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Chapter 4 Field-Effect Transistors

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1 Chapter 4 Field-Effect Transistors
Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap4-1

2 Microelectronic Circuit Design
Chapter Goals Describe operation of MOSFETs and JFETs. Define FET characteristics in operation regions of cutoff, triode and saturation. Develop mathematical models for i-v characteristics of MOSFETs and JFETs. Introduce graphical representations for output and transfer characteristic descriptions of electron devices. Define and contrast characteristics of enhancement-mode and depletion-mode FETs. Define symbols to represent FETs in circuit schematics. Investigate circuits that bias transistors into different operating regions. Learn basic structure and mask layout for MOS transistors and circuits. Explore MOS device scaling Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-2

3 Microelectronic Circuit Design
Chapter Goals (contd.) Contrast 3 and 4 terminal device behavior. Descibe sources of capacitance in MOSFETs and JFETs. Explore FET modeling in SPICE. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-3

4 Microelectronic Circuit Design
Moore’s Law Intel co-founder Gordon Moore is a visionary. In 1965, his prediction, popularly known as Moore's Law, states that the number of transistors on a chip will double about every two years. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

5 Microelectronic Circuit Design
Moore’s Law (cont.) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

6 Microelectronic Circuit Design
Moore’s Law (cont.) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

7 Microelectronic Circuit Design
Moore’s Law (cont.) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

8 Microelectronic Circuit Design
Moore’s Law (cont.) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

9 Microelectronic Circuit Design
Moore’s Law (cont.) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

10 Microelectronic Circuit Design
Transistor History Nobel prize for transistor (William Shockley) – link History of transistors - link Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

11 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 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-11

12 MOS Capacitor Structure
First electrode- Gate : Consists of low-resistivity material such as polycrystalline silicon Second electrode- Substrate or Body: n- or p-type semiconductor Dielectric-Silicon dioxide:stable high-quality electrical insulator between gate and substrate. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-12

13 Substrate Conditions for Different Biases
Accumulation VG<<VTN Depletion VG<VTN Inversion VG>VTN Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-13

14 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. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-14

15 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 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-15

16 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. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-16

17 NMOS Transistor: Triode Region Characteristics
for where, Kn= Kn’W/L Kn’=μnCox’’ (A/V2) Cox’’=εox/Tox εox=oxide permittivity (F/cm) Tox=oxide thickness (cm) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-17

18 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 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-18

19 MOSFET as Voltage-Controlled Resistor
Example 1: Voltage-Controlled Attenuator If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then, If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then, To maintain triode region operation, or or Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-19

20 MOSFET as Voltage-Controlled Resistor (contd.)
Example 2: Voltage-Controlled High-Pass Filter Voltage Transfer function, where, cut-off frequency If Kn=500μA/V2, VTN=1V, C=0.02μF and VGG=1.5V, then, To maintain triode region operation, Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-20

21 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. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-21

22 NMOS Transistor: Saturation Region (contd.)
for is also called saturation or pinch-off voltage Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-22

23 Microelectronic Circuit Design
State of the Art – 32 nm Intel – power of smaller - link 32nm high k metal gate transistor - link Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

24 Transconductance of a MOS Device
Transconductance relates the change in drain current to a change in gate-source voltage Taking derivative of the expression for the drain current in saturation region, Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-24

25 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 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-25

26 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. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-26

27 Transfer Characteristics of MOSFETS
Plots drain current versus gate-source voltage for a fixed drain-source voltage Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-27

28 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) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-28

29 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 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-29

30 Enhancement-Mode PMOS Transistors: Output Characteristics
For , transistor is off. For more negative vGS, drain current increases in magnitude. PMOS is in triode region for small values of VDS and in saturation for larger values. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-30

31 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 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-31

32 40 years of Moore’s Law and CPU
The history of microprocessor- link The history of intel CPU – link What did you do 40 years ago? - link 45nm : biggest change to transistor in 40 years - link Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

33 Process-defining Factors
Minimum Feature Size, F : Width of smallest line or space that can be reliably transferred to wafer surface using given generation of lithographic manufacturing tools Alignment Tolerance, T: Maximum misalignment that can occur between two mask levels during fabrication Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-33

