1. Chapter 2
Field-Effect
Transistors (FETs)
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2. Outline Introduction Device Structure and Physical Operation
Current-Voltage Characteristics
MOSFET Circuit at DC
The MOSFET as an amplifier
Biasing in MOS Amplifier Circuits
Small-signal Operation and Models
Single-Stage MOS amplifier
The MOSFET Internal Capacitance and High-Frequency Model
The depletion-type MOSFET
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3. Introduction Characteristics Far more useful than two-terminal device.
Voltage between two terminals can control the current flows in third terminal.
Quite small.
Low power.
Simple manufacturing process.
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4. Introduction Classification of MOSFET Widely used in IC circuits
P channel
Enhancement type
Depletion type
N channel
JFET
Widely used in IC circuits
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5. Device Structure and Physical Operation
Device structure of the enhancement NMOS
Physical operation
p channel device
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6. Device Structure of the Enhancement-Type NMOS
Perspective view
Four terminals
Channel length and width
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7. Device Structure of the Enhancement-Type NMOS
Cross-section view.
L = 0.1 to 3 mm
W = 0.2 to 100 mm
Tox= 2 to 50 nm
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8. Drain current controlled by vDS Drain current controlled by vGS
Physical Operation
Creating an n channel
Drain current controlled by vDS
Drain current controlled by vGS
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9. Creating a Channel for Current Flow
The enhancement-type NMOS transistor with a positive voltage applied to the gate.
An n channel is induced at the top of the substrate beneath the gate.
Inversion layer
Threshold voltage
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10. Drain Current Controlled by Small Voltage vDS
An NMOS transistor with vGS > Vt and with a small vDS applied.
The channel depth is uniform.
The device acts as a resistance.
The channel conductance is proportional to effective voltage.
Drain current is proportional to (vGS – Vt) vDS.
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11. vDS Increased
Operation of the enhancement NMOS transistor as vDS is increased.
The induced channel acquires a tapered shape.
Channel resistance increases as vDS is increased.
Drain current is controlled by both of the two voltages.
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12. Channel Pinched Off Channel is pinched off
Inversion layer disappeared at the drain point
Drain current isn’t disappeared
Drain current is saturated and only controlled by the vGS
Triode region and saturation region
Channel length modulation
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13. Drain Current Controlled by vGS
vGS creates the channel.
Increasing vGS will increase the conductance of the channel.
At saturation region only the vGS controls the drain current.
At subthreshold region, drain current has the exponential relationship with vGS
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14. p Channel Device
Two reasons for readers to be familiar with p channel device
Existence in discrete-circuit.
More important reason is the utilization of CMOS circuits.
Structure of p channel device
The substrate is n type and the inversion layer is p type.
Carrier is hole.
Threshold voltage is negative.
All the voltages and currents are opposite to the ones of n channel device.
Physical operation is similar to that of n channel device.
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15. Complementary MOS or CMOS
The PMOS transistor is formed in n well.
Another arrangement is also possible in which an n-type body is used and the n device is formed in a p well.
CMOS is the most widely used of all the analog and digital IC circuits.
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16. Current-Voltage Characteristics
Circuit symbol
Output characteristic curves
Channel length modulation
Characteristics of p channel device
Body effect
Temperature effects and Breakdown Region
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17. Circuit Symbol
Circuit symbol for the n-channel enhancement-type MOSFET.
Modified circuit symbol with an arrowhead on the source terminal to distinguish it from the drain and to indicate device polarity (i.e., n channel).
(c) Simplified circuit symbol to be used when the source is connected to the body or when the effect of the body on device operation is unimportant.
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18. Output Characteristic Curves
An n-channel enhancement-type MOSFET with vGS and vDS applied and with the normal directions of current flow indicated.
The iD–vDS characteristics for a device with k’n (W/L) = 1.0 mA/V2.
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19. Output Characteristic Curves
Three distinct region
Cutoff region
Triode region
Saturation region
Characteristic equations
Circuit model
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20. Cutoff Region Biased voltage The transistor is turned off.
Operating in cutoff region as a switch.
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21. Triode Region Biased voltage
The channel depth from uniform to tapered shape.
Drain current is controlled not only by vDS but also by vGS
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22. Triode Region
Assuming that the drain-t-source voltage is sufficiently small.
The MOS operates as a linear resistance
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23. Saturation Region Biased voltage The channel is pinched off.
Drain current is controlled only by vGS
Drain current is independent of vDS and behaves as an ideal current source.
