MOSFET

1. Chapter 5
Field-Effect Transistors

2. Chapter Goals
• Describe operation of MOSFETs and JFETs.
• Define MOSFET characteristics in operation regions of cutoff,
triode and saturation.
• Discuss mathematical models for i-v characteristics of MOSFETs
and JFETs.
• Introduce graphical representations for output and transfer
characteristic descriptions of electronic devices.
• Define and contrast characteristics of enhancement-mode and
depletion-mode MOFETs.
• Define symbols to represent MOSFETs in circuit schematics.
• Investigate circuits that bias transistors into different operating
regions.
• MOSFET and JFET DC circuit analysis
• Explore MOSFET modeling in SPICE

4. Types of Field-Effect Transistors
• MOSFET (Metal-Oxide Semiconductor Field-Effect
Transistor)
– Primary component in high-density VLSI chips such
as memories and microprocessors
• JFET (Junction Field-Effect Transistor)
– Finds application especially in analog and RF circuit
design

5. The MOS Transistor
Polysilicon
Aluminum

6. The NMOS Transistor Cross Section
n areas have been doped with donor ions
(arsenic) of concentration ND - electrons
are the majority carriers
p areas have been doped with acceptor
ions (boron) of concentration NA - holes
are the majority carriers
Gate oxide
n+
Source Drain
p substrate
Bulk (Body)
p+ stopper
Field-Oxide
(SiO2)n+
Polysilicon
Gate
L
W

7. MOS Capacitor Structure
• First electrode - Gate :
Consists of low-resistivity
material such as highly-doped
polycrystalline silicon,
aluminum or tungsten
• Second electrode -
Substrate or Body: n- or p-
type semiconductor
• Dielectric - Silicon dioxide:
stable high-quality electrical
insulator between gate and
substrate.

8. Substrate Conditions for Different
Biases
Accumulation
VG << VTN
Depletion
VG < VTN
Inversion
VG > VTN

9. Low-frequency C-V Characteristics for MOS Capacitor
on P-type Substrate
• MOS capacitance is non-
linear function of voltage.
• Total capacitance in any
region dictated by the
separation between
capacitor plates.
• Total capacitance modeled
as series combination of
fixed oxide capacitance
and voltage-dependent
depletion layer
capacitance.

10. NMOS Transistor: Structure
• 4 device terminals:
Gate(G), Drain(D),
Source(S) and Body(B).
• Source and drain regions
form pn junctions with
substrate.
• vSB, vDS and vGS always
positive during normal
operation.
• vSB must always reverse

14. The Threshold Voltage
VT = VT0 + γ(√|-2φF + VSB| - √|-2φF|)
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
φF = -φTln(NA/ni) is the Fermi potential (φT = 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)
γ = √(2qεsiNA)/Cox is the body-effect coefficient (impact of changes in
VSB) (εsi=1.053x10-10
F/m is the permittivity of silicon; Cox = εox/tox is
-11

15. The Body Effect
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
-2.5 -2 -1.5 -1 -0.5 0
VBS (V)
VT(V)
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

16. NMOS Transistor: Triode Region Characteristics

17. • It is to be noted that the VDS measured relative to the source increases from 0 to VDS as we
travel along the channel from source to drain. This is because the voltage between the gate
and points along the channel decreases from VGS at the source end to VGS-VDS.
• When VDS is increased to the value that reduces the voltage between the gate and channel at the
drain end to Vt that is ,
• VGS-VDS=Vt or VDS= VGS-Vt or VDS(sat) ≥ VGS-Vt
Concept of Asymmetric Channel

18. Transistor in Saturation Mode
S
D
B
G
VGS VDS > VGS - VT
ID
VGS - VT
- +n+ n+
Pinch-off
Assuming VGS > VT
VDS
The current remains constant (saturates).

19. NMOS Transistor: Saturation Region
TNGSDSTNGSD
VvvVv
L
WnK
i −≥−= 



 for
2
'
2
vDSAT
=vGS
−VTN is called the saturation or pinch-off voltage

20. Channel-Length Modulation
• As vDS increases above
vDSAT, thelength of the
depleted channel beyond
pinch-off point, DL,
increases and actual L
decreases.
• iD increases slightly with
vDS instead of being
constant.
iD
=
Kn
'
2
W
L
vGS
−VTN


 


2
1+λvDS





λ = channel length modulation
parameter

23. (µpCox)

26. Enhancement-Mode PMOS Transistors:
Structure
• p-type source and drain regions in
n-type substrate.
• vGS < 0 required to create p-type
inversion layer in channel region
• For current flow, vGS < vTP
• To maintain reverse bias on
source-substrate and drain-
substrate junctions, vSB < 0 and vDB
< 0
• Positive bulk-source potential
causes VTP to become more
negative

30. Depletion-Mode MOSFETS
• NMOS transistors with
• Ion implantation process is used to form a built-in
n-type channel in the device to connect source
and drain by a resistive channel
• Non-zero drain current for vGS = 0; negative vGS
required to turn device off.
VTN
≤0

33. Problem-solving Technique :MOSFET
DC Analysis
• STep1: Requires knowing the bias
condition of the transistor such as
cutoff or saturation or nonsaturation.
• Step2: If the bias condition is not
obvious, one must guess the bias
condition before analyzing the circuit.
• Step3 How can we Guess?
(i) Assume that the transistor is biased in
the saturation region, which implies
that:
VGS>VTN, ID>0, and VDS≥VDS(sat)
If all the above conditions are satisfied,
analyze the circuit using the saturation
current voltage relations.
(ii) If VGS<VTN, then transistor is probably in
cutoff mode.
(iii) If VDS<VDS(sat), the transistor is likely
biased in nonsaturation region, analyze
the circuit using nonsaturation current
voltage relations.

34. MOSFET Circuit Symbols
• (g) and (i) are the
most commonly used
symbols in VLSI logic
design.
• MOS devices are
symmetric.
• In NMOS, n+
region at
higher voltage is the
drain.
• In PMOS p+
region at
lower voltage is the
drain

35. Summary of the MOSFET
Current-Voltage relationship
Table 5.1