34 Mask Sequence for a Polysilicon-Gate Transistor
Mask 1: Defines active area or thin oxide region of transistor Mask 2: Defines polysilicon gate of transistor, aligns to mask 1 Mask 3: Delineates the contact window, aligns to mask 2. Mask 4: Delineates the metal pattern, aligns to mask 3. Channel region of transistor formed by intersection of first two mask layers. Source and Drain regions formed wherever mask 1 is not covered by mask 2 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-34

35 Basic Ground Rules for Layout
T=F/2=L, L could be 1, 0.5, 0.25 mm, etc. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-35

36 Internal Capacitances in Electronic Devices
Limit high-frequency performance of the electronic device they are associated with. Limit switching speed of circuits in logic applications Limit frequency at which useful amplification can be obtained in amplifiers. MOSFET capacitances depend on operation region and are non-linear functions of voltages at device terminals. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-36

37 NMOS Transistor Capacitances: Triode Region
Cox” =Gate-channel capacitance per unit area(F/m2). CGC =Total gate channel capacitance. CGS = Gate-source capacitance. CGD =Gate-drain capacitance. CGSO and CGDO = overlap capacitances (F/m). Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-37

38 NMOS Transistor Capacitances: Triode Region (contd.)
CSB = Source-bulk capacitance. CDB = Drain-bulk capacitance. AS and AD = Junction bottom area capacitance of the source and drain regions. PS and PD = Perimeter of the source and drain junction regions. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-38

39 NMOS Transistor Capacitances: Saturation Region
Drain no longer connected to channel Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-39

40 NMOS Transistor Capacitances: Cutoff Region
Conducting channel region completely gone. CGB = Gate-bulk capacitance CGBO = gate-bulk capacitance per unit width. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-40

41 SPICE Model for NMOS Transistor
Typical default values used by SPICE: Kn or Kp = 20 mA/V2 g = 0 l = 0 VTO = 1 V mn or mp = 600 cm2/V.s 2FF = 0.6 V CGDO=CGSO=CGBO=CJSW= 0 Tox= 100 nm Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-41

42 Bias Analysis Approach
Assume an operation region (generally the saturation region) Use circuit analysis to find VGS Use VGS to calculate ID, and ID to find VDS Check validity of operation region assumptions Change assumptions and analyze again if required. NOTE :An enhancement-mode device with VDS = VGS is always in saturation Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-42

43 Constant Gate-Source MOSFET Bias Circuit
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

44 Simplified MOSFET Bias Circuit
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

45 Microelectronic Circuit Design
Loadline Analysis Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

46 Four-Resistor and Two-Resistor Biasing
Provide excellent bias for transistors in discrete circuits. Stabilize bias point with respect to device parameter and temperature variations using negative feedback. Use single voltage source to supply both gate-bias voltage and drain current. Generally used to bias transistors in saturation region. Two-resistor biasing uses lesser components that four-resistor biasing and also isolates drain and gate terminals Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-46

47 Bias Analysis: Example 1 (Four-Resistor Biasing)
Assumption: Transistor is saturated, IG=IB=0 Analysis: First, simplify circuit, split VDD into two equal-valued sources and apply Thevenin transformation to find VEQ and REQ for gate-bias voltage Problem: Find Q-pt (ID, VDS) Approach: Assume operation region, find Q-point, check to see if result is consistent with operation region Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-47

48 Bias Analysis: Example 1 (Four-Resistor Biasing) (contd.)
Since VGS<VTN for VGS= V and MOSFET will be cut-off, and ID= 34.4 mA Also, Since IG=0, VDS>VGS-VTN. Hence saturation region assumption is correct. Q-pt: (34.4 mA, 6.08 V) with VGS= 2.66 V Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-48

49 Bias Analysis: Example 2 (Four-Resistor Biasing)
Analysis with body effect using same assumptions as in example 1: Iterative solution can be found by following steps: Estimate value of ID and use it to find VGS and VSB Use VSB to calculate VTN Find ID’ using above 2 steps If ID’ is not same as original ID estimate, start again. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-49

50 Bias Analysis: Example 2 (Four-Resistor Biasing) (contd.)
The iteration sequence leads to ID= 88.0 mA VDS>VGS-VTN. Hence saturation region assumption is correct. Q-pt: (88.0 mA, 6.48 V) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-50

