝑜𝑛
𝑔𝑚 = 𝛽(𝑉𝑔𝑠 − 𝑉𝑡ℎ)
𝑔𝑚 = √2𝛽𝐼𝑑𝑠
***
[Type text] [Type text] Dr. D. V. Kamath
Basic MOS small-signal model (at low frequencies)
𝑖𝑑 = 𝑔𝑚𝑣𝑔𝑠
MOS device is basically characterized by transconductance 𝑔𝑚
MOS device is modeled by VCCS
MOS output conductance 𝒈𝒅𝒔
The output conductance 𝑔𝑑𝑠 is expressed as
gds =δIds
δVds= λ𝐼𝑑𝑠
λ α 1
L and Ids α
1
L
gds α 1
L2
The output conductance 𝑔𝑑𝑠 is strongly dependent on the channel length
MOS transistor figure of merit 𝝎𝑶
The parameter 𝜔𝑂 is used as a measure of frequency response or switching speed ωO = 1 τsd⁄
ωO =gm
Cg=
µnCoxWL
(Vgs − Vth)
CoxWL=
µn(Vgs − Vth)
L2=
1
τsd
A fast switching speed requires that 𝑔𝑚 be as high as possible
The switching speed depends on effective gate voltage and on carrier mobility and inversely as
the square of channel length
***
vm
v
G D
ids
v g
- -
gs
+
S
+
dsgs