MOS Transistor

1
MOS Transistor

• ECE442 Digital Electronics

2
Review Design Abstraction Levels
3
The MOS Transistor
Polysilicon
Aluminum
4
The NMOS Transistor Cross Section
Gate oxide
Polysilicon Gate
Source
Drain
Field-Oxide (SiO2)
n
n
p substrate
p stopper
Bulk (Body)
5
NMOS Transistor Layout
6
NMOS Region of Operations
7
NMOS Transistor Operation
8
NMOS Triode Region-1
9
NMOS Triode Region-2
10
NMOS Active (Saturation) Region
11
Switch Model of NMOS Transistor
Gate
VGS
Source (of carriers)
Drain (of carriers)
VGS lt VT
VGS gt VT
12
Switch Model of PMOS Transistor
Gate
VGS
Source (of carriers)
Drain (of carriers)
VGS gt VDD VT
VGS lt VDD VT
13
Threshold Voltage Concept
G
D
S
n
p substrate
B
The value of VGS where strong inversion occurs is
called the threshold voltage, VT
14
The 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)

15
The 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)
16
Transistor 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
17
Voltage-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

18
Transistor in Saturation Mode
Assuming VGS gt VT
VDS gt VGS - VT
VGS
VDS
G
S
D
B
The current remains constant (transistor
saturates)
19
Voltage-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)

20
Channel Length Modulation
21
Channel Length Modulation
22
Current 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

23
Long 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
24
Long 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
25
Short 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

26
Voltage-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

27
Velocity 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

28
Short 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
29
MOS 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)
30
Short 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
31
The 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
32
MOS Capacitance