51 Microelectronic Circuit Design
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

52 Four Transistor Bias Summary
VEQ = VGS + IDS RS (4.48) VDD = IDS RD + VDS + IDS RS (4.50) IDS   VGS   IDS  VDS   IDS   VGS   IDS   VDS  Assume that the transistor is operating in the saturation region with IDS = Kn/2 (VGS – VTN) (4.47) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

53 Microelectronic Circuit Design
VEQ=VGS+ RsKn/2 (VGS – VTN)2 (4.49) Solve VGS and check to make sure transistor is not cutoff. Use (4.53) to compute IDS, (4.52) to get VDS Verify that transistor is in saturation mode because VDS ≥ VGS-VTN Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

54 Bias Analysis: Example 3 (Two-Resistor Biasing)
Since VGS<VTN for VGS= V and MOSFET will be cut-off, Assumption: IG=IB=0, transistor is saturated (since VDS= VGS) Analysis: and ID= 130 mA VDS>VGS-VTN. Hence saturation region assumption is correct. Q-pt: (130 mA, 2.00 V) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-54

55 Bias Analysis: Example 4 ( Biasing in Triode Region)
Also But VDS<VGS-VTN. Hence, saturation region assumption is incorrect Using triode region equation, Assumption: IG=IB=0, transistor is saturated (since VDS= VGS) Analysis: VGS=VDD=4 V and ID=1.06 mA VDS<VGS-VTN, transistor is in triode region Q-pt:(1.06 mA, 2.3 V) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-55

56 Bias Analysis: Example 5 (Two-Resistor biasing for PMOS Transistor)
Also Since VGS= V is less than VTP= -2 V, VGS = V ID = 52.5 mA and VGS = V Assumption: IG=IB=0, transistor is saturated (since VDS= VGS) Analysis: Hence saturation assumption is correct. Q-pt: (52.5 mA, V) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-56

57 Microelectronic Circuit Design
Feedback Analysis Negative Feedback is used to stabilize the operating point. ID   -VDS   -VGS   ID  Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

58 MOSFET as Current Source
Ideal current source gives fixed output current regardless of voltage across it. MOSFET behaves as as an ideal current source if biased in the pinch-off region (output current depends on terminal voltage). Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-58

59 Microelectronic Circuit Design
NMOS Current Mirror But VGS2=VGS1 Assumption: M1 and M2 have identical VTN, Kn’, l and W/L and are in saturation. Thus, output current mirrors reference current if VDS1=VDS2. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-59

60 NMOS Current Mirror: Example
Given data: IREF= 50 mA, VO= 12 V, VTN= 1 V, Kn= 150 mA/V2, l= V-1 Determine: VGS, VDS1, IO Analysis: Using trial-and-error, Hence, VDS1 =1.81 V. Also, VDS2=12 V Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-60

61 MOS Current Mirror Ratio
Thus, ratio between IO and IREF can be modified by changing W/L ratios of the current mirror transistors (ignoring differences due to VDS mismatch) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-61

62 MOS Current Mirror Output Resistance
Output current changes with vDS due to channel length modulation. Output resistance is given by In the current mirror, vO = vDS2 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-62

63 Microelectronic Circuit Design
Current Mirror Layout Two possible layouts of a current mirror Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-63

64 Design of Multiple Current Mirrors: Example
Choose R to set IREF= 25 mA Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-64

65 Design of Multiple Current Mirrors (contd).
and R can be replaced by transistor M6 for better integration. We know that VGS6 = V and ID = 25 mA and M6 is in saturation Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-65

66 MOS Transistor Scaling
Drain current: Gate Capacitance: wheret is the circuit delay in a logic circuit. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-66

67 MOS Transistor Scaling (contd.)
Circuit and Power Densities: Power-Delay Product: Cutoff Frequency: fT improves with square of channel length reduction Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-67

68 MOS Transistor Scaling (contd.)
High Field Limitations: High electric fields arise if technology is scaled down with supply voltage constant. Cause reduction in mobility of MOS transistor, breakdown of linear relationship between mobility and electric field and carrier velocity saturation. Ultimately results in reduced long-term reliability and breakdown of gate oxide or pn junction. Drain current in saturation region is linearised to where, vSAT is carrier saturation velocity Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-68

69 MOS Transistor Scaling (contd.)
Sub-threshold Conduction: ID decreases exponentially for VGS<VTN. Reciprocal of the slope in mV/decade gives the turn off rate for the MOSFET. VTN should be reduced if dimensions are scaled down, but curve in sub-threshold region shifts horizontally instead of scaling with VTN.. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-69

