1. Chapter 4 Field-Effect Transistors
Microelectronic Circuit Design
Richard C. Jaeger
Travis N. Blalock
Microelectronic Circuit Design
McGraw-Hill

2. MOS Capacitor Structure
First electrode- Gate: Consists of low-resistivity material such as metal or polycrystalline silicon
Second electrode- Substrate or Body: n- or p-type semiconductor
Dielectric-Silicon dioxide:stable high-quality electrical insulator between gate and substrate.
Microelectronic Circuit Design
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3. Substrate Conditions for Different Biases
Accumulation
Depletion
Accumulation
VG<<VTN
Depletion
VG<VTN
Inversion
VG>VTN
Inversion
Microelectronic Circuit Design
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4. Microelectronic Circuit Design
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.
Microelectronic Circuit Design
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5. 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 always < vDS and vGS to reverse bias pn junctions
Microelectronic Circuit Design
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6. NMOS Transistor: Qualitative I-V Behavior
VGS<<VTN : Only small leakage current flows.
VGS<VTN: Depletion region formed under gate merges with source and drain depletion regions. No current flows between source and drain.
VGS>VTN: Channel formed between source and drain. If vDS>0,, finite iD flows from drain to source.
iB=0 and iG=0.
Microelectronic Circuit Design
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7. NMOS Transistor: Triode Region Characteristics
for
where, Kn= Kn’W/L
Kn’=mnCox’’ (A/V2)
Cox’’=ox/Tox
 ox=oxide permittivity (F/cm)
Tox=oxide thickness (cm)
Microelectronic Circuit Design
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8. NMOS Transistor: Triode Region Characteristics (contd.)
Output characteristics appear to be linear.
FET behaves like a gate-source voltage-controlled resistor between source and drain with
Microelectronic Circuit Design
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9. MOSFET as Voltage-Controlled Resistor
Example 1: Voltage-Controlled Attenuator
If Kn=500mA/V2, VTN=1V, R=2k and VGG=1.5V, then,
To maintain triode region operation, or or
Microelectronic Circuit Design
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10. MOSFET as Voltage-Controlled Resistor (contd.)
Example 2: Voltage-Controlled High-Pass Filter
Voltage Transfer function,
where, cut-off frequency
If Kn=500mA/V2, VTN=1V, C=0.02mF and VGG=1.5V, then,
To maintain triode region operation,
Microelectronic Circuit Design
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11. NMOS Transistor: Saturation Region
If vDS increases above triode region limit, channel region disappears, also said to be pinched-off.
Current saturates at constant value, independent of vDS.
Saturation region operation mostly used for analog amplification.
Microelectronic Circuit Design
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12. NMOS Transistor: Saturation Region (contd.)
for
is also called saturation or pinch-off voltage
Microelectronic Circuit Design
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13. Transconductance of a MOS Device
Transconductance relates the change in drain current to a change in gate-source voltage
Taking the derivative of the expression for the drain current in saturation region,
Microelectronic Circuit Design
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14. Channel-Length Modulation
As vDS increases above vDSAT, length of 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
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15. Depletion-Mode MOSFETS
NMOS transistors with
Ion implantation process used to form a built-in n-type channel in device to connect source and drain by a resistive channel
Non-zero drain current for vGS=0, negative vGS required to turn device off.
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16. Transfer Characteristics of MOSFETS
Plots drain current versus gate-source voltage for a fixed drain-source voltage
Microelectronic Circuit Design
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17. Body Effect or Substrate Sensitivity
Non-zero vSB changes threshold voltage, causing substrate sensitivity modeled by
where
VTO= zero substrate bias for VTN (V)
g= body-effect parameter ( )
2FF= surface potential parameter (V)
Microelectronic Circuit Design
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