1. SJTU J. Chen 1 2015-8-16 Chapter 5 Field-Effect Transistors (FETs)

2. 2015-8-16 SJTU J. Chen 2 Content Physical operation and current-voltage characteristics DC analysis Biasing in MOS amplifier circuit and basic configuration

3. SJTU J. Chen 3 2015-8-16 Physical operation and current -voltage characteristics

4. 2015-8-16 SJTU J. Chen 4 FET: Field Effect Transistor There are two types  MOSFET: metal-oxide-semiconductor FET  JFET: Junction FET MOSFET is also called the insulated-gate FET or IGFET.  Quite small  Simple manufacturing process  Low power consumption  Widely used in VLSI circuits( >800 million on a single IC chip) Introduction to FET

5. 2015-8-16 SJTU J. Chen 5 Device structure of MOSFET (n-type) p-type Semiconductor Substrate (Body) Body(B) n+n+ n+n+ Oxide (SiO 2 ) Source(S) Gate(G) Drain(D) Metal For normal operation, it is needed to create a conducting channel between Source and Drain Channel area

6. 2015-8-16 SJTU J. Chen 6  An n channel can be induced at the top of the substrate beneath the gate by applying a positive voltage to the gate  The channel is an inversion layerinversion layer  The value of V GS at which a sufficient number of mobile electrons accumulate to form a conducting channel is called the threshold voltage (V t ) Creating a channel for current flow

7. 2015-8-16 SJTU J. Chen 7  L = 0.1 to 3  m  W = 0.2 to 100  m  T ox = 2 to 50 nm Device structure of MOSFET (n-type) Cross-section view

8. 2015-8-16 SJTU J. Chen 8 According to the type of the channel, FETs can be classified as  MOSFET  N channel  P channel  JFET  P channel  N channel Classification of FET Enhancement type Depletion type Enhancement type Depletion type

9. 2015-8-16 SJTU J. Chen 9 Drain current under small voltage v DS  An NMOS transistor with v GS > V t and with a small v DS applied.  The channel depth is uniform and the device acts as a resistance.  The channel conductance is proportional to effective voltage, or excess gate voltage, ( v GS – V t ).  Drain current is proportional to ( v GS – V t ) and v DS.

10. 2015-8-16 SJTU J. Chen 10 Drain current under small voltage v DS

11. 2015-8-16 SJTU J. Chen 11  The induced channel acquires a tapered shape.  Channel resistance increases as v DS is increased.  Drain current is controlled by both of the two voltages. Operation as v DS is increased B

12. 2015-8-16 SJTU J. Chen 12 When V GD = V t or V GS - V DS = V t, the channel is pinched off  Inversion layer disappeared at the drain point  Drain current does not disappeared! Channel pinched off

13. 2015-8-16 SJTU J. Chen 13 Drain current under pinch off The electrons pass through the pinch off area at very high speed so as the current continuity holds, similar to the water flow at the Yangtze Gorges Pinched-off channel

14. 2015-8-16 SJTU J. Chen 14 Drain current is saturated and only controlled by the v GS Drain current under pinch off

15. 2015-8-16 SJTU J. Chen 15 v GS creates the channel. Increasing v GS will increase the conductance of the channel. At saturation region only the v GS controls the drain current. At subthreshold region, drain current has the exponential relationship with v GS Drain current controlled by v GS

16. 2015-8-16 SJTU J. Chen 16 Two reasons for readers to be familiar with p channel device p channel device  Existence in discrete-circuit.  More important is the utilization of complementary MOS or CMOS circuits.CMOS

17. 2015-8-16 SJTU J. Chen 17 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. p channel device

18. 2015-8-16 SJTU J. Chen 18  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. Complementary MOS or CMOS

19. 2015-8-16 SJTU J. Chen 19 Circuit symbol Output characteristic curves Channel length modulation Characteristics of p channel device Body effect Temperature effects and Breakdown Region Current-voltage characteristics