70 Microelectronic Circuit Design
MOS Scaling Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

71 SIA MOS Technology Roadmap
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

72 Worldwide Semiconductor Sales Trend
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

73 Technology Cross-over
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

74 Linear Technology Shrink
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

75 MOS Technology Scaling Ratio
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

76 Microelectronic Circuit Design
Linear Shrink Cost Per Function : 40% reduction Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

77 Re-Design Scale Factors
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

78 Microelectronic Circuit Design
Intel CPU Scaling Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

79 Wafer Diameter Increase
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

80 Microelectronic Circuit Design
Dies on a Wafer Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

81 Microelectronic Circuit Design
Gross Die Per Wafer Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

82 Microelectronic Circuit Design
Gate Cost Die Size too small – I/O overhead Die Size too big - Complexity Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

83 Microelectronic Circuit Design
Gate Cost Pentium 4 has 14M Gates – Near Optimum Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

84 Microelectronic Circuit Design
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85 Microelectronic Circuit Design
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86 Microelectronic Circuit Design
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87 Microelectronic Circuit Design
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88 Microelectronic Circuit Design
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89 Microelectronic Circuit Design
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90 Microelectronic Circuit Design
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91 Microelectronic Circuit Design
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92 Microelectronic Circuit Design
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94 Microelectronic Circuit Design
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95 Microelectronic Circuit Design
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96 Microelectronic Circuit Design
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97 Microelectronic Circuit Design
Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill

98 Junction Field-Effect Transistor (JFET) Structure
Much lower input current and much higher input impedance than BJT. In triode region, JFET is a voltage-controlled resistor, r = resistivity of channel L = channel length W = channel width between pn junction depletion regions t = channel depth Inherently a depletion-mode device N-type semiconductor block that houses the channel region in n-channel JFET. Two pn junctions to form the gate. Current enters channel at drain and exits at source. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-98

99 JFET with Gate-Source Bias
vGS =0, gate isolated from channel. VP < vGS <0, W’<W, channel resistance increases, gate-source junction reverse-biased, iG almost 0. vGS = VP <0, channel region pinched-off, channel resistance infinite. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-99

100 JFET Channel with Drain-Source Bias
With constant vGS, depletion region near drain increases with vDS. At vDSP = vGS - VP,channel is totally pinched-off, iD is saturated. JFET also suffers from channel length modulation like MOSFET at larger values of vDS. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-100

101 N-Channel JFET: i-v Characteristics
Transfer Characteristics Output Characteristics Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-101

102 N-Channel JFET: i-v Characteristics (contd.)
For all regions : iG=0 for In cutoff region: iD =0 for (VP <0). In Triode region: for and In pinch-off region: for Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-102

103 Microelectronic Circuit Design
P-Channel JFET Polarities of n- and p-type regions of n-channel JFET are reversed to get the p-Channel JFET. Channel current direction and operating bias voltages are also reversed. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-103

104 Microelectronic Circuit Design
JFET Circuit Symbols JFET structures are symmetric like MOSFET. Source and drain determined by circuit voltages. Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-104

105 JFET Capacitances and SPICE Modeling
CGD and CGS are determined by depletion-layer capacitances of reverse-biased pn junctions forming gate and are bias dependent. Typical default values used by SPICE: Vp = -2 V l = CGD = CGD =0 Transconductance parameter= BETA =IDSS/VP2= 100 mA/V2 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-105

106 Biasing JFET and Depletion-Mode MOSFET: Example
N-channel JFET Depletion-mode MOSFET Assumptions: JFET is pinched-off, gate-channel junction is reverse-biased, reverse leakage current of gate, IG = 0 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-106

107 Biasing JFET and Depletion-Mode MOSFET: Example (contd.)
Analysis: Since IS = ID , Since VGS= V is less than VP= -5 V, VGS = V and, ID = IS = 1.91 mA. Also, VDS>VGS-VP. Hence pinch-off region assumption is correct and gate-source junction is reverse-biased by 1.91V. Q-pt: (1.91 mA, 6.27 V) Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill Chap1-107

108 Microelectronic Circuit Design
Homework 4.22 4.81 4.85(a) 4.103(b) 4.145 Jaeger/Blalock 7/1/03 Microelectronic Circuit Design McGraw-Hill


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