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24. Saturation Region
The iD–vGS characteristic for an enhancement-type NMOS transistor in saturation
Vt = 1 V, k’n W/L = 1.0 mA/V2
Square law of iD–vGS characteristic curve.
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25. Channel Length Modulation
Explanation for channel length modulation
Pinched point moves to source terminal with the voltage vDS increased.
Effective channel length reduced
Channel resistance decreased
Drain current increases with the voltage vDS increased.
Current drain is modified by the channel length modulation
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26. Channel Length Modulation
The MOSFET parameter VA depends on the process technology and, for a given process, is proportional to the channel length L.
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27. Channel Length Modulation
MOS transistors don’t behave an ideal current source due to channel length modulation.
The output resistance is finite.
The output resistance is inversely proportional to the drain current.
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28. Characteristics of p Channel Device
Circuit symbol for the p-channel enhancement-type MOSFET.
Modified symbol with an arrowhead on the source lead.
Simplified circuit symbol for the case where the source is connected to the body.
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29. Characteristics of p Channel Device
The MOSFET with voltages applied and the directions of current flow indicated.
The relative levels of the terminal voltages of the enhancement-type PMOS transistor for operation in the triode region and in the saturation region.
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30. MOSFET Circuit at DC
Assuming device operates in saturation thus iD satisfies with iD~vGS equation.
According to biasing method, write voltage loop equation.
Combining above two equations and solve these equations.
Usually we can get two value of vGS, only the one of two has physical meaning.
Checking the value of vDS
if vDS≥vGS-Vt, the assuming is correct.
if vDS≤vGS-Vt, the assuming is not correct. We shall use triode region equation to solve the problem again.
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31. MOSFET Circuit at DC
The NMOS transistor is operating in the saturation region due to
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32. MOSFET Circuit at DC
Assuming the MOSFET operate in the saturation region
Checking the validity of the assumption
If not to be valid, solve the problem again for triode region
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33. The MOSFET As an Amplifier
Basic structure of the common-source amplifier.
Graphical construction to determine the transfer characteristic of the amplifier in (a).
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34. The MOSFET As an Amplifier
Transfer characteristic showing operation as an amplifier biased at point Q.
Three segments:
XA---the cutoff region segment
AQB---the saturation region segment
BC---the triode region segment
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35. Biasing in MOS Amplifier Circuits
Voltage biasing scheme
Current-source biasing scheme
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36. Biasing in MOS Amplifier Circuits
(d) coupling of a signal source to the gate using a capacitor CC1;
(e) practical implementation using two supplies.
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37. Biasing in MOS Amplifier Circuits
Biasing the MOSFET using a constant-current source I.
Implementation of the constant-current source I using a current mirror.
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38. Small-Signal Operation and Models
The ac characteristic
Definition of transconductance
Definition of output resistance
Definition of voltage gain
Small-signal model
Hybrid π model
T model
Modeling the body effect
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39. The ac Characteristics
The definition of transconductance
The definition of output resistance
The definition of voltage gain
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40. The Small-Signal Models
neglecting the the channel-length modulation effect
including the effect of channel-length modulation, modeled by output resistance ro = |VA| /ID.
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41. The Small-Signal Models
The T model of the MOSFET augmented with the drain-to-source resistance ro.
An alternative representation of the T model.
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42. Single-Stage MOS Amplifier
Characteristic parameters
Basic structure
Three configurations
Common-source configuration
Common-drain configuration
Common-gate configuration
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43. Characteristic Parameters of Amplifier
This is the two-port network of amplifier.
Voltage signal source.
Output signal is obtained from the load resistor.
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44. Definitions Input resistance with no load Input resistance
Open-circuit voltage gain
Voltage gain
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45. Definitions(cont’d) Short-circuit current gain Current gain
Short-circuit transconductance gain
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46. Definitions(cont’d) Open-circuit overall voltage gain
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47. Definitions(cont’d) Output resistance of amplifier proper
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48. Definitions(cont’d)
Voltage amplifier
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49. Definitions(cont’d)
Transconductance amplifier
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50. Relationships
Voltage divided coefficient
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51. Basic Structure of the Circuit
Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations.
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52. The Common-Source Amplifier
Common-source amplifier based on the circuit of basic structure.
Biasing with constant-current source.
CC1 And CC2 are coupling capacitors.
CS is the bypass capacitor.
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53. Equivalent Circuit of the CS Amplifier
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54. Equivalent Circuit of the CS Amplifier
Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized.