20. 2015-8-16 SJTU J. Chen 20 (a)Circuit symbol for the n-channel enhancement-type MOSFET. (b)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. Circuit symbol

21. 2015-8-16 SJTU J. Chen 21 (a)An n-channel enhancement- type MOSFET with v GS and v DS applied and with the normal directions of current flow indicated. (b)The i D – v DS characteristics for a device with k’ n (W/L) = 1.0 mA/V 2. Output characteristic curves of NMOS

22. 2015-8-16 SJTU J. Chen 22 Three distinct region  Cutoff region  Triode region  Saturation region Characteristic equations Circuit model Output characteristic curves of NMOS

23. 2015-8-16 SJTU J. Chen 23 Biased voltage The transistor is turned off. Operating in cutoff region as a switch. Cutoff region

24. 2015-8-16 SJTU J. Chen 24 Biased voltage The channel depth changes from uniform to tapered shape. Drain current is controlled not only by v DS but also by v GS Triode region process transcon- ductance parameter

25. 2015-8-16 SJTU J. Chen 25 Assuming that the draint-source voltage is sufficiently small, the MOS operates as a linear resistance Triode region

26. 2015-8-16 SJTU J. Chen 26 Biased voltage The channel is pinched off. Drain current is controlled only by v GS Drain current is independent of v DS and behaves as an ideal current source. Saturation region

27. 2015-8-16 SJTU J. Chen 27  The i D – v GS characteristic for an enhancement-type NMOS transistor in saturation  V t = 1 V, k’ n W/L = 1.0 mA/V 2  Square law of i D – v GS characteristic curve. Saturation region

28. 2015-8-16 SJTU J. Chen 28 Explanation for channel length modulation  Pinched point moves to source terminal with the voltage v DS increased.  Effective channel length reduced  Channel resistance decreased  Drain current increases with the voltage v DS increased. Current drain is modified by the channel length modulation Channel length modulation

29. 2015-8-16 SJTU J. Chen 29 The MOSFET parameter V A depends on the process technology and, for a given process, is proportional to the channel length L. Channel length modulation

30. 2015-8-16 SJTU J. Chen 30 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. Channel length modulation

31. 2015-8-16 SJTU J. Chen 31 Large-signal equivalent circuit model of the n-channel MOSFET in saturation, incorporating the output resistance r o. The output resistance models the linear dependence of i D on v DS Large-signal equivalent circuit model

32. 2015-8-16 SJTU J. Chen 32 (a)Circuit symbol for the p-channel enhancement-type MOSFET. (b)Modified symbol with an arrowhead on the source lead. (c)Simplified circuit symbol for the case where the source is connected to the body. Characteristics of p channel device

33. 2015-8-16 SJTU J. Chen 33  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. Characteristics of p channel device

34. 2015-8-16 SJTU J. Chen 34 Large-signal equivalent circuit model of the p-channel MOSFET in saturation, incorporating the output resistance r o. The output resistance models the linear dependence of i D on v DS Characteristics of p channel device

35. 2015-8-16 SJTU J. Chen 35 In discrete circuit usually there is no body effect due to the connection between body and source terminal. In IC circuit the substrate is connected to the most negative power supply for NMOS circuit in order to maintain the pn junction reversed biased. The body effect---the body voltage can control i D  Widen the depletion layer  Reduce the channel depth  Threshold voltage is increased  Drain current is reduced The body effect can cause the performance degradation. The body effect

36. 2015-8-16 SJTU J. Chen 36 Temperature effects and breakdown region Drain current will decrease when the temperature increase. Breakdown  Avalanche breakdown  Punched-through  Gate oxide breakdown

37. 2015-8-16 SJTU J. Chen 37 MOS 管注意事项 MOS 管栅 - 衬之间的电容很小，只要有少量的 感应电荷就可产生很高的电压。 由于 R GS(DC) 很大，感应电荷难于释放，感应电 荷所产生的高压会使很薄的绝缘层击穿，造成 管子损坏。 因此，在存放、焊接和电路设计时要多加注意， 应给栅 - 源之间提供直流通路，避免栅极悬空。

