Title: MOS Transistor
1MOS Transistor
- ECE442 Digital Electronics
2Review Design Abstraction Levels
3The MOS Transistor
Polysilicon
Aluminum
4The NMOS Transistor Cross Section
Gate oxide
Polysilicon Gate
Source
Drain
Field-Oxide (SiO2)
n
n
p substrate
p stopper
Bulk (Body)
5NMOS Transistor Layout
6NMOS Region of Operations
7NMOS Transistor Operation
8NMOS Triode Region-1
9NMOS Triode Region-2
10NMOS Active (Saturation) Region
11Switch Model of NMOS Transistor
Gate
VGS
Source (of carriers)
Drain (of carriers)
VGS lt VT
VGS gt VT
12Switch Model of PMOS Transistor
Gate
VGS
Source (of carriers)
Drain (of carriers)
VGS gt VDD VT
VGS lt VDD VT
13Threshold Voltage Concept
G
D
S
n
p substrate
B
The value of VGS where strong inversion occurs is
called the threshold voltage, VT
14The Threshold Voltage
- VT VT0 ?(?-2?F VSB - ?-2?F)
- where
- VT0 is the threshold voltage at VSB 0 and is
mostly a function of the manufacturing process - Difference in work-function between gate and
substrate material, oxide thickness, Fermi
voltage, charge of impurities trapped at the
surface, dosage of implanted ions, etc. - VSB is the source-bulk voltage
- ?F -?Tln(NA/ni) is the Fermi potential (?T
kT/q 26mV at 300K is the thermal voltage NA is
the acceptor ion concentration ni ? 1.5x1010
cm-3 at 300K is the intrinsic carrier
concentration in pure silicon) - ? ?(2q?siNA)/Cox is the body-effect coefficient
(impact of changes in VSB) (?si1.053x10-10F/m is
the permittivity of silicon Cox ?ox/tox is the
gate oxide capacitance with ?ox3.5x10-11F/m)
15The Body Effect
- VSB is the substrate bias voltage (normally
positive for n-channel devices with the body tied
to ground) - A negative bias causes VT to increase from
0.45V to 0.85V
VT (V)
VBS (V)
16Transistor in Linear Mode
Assuming VGS gt VT
VGS
VDS
G
S
D
n
B
The current is a linear function of both VGS and
VDS
17Voltage-Current Relation Linear Mode
- For long-channel devices (L gt 0.25 micron)
- When VDS ? VGS VT
- ID kn W/L (VGS VT)VDS VDS2/2
- where
- kn ?nCox ?n?ox/tox is the process
transconductance parameter (?n is the carrier
mobility (m2/Vsec)) - kn kn W/L is the gain factor of the device
- For small VDS, there is a linear dependence
between VDS and ID, hence the name resistive or
linear region
18Transistor in Saturation Mode
Assuming VGS gt VT
VDS gt VGS - VT
VGS
VDS
G
S
D
B
The current remains constant (transistor
saturates)
19Voltage-Current Relation Saturation Mode
- For long channel devices
- When VDS ? VGS VT
- ID (kn/2) W/L (VGS VT) 2
- since the voltage difference over the induced
channel (from the pinch-off point to the source)
remains fixed at VGS VT - However, the effective length of the conductive
channel is modulated by the applied VDS, so - ID ID (1 ?VDS)
- where ? is the channel-length modulation
(varies with the inverse of the channel length)
20Channel Length Modulation
21Channel Length Modulation
22Current Determinates
- For a fixed VDS and VGS (gt VT), IDS is a function
of - the distance between the source and drain L
- the channel width W
- the threshold voltage VT
- the thickness of the SiO2 tox
- the dielectric of the gate insulator (e.g., SiO2)
?ox - the carrier mobility
- for NMOS ?n 500 cm2/V-sec
- for PMOS ?p 180 cm2/V-sec
23Long Channel I-V Plot (NMOS)
X 10-4
VGS 2.5V
VGS 2.0V
ID (A)
VGS 1.5V
VGS 1.0V
VDS (V)
NMOS transistor, 0.25um, Ld 10um, W/L 1.5,
VDD 2.5V, VT 0.43V
24Long Channel I-V Plot (NMOS)
X 10-4
VGS 2.5V
VGS 2.0V
ID (A)
VGS 1.5V
VGS 1.0V
VDS (V)
NMOS transistor, 0.25um, Ld 10um, W/L 1.5,
VDD 2.5V, VT 0.43V
25Short Channel Effects
- Behavior of short channel device mainly due to
?sat 105
5
- Velocity saturation the velocity of the
carriers saturates due to scattering (collisions
suffered by the carriers)
Constant velocity
?n (m/s)
Constant mobility (slope ?)
?c
?(V/?m)
- For an NMOS device with L of .25?m, only a
couple of volts difference between D and S are
needed to reach velocity saturation
26Voltage-Current Relation Velocity Saturation
- For short channel devices
- Linear When VDS ? VGS VT
- ID ?(VDS) kn W/L (VGS VT)VDS VDS2/2
- where
- ?(V) 1/(1 (V/?cL)) is a measure of the
degree of velocity saturation - Saturation When VDS VDSAT ? VGS VT
- IDSat ?(VDSAT) kn W/L (VGS VT)VDSAT
VDSAT2/2
27Velocity Saturation Effects
For short channel devices and large enough VGS
VT
Long channel devices
- VDSAT lt VGS VT so the device enters
saturation before VDS reaches VGS VT and
operates more often in saturation
Short channel devices
VDSAT
VGS-VT
- IDSAT has a linear dependence w.r.t VGS so a
reduced amount of current is delivered for a
given control voltage
28Short Channel I-V Plot (NMOS)
X 10-4
Early Velocity Saturation
VGS 2.5V
VGS 2.0V
ID (A)
VGS 1.5V
VGS 1.0V
VDS (V)
NMOS transistor, 0.25um, Ld 0.25um, W/L 1.5,
VDD 2.5V, VT 0.43V
29MOS ID-VGS Characteristics
- Linear (short-channel) versus quadratic
(long-channel) dependence of ID on VGS in
saturation - Velocity-saturation causes the short-channel
device to saturate at substantially smaller
values of VDS resulting in a substantial drop in
current drive
X 10-4
long-channel quadratic
ID (A)
short-channel linear
VGS (V)
(for VDS 2.5V, W/L 1.5)
30Short Channel I-V Plot (PMOS)
- All polarities of all voltages and currents are
reversed
VDS (V)
VGS -1.0V
VGS -1.5V
ID (A)
VGS -2.0V
VGS -2.5V
X 10-4
PMOS transistor, 0.25um, Ld 0.25um, W/L 1.5,
VDD 2.5V, VT -0.4V
31The MOS Current-Source Model
ID 0 for VGS VT ? 0 ID k W/L (VGS
VT)VminVmin2/2(1?VDS) for VGS VT ? 0
with Vmin min(VGS VT, VDS, VDSAT) and VGT
VGS - VT
G
ID
S
D
B
- Determined by the voltages at the four
terminals and a set of five device parameters
VT0(V) ?(V0.5) VDSAT(V) k(A/V2) ?(V-1)
NMOS 0.43 0.4 0.63 115 x 10-6 0.06
PMOS -0.4 -0.4 -1 -30 x 10-6 -0.1
32MOS Capacitance