Chapter 5 FieldEffect Transistors FETs 2022125 SJTU J
- Slides: 96
Chapter 5 Field-Effect Transistors (FETs) 2022/1/25 SJTU J. Chen 1
2 Content l Physical operation and current-voltage characteristics l DC analysis l Biasing in MOS amplifier circuit and basic configuration 2022/1/25 SJTU J. Chen
Physical operation and current -voltage characteristics 2022/1/25 SJTU J. Chen 3
4 Introduction to FET: Field Effect Transistor l There are two types l Ø Ø l MOSFET: metal-oxide-semiconductor FET JFET: Junction FET MOSFET is also called the insulated-gate FET or IGFET. Ø Ø Quite small Simple manufacturing process Low power consumption Widely used in VLSI circuits(>800 million on a single IC chip) 2022/1/25 SJTU J. Chen
Device structure of MOSFET (n-type) Source(S) Oxide Gate(G) (Si. O 2) n+ Drain(D) Metal Channel area n+ p-type Semiconductor Substrate (Body) Body(B) l For normal operation, it is needed to create a conducting channel between Source and Drain 2022/1/25 SJTU J. Chen 5
Creating a channel for current flow Ø An n channel can be induced at the top of the substrate beneath the gate by applying a positive voltage to the gate Ø The channel is an inversion layer Ø The value of VGS at which a sufficient number of mobile electrons accumulate to form a conducting channel is called the threshold voltage (Vt) 2022/1/25 SJTU J. Chen 6
Device structure of MOSFET (n-type) ØL = 0. 1 to 3 mm ØW = 0. 2 to 100 mm ØTox= 2 to 50 nm Cross-section view 2022/1/25 SJTU J. Chen 7
8 Classification of FET l According to the type of the channel, FETs can be classified as Ø MOSFET § § Ø N channel P channel • Enhancement type • Depletion type JFET 2022/1/25 § P channel § N channel SJTU J. Chen
9 Drain current under small voltage v. DS Ø An NMOS transistor with v. GS > Vt and with a small v. DS applied. - The channel depth is uniform and the device acts as a resistance. Ø The channel conductance is proportional to effective voltage, or excess gate voltage, (v. GS – Vt). Ø Drain current is proportional to (v. GS – Vt) and v. DS. 2022/1/25 SJTU J. Chen
10 Drain current under small voltage v. DS 2022/1/25 SJTU J. Chen
Operation as v. DS is increased Ø The induced channel acquires a tapered shape. Ø Channel resistance increases as v. DS is increased. Ø Drain current is controlled by both of the two voltages. B 2022/1/25 SJTU J. Chen 11
12 Channel pinched off l When VGD = Vt or VGS - VDS = Vt , the channel is pinched off Ø Ø Inversion layer disappeared at the drain point Drain current does not disappeared! 2022/1/25 SJTU J. Chen
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 2022/1/25 SJTU J. Chen 13
Drain current under pinch off l Drain current is saturated and only controlled by the v. GS 2022/1/25 SJTU J. Chen 14
Drain current controlled by v. GS l v. GS creates the channel. l Increasing v. GS will increase the conductance of the channel. l At saturation region only the v. GS controls the drain current. l At subthreshold region, drain current has the exponential relationship with v. GS 2022/1/25 SJTU J. Chen 15
16 p channel device l Two reasons for readers to be familiar with p channel device Existence in discrete-circuit. Ø More important is the utilization of complementary MOS or CMOS circuits. Ø 2022/1/25 SJTU J. Chen
17 p channel device l Structure of p channel device Ø The substrate is n type and the inversion layer is p type. Ø Carrier is hole. Ø Threshold voltage is negative. Ø All the voltages and currents are opposite to the ones of n channel device. Ø Physical operation is similar to that of n channel device. 2022/1/25 SJTU J. Chen
Complementary MOS or CMOS Ø The PMOS transistor is formed in n well. Ø Another arrangement is also possible in which an n-type body is used and the n device is formed in a p well. Ø CMOS is the most widely used of all the analog and digital IC circuits. 2022/1/25 SJTU J. Chen 18
Current-voltage characteristics l Circuit symbol l Output characteristic curves l Channel length modulation l Characteristics of p channel device l Body effect l Temperature effects and Breakdown Region 2022/1/25 SJTU J. Chen 19
20 Circuit symbol (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. 2022/1/25 SJTU J. Chen
Output characteristic curves of NMOS (a) An n-channel enhancementtype 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 m. A/V 2. 2022/1/25 SJTU J. Chen 21
Output characteristic curves of NMOS • Three distinct region Ø Cutoff region Ø Triode region Ø Saturation region • Characteristic equations • Circuit model 2022/1/25 SJTU J. Chen 22
23 Cutoff region • Biased voltage • The transistor is turned off. • Operating in cutoff region as a switch. 2022/1/25 SJTU J. Chen
24 Triode region • 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 process transconductance parameter 2022/1/25 SJTU J. Chen
25 Triode region • Assuming that the draint-source voltage is sufficiently small, the MOS operates as a linear resistance 2022/1/25 SJTU J. Chen
26 Saturation region • 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. 2022/1/25 SJTU J. Chen
27 Saturation region Ø The i. D–v. GS characteristic for an enhancement-type NMOS transistor in saturation Ø Vt = 1 V, k’n W/L = 1. 0 m. A/V 2 Ø Square law of i. D–v. GS characteristic curve. 2022/1/25 SJTU J. Chen