38. 2015-8-16 SJTU J. Chen 38 MOS 器件出厂时通常装在黑色的导电泡沫 塑料袋中，切勿自行随便拿个塑料袋装。 可用细铜线把各个引脚连接在一起，或用 锡纸包装。 取出的 MOS 器件不能在塑料板上滑动，应 用金属盘来盛放待用器件。 焊接用的电烙铁必须良好接地。 在焊接前应把电路板的电源线与地线短接， 待 MOS 器件焊接完成后再分开。 MOS 管注意事项

39. 2015-8-16 SJTU J. Chen 39 MOS 器件各引脚的焊接顺序是漏极、源极、 栅极。拆机时顺序相反。 电路板在装机之前，要用接地的线夹子去 碰一下机器的各接线端子，再把电路板接 上去。 MOS 场效应晶体管的栅极在允许条件下， 最好接入保护二极管。在检修电路时应注 意查证原有的保护二极管是否损坏。 MOS 管注意事项

40. SJTU J. Chen 40 2015-8-16 DC analysis Biasing in MOS amplifier circuit and basic configuration

41. 2015-8-16 SJTU J. Chen 41 1. Assuming device operates in saturation thus i D satisfies with i D ~v GS equation. 2. According to biasing method, write voltage loop equation. 3. Combining above two equations and solve these equations. 4. Usually we can get two value of v GS, only the one of two has physical meaning. MOSFET amplifier: DC analysis

42. 2015-8-16 SJTU J. Chen 42 5. Checking the value of v DS i. if v DS ≥v GS -V t, the assuming is correct. ii. if v DS ≤v GS -V t, the assuming is not correct. We shall use triode region equation to solve the problem again. DC analysis

43. 2015-8-16 SJTU J. Chen 43 The NMOS transistor is operating in the saturation region due to Examples of DC analysis

44. 2015-8-16 SJTU J. Chen 44  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 Examples of DC analysis

45. 2015-8-16 SJTU J. Chen 45 The MOSFET as an amplifier Graph determining the transfer characteristic of the amplifier Basic structure of the common-source amplifier

46. 2015-8-16 SJTU J. Chen 46 The MOSFET as an amplifier and as a switch vivi Time vIvI vovo  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

47. 2015-8-16 SJTU J. Chen 47 Homework April 2, 2008:  5.2 ； 5.4 ； 5.9 ； 5.10 ；

48. 2015-8-16 SJTU J. Chen 48 Voltage biasing scheme  Biasing by fixing voltage (constant V GS )  Biasing with feedback resistor Current-source biasing scheme Biasing in MOS amplifier circuits  Disadvantage of fixing biasing  Fixing biasing may result in large I D variability due to deviation in device performance  Current becomes temperature dependent  Unsuitable biasing method

49. 2015-8-16 SJTU J. Chen 49  Biasing using a resistance in the source lead can reduce the variability in I D  Coupling of a signal source to the gate using a capacitor C C1 Biasing in MOS with feedback resistor

50. 2015-8-16 SJTU J. Chen 50 Implementing a constant-current source using a current mirror Biasing in MOS with current-source Biasing the MOSFET using a constant-current source I

51. 2015-8-16 SJTU J. Chen 51 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 Small-signal operation and models

52. 2015-8-16 SJTU J. Chen 52  Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier.  Small signal condition The conceptual circuit

53. 2015-8-16 SJTU J. Chen 53 With the channel-length modulation the effect by including an output resistance The small-signal models Without the channel-length modulation effect — transconductance

54. 2015-8-16 SJTU J. Chen 54 An alternative representation of the T model The small-signal models The T model of the MOSFET augmented with the drain-to- source resistance r o

55. 2015-8-16 SJTU J. Chen 55 Small-signal equivalent-circuit model of a MOSFET in which the source is not connected to the body. Modeling the body effect

