Chapter 5 Field-Effect Transistors

1
Chapter 5Field-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

3
(No Transcript)
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
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 ltlt VTN
Depletion VG lt VTN
Inversion VG gt 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 bias the pn junctions

11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
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
NMOS Transistor Triode Region Characteristics
17
Concept of Asymmetric Channel

• 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-VDSVt or VDS
  VGS-Vt or VDS(sat) VGS-Vt

18
Transistor in Saturation Mode
Assuming VGS gt VT
VDS gt VGS - VT
VGS
VDS
G
S
D
B
The current remains constant (saturates).
19
NMOS Transistor Saturation Region
is called the saturation or pinch-off voltage
20
Channel-Length Modulation

• As vDS increases above vDSAT, the length of the
  depleted channel beyond pinch-off point, DL,
  increases and actual L decreases.
• iD increases slightly with vDS instead of being
  constant.

l channel length modulation parameter
21
(No Transcript)
22
(No Transcript)
23
(?pCox)