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55. Characteristics of CS Amplifier
Input resistance
Voltage gain
Overall voltage gain
Output resistance
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56. Summary of CS Amplifier
Very high input resistance
Moderately high voltage gain
Relatively high output resistance
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57. The Common-Source Amplifier with a Source Resistance
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58. Small-signal Equivalent Circuit with ro Neglected
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59. Characteristics of CS Amplifier with a Source Resistance
Input resistance
Voltage gain
Overall voltage gain
Output resistance
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60. Summary of CS Amplifier with a Source Resistance
Including RS results in a gain reduction by the factor (1+gmRS)
RS takes the effect of negative feedback.
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61. The Common-Gate Amplifier
Biasing with constant current source I
Input signal vsig is applied to the source
Output is taken at the drain
Gate is signal grounded
CC1 and CC2 are coupling capacitors
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62. The Common-Gate Amplifier
A small-signal equivalent circuit of the amplifier in fig. (a).
T model is used in preference to the π model
Neglecting ro
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63. The Common-Gate Amplifier Fed with a Current-Signal Input
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64. Characteristics of CG Amplifier
Input resistance
Voltage gain
Overall voltage gain
Output resistance
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65. Summary of CG Amplifier
Noninverting amplifier
Low input resistance
Has nearly identical voltage gain of CS amplifier, but the overall voltage gain is smaller by the factor (1+gmRsig）
Relatively high output resistance
Current follower
Superior high-frequency performance
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66. The Common-Drain or Source-Follower Amplifier
Biasing with current source
Input signal is applied to gate, output signal is taken at the source.
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67. The Common-Drain or Source-Follower Amplifier
Small-signal equivalent-circuit model
T model makes analysis simpler
Drain is signal grounded
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68. Small-Signal Analysis Performed Directly on the Circuit
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69. Circuit for Determining the Output Resistance of CD Amplifier
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70. Characteristics of CD Amplifier
Input resistance
Voltage gain
Overall voltage gain
Output resistance
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71. Summary of CD or Source-Follow Amplifier
Very high input resistance
Voltage gain is less than but close to unity
Relatively low output resistance
Voltage buffer amplifier
Power amplifier
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72. Summary and Comparisons
The CS amplifier is the best suited for obtaining the bulk of gain required in an amplifier.
Including resistance RS in the source lead of CS amplifier provides a number of improvements in its performance.
The low input resistance of CG amplifier makes it useful only in specific application. It has excellent high-frequency response. Can be used as a current buffer.
Source follower finds application as a voltage buffer and as the output stage in a multistage amplifier.
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73. High-Frequency Model
(b) The equivalent circuit for the case in which the source is connected to the substrate (body).
(c) The equivalent circuit model of (b) with Cdb neglected (to simplify analysis).
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74. The MOSFET Unity-Gain Frequency
Current gain
Unity-gain frequency
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75. Frequency Response of the CS Amplifier
Capacitively coupled common-source amplifier.
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76. Frequency Response of the CS Amplifier
A sketch of the frequency response of the amplifier in (a) delineating the three frequency bands of interest.
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77. The High-frequency Response
Equivalent Circuit
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78. The High-frequency Response
The circuit of (a) simplified at the input and the output
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79. The High-frequency Response
The equivalent circuit with Cgd replaced at the input side with the equivalent capacitance Ceq;
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80. The High-frequency Response
The frequency response plot, which is that of a low-pass single-time-constant circuit.
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81. The High-frequency Response
A large value of will cause to be lowered.
Although is a very small capacitance, its effect on the amplifier frequency response can be very significant as a result of its multiplication by the factor This effect is known as Miller effect.
To extend the high-frequency response of a MOSFET amplifier, we have to find configuration in which the Miller effect is absent or at least reduced.
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82. The Depletion-Type MOSFET
Circuits symbol
Structure
Characteristic curves
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83. Circuit Symbol for the n-Channel Depletion-Type MOSFET
Simplified circuit symbol applicable for the case the substrate (B) is connected to the source (S).
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84. Physical Structure
The structure of depletion-type MOSFET is similar to that of enhancement-type MOSFET with one important difference: the depletion-type MOSFET has a physically implanted channel.
There is no need to induce a channel.
The depletion MOSFET can be operated at both enhancement mode and depletion mode.
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85. Characteristic Curves
Transistor with current and voltage polarities indicated.
Typical value for discrete transistor: Vt = –4 V and k¢n(W/L) = 2 mA/V2
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86. The Output Characteristic Curves
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87. The iD–vGS Characteristic in Saturation
Expression of characteristic equation
Drain current with
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88. The iD–vGS Characteristic in Saturation
Sketches of the iD–vGS characteristics for MOSFETs of enhancement and depletion types
The characteristic curves intersect the vGS axis at Vt.
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