56. 2015-8-16 SJTU J. Chen 56 Characteristic parameters Three configurations  Common-source configuration  Common-drain configuration  Common-gate configuration Single-stage MOS amplifier

57. 2015-8-16 SJTU J. Chen 57 Input resistance with no load Input resistance Open-circuit voltage gain Voltage gain Definitions

58. 2015-8-16 SJTU J. Chen 58 Short-circuit current gain Current gain Short-circuit transconductance gain Open-circuit overall voltage gain Overall voltage gain Output resistance Definitions

59. 2015-8-16 SJTU J. Chen 59 Voltage divided coefficient Hence the appropriate configuration should be chosen according to the signal source and load properties, such as source resistance, load resistance, etc Relationships

60. 2015-8-16 SJTU J. Chen 60 Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations. Basic structure of the circuit

61. 2015-8-16 SJTU J. Chen 61  The simplest common-source amplifier biased with constant- current source.  C C1 And C C2 are coupling capacitors.  C S is the bypass capacitor. The common-source amplifier

62. 2015-8-16 SJTU J. Chen 62 Equivalent circuit of the CS amplifier

63. 2015-8-16 SJTU J. Chen 63 Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized. Equivalent circuit of the CS amplifier

64. 2015-8-16 SJTU J. Chen 64 Input resistance Voltage gain Overall voltage gain Output resistance Characteristics of CS amplifier  Summary of CS amplifier  Very high input resistance  Moderately high voltage gain  Relatively high output resistance

65. 2015-8-16 SJTU J. Chen 65 The CS amplifier with a source resistance

66. 2015-8-16 SJTU J. Chen 66 Small-signal equivalent circuit with r o neglected  Voltage gain  Overall voltage gain R S takes the effect of negative feedback Gain is reduction by (1+g m R S )

67. 2015-8-16 SJTU J. Chen 67  Biasing with constant current source I  Input signal v sig is applied to the source  Output is taken at the drain  Gate is signal grounded  C C1 and C C2 are coupling capacitors The Common-Gate amplifier

68. 2015-8-16 SJTU J. Chen 68 The CG amplifier  A small-signal equivalent circuit  T model is used in preference to the π model  R o is neglecting

69. 2015-8-16 SJTU J. Chen 69 The CG amplifier fed with a current-signal input Voltage gain Overall voltage gain

70. 2015-8-16 SJTU J. Chen 70 Noninverting amplifier Low input resistance Relatively high output resistance Current follower Superior high-frequency performance Summary of CG amplifier

71. 2015-8-16 SJTU J. Chen 71  Biasing with current source  Input signal is applied to gate, output signal is taken at the source The common-drain or source-follower amplifier

72. 2015-8-16 SJTU J. Chen 72 The CD or source-follower amplifier  Small-signal equivalent- circuit model  T model makes analysis simpler  Drain is signal grounded Overall voltage gain

73. 2015-8-16 SJTU J. Chen 73 Circuit for determining the output resistance

74. 2015-8-16 SJTU J. Chen 74 Very high input resistance Voltage gain is less than but close to unity Relatively low output resistance Voltage buffer amplifier Power amplifier Summary of CD or source-follow amplifier

75. 2015-8-16 SJTU J. Chen 75 The CS amplifier is the best suited for obtaining the bulk of gain required in an amplifier. Including resistance R S 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. It can be used as a current buffer. Source follower finds application as a voltage buffer and as the output stage in a multistage amplifier. Summary and comparisons

76. 2015-8-16 SJTU J. Chen 76 Internal capacitances  The gate capacitive effect  Triode region  Saturation region  Cutoff region  Overlap capacitance  The junction capacitances  Source-body depletion-layer capacitance  drain-body depletion-layer capacitance High-frequency model The internal capacitance and high-frequency model

77. 2015-8-16 SJTU J. Chen 77 MOSFET operates at triode region MOSFET operates at saturation region MOSFET operates at cutoff region The gate capacitive effect

78. 2015-8-16 SJTU J. Chen 78 Overlap capacitance results from the fact that the source and drain diffusions extend slightly under the gate oxide. The expression for overlap capacitance Typical value Overlap capacitance This additional component should be added to C gs and C gd in all preceding formulas

79. 2015-8-16 SJTU J. Chen 79 Source-body depletion-layer capacitance drain-body depletion-layer capacitance The junction capacitances

80. 2015-8-16 SJTU J. Chen 80 High-frequency model

81. 2015-8-16 SJTU J. Chen 81 The equivalent circuit model with C db neglected (to simplify analysis) High-frequency model The equivalent circuit for the case in which the source is connected to the substrate (body)

82. 2015-8-16 SJTU J. Chen 82 Current gain Unity-gain frequency The MOSFET unity-gain frequency

83. 2015-8-16 SJTU J. Chen 83 The depletion-type MOSFET 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

84. 2015-8-16 SJTU J. Chen 84 Simplified circuit symbol applicable for the case the substrate (B) is connected to the source (S). Circuit symbol for the n-channel depletion-MOS Circuit symbol for the n- channel depletion-type MOSFET

85. 2015-8-16 SJTU J. Chen 85 Characteristic curves  Expression of characteristic equation  Drain current with the i D –v GS characteristic in saturation

86. 2015-8-16 SJTU J. Chen 86  Sketches of the i D – v GS characteristics for MOSFETs of enhancement and depletion types  The characteristic curves intersect the v GS axis at V t. The i D – v GS characteristic in saturation

87. 2015-8-16 SJTU J. Chen 87 The output characteristic curves

88. 2015-8-16 SJTU J. Chen 88N - c h a n n e l Depletion layer G D S GDS n-type Semiconductor The junction FET P+P+P+P+ P+P+P+P+

89. 2015-8-16 SJTU J. Chen 89 U GS = 0 U GS < 0 U GS = U GS(off) D S P+P+P+P+ Physical operation under v DS =0 G P+P+P+P+DS P+P+P+P+ G P+P+P+P+ DS G P+P+P+P+ P+P+P+P+

90. 2015-8-16 SJTU J. Chen 90 The effect of U DS on I D for U GS(off) <U GS < 0 动画

91. SJTU J. Chen 91 2015-8-16 Summary of semiconductor devices

92. 2015-8-16 SJTU J. Chen 92 Diode, BJT and FET are nonlinear devices made of semiconductor, mostly silicon Diode  A diode allows current to flow in forward direction and hence can perform functions such as rectification, demodulation/detection, switch etc.  The reverse current may become dramatically large at breakdown, such phenomena can be used as voltage regulator

93. 2015-8-16 SJTU J. Chen 93 Bipolar Junction Transistor  A BJT has three terminals: base, emitter and collector  The collector current is controlled by voltage/ current on the base-emitter junction and is almost independent on collector voltage.  It can perform functions such as amplification and switch, etc.  A BJT should be properly biased for normal operation  There are three basic configurations, each has different performance (input/output resistance, gain, high frequency response, etc)

94. 2015-8-16 SJTU J. Chen 94 Field Effect Transistor  A FET has three terminals: gate, source and drain  The drain current is controlled by gate voltage and is almost independent on drain voltage.  It can perform functions such as amplification, logic calculation and switch, etc.  A FET should be properly biased for normal operation  There are three basic configurations, each has different performance (input/output resistance, gain, high frequency response, etc)

95. 2015-8-16 SJTU J. Chen 95 As the microelectronics develops, more and more functions are fulfilled by IC chips The discrete devices and circuits, however, are still very important not only for practical applications, but also for better understanding and design of LSICs Quantitative calculation is sometimes complicated but not difficult As long as you know the parameter definitions clearly, results can be derived KCL, KVL, etc

96. 2015-8-16 SJTU J. Chen 96 Homework April 6, 2010:  5.25 ； 5.40 ； 5.47 ； 5.63; 5.116