Channel length modulation • Explanation for channel length modulation Ø Pinched point moves to source terminal with the voltage v. DS increased. Ø Effective channel length reduced Ø Channel resistance decreased Ø Drain current increases with the voltage v. DS increased. • Current drain is modified by the channel length modulation 2022/1/25 SJTU J. Chen 28
Channel length modulation The MOSFET parameter VA depends on the process technology and, for a given process, is proportional to the channel length L. 2022/1/25 SJTU J. Chen 29
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. 2022/1/25 SJTU J. Chen 30
Large-signal equivalent circuit model of the n-channel MOSFET in saturation, incorporating the output resistance ro. The output resistance models the linear dependence of i. D on v. DS 2022/1/25 SJTU J. Chen 31
Characteristics of p channel device (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. 2022/1/25 SJTU J. Chen 32
Characteristics of p channel device Ø The MOSFET with voltages applied and the directions of current flow indicated. Ø The relative levels of the terminal voltages of the enhancement -type PMOS transistor for operation in the triode region and in the saturation region. 2022/1/25 SJTU J. Chen 33
Characteristics of p channel device Large-signal equivalent circuit model of the p-channel MOSFET in saturation, incorporating the output resistance ro. The output resistance models the linear dependence of i. D on v. DS 2022/1/25 SJTU J. Chen 34
35 The body effect In discrete circuit usually there is no body effect due to the connection between body and source terminal. l 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. l The body effect---the body voltage can control i. D l Ø Ø l Widen the depletion layer Reduce the channel depth Threshold voltage is increased Drain current is reduced The body effect can cause the performance degradation. 2022/1/25 SJTU J. Chen
Temperature effects and breakdown region l Drain current will decrease when the temperature increase. l Breakdown Ø Avalanche breakdown Ø Punched-through Ø Gate oxide breakdown 2022/1/25 SJTU J. Chen 36
• DC analysis • Biasing in MOS amplifier circuit and basic configuration 2022/1/25 SJTU J. Chen 40
MOSFET amplifier: DC analysis 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. 2022/1/25 SJTU J. Chen 41
42 DC analysis 5. Checking i. the value of v. DS if v. DS≥v. GS-Vt, the assuming is correct. ii. if v. DS≤v. GS-Vt, the assuming is not correct. We shall use triode region equation to solve the problem again. 2022/1/25 SJTU J. Chen
Examples of DC analysis The NMOS transistor is operating in the saturation region due to 2022/1/25 SJTU J. Chen 43
Examples of DC analysis ØAssuming the MOSFET operate in the saturation region ØChecking the validity of the assumption ØIf not to be valid, solve the problem again for triode region 2022/1/25 SJTU J. Chen 44
The MOSFET as an amplifier Basic structure of the common -source amplifier Graph determining the transfer characteristic of the amplifier 2022/1/25 SJTU J. Chen 45
46 The MOSFET as an amplifier and as a switch Ø Transfer characteristic showing operation as an amplifier biased at point Q. Ø Three segments: vo Time § XA---the cutoff region segment § AQB---the saturation region segment § BC---the triode region segment v. I vi 2022/1/25 Time SJTU J. Chen
47 Homework l April Ø 2, 2008: 5. 2; 5. 4; 5. 9; 5. 10; 2022/1/25 SJTU J. Chen
Biasing in MOS amplifier circuits l Voltage biasing scheme Ø Ø l Biasing by fixing voltage (constant VGS) Biasing with feedback resistor Current-source biasing scheme ØDisadvantage of fixing biasing LFixing biasing may result in large ID variability due to deviation in device performance LCurrent becomes temperature dependent - Unsuitable biasing method 2022/1/25 SJTU J. Chen 48
Biasing in MOS with feedback resistor Ø Biasing using a resistance in the source lead can reduce the variability in ID Ø Coupling of a signal source to the gate using a capacitor CC 1 2022/1/25 SJTU J. Chen 49
Biasing in MOS with current-source Implementing a constant-current source using a current mirror Biasing the MOSFET using a constant-current source I 2022/1/25 SJTU J. Chen 50
Small-signal operation and models l The ac characteristic Ø Definition of transconductance Ø Definition of output resistance Ø Definition of voltage gain l Small-signal 2022/1/25 model Ø Hybrid π model Ø T model Ø Modeling the body effect SJTU J. Chen 51
52 The conceptual circuit Ø Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier. Ø Small signal condition 2022/1/25 SJTU J. Chen
The small-signal models With the channel-length modulation the effect by including an output resistance Without the channel-length modulation effect —transconductance 2022/1/25 SJTU J. Chen 53
The small-signal models The T model of the MOSFET augmented with the drain-tosource resistance ro 2022/1/25 SJTU An alternative representation of the T model J. Chen 54
Modeling the body effect Small-signal equivalent-circuit model of a MOSFET in which the source is not connected to the body. 2022/1/25 SJTU J. Chen 55
Single-stage MOS amplifier l Characteristic parameters l Three configurations 2022/1/25 Ø Common-source configuration Ø Common-drain configuration Ø Common-gate configuration SJTU J. Chen 56
57 Definitions l Input resistance with no load l Input resistance l Open-circuit voltage gain l Voltage gain 2022/1/25 SJTU J. Chen
58 Definitions l Short-circuit current gain l Current gain l Short-circuit transconductance gain l Open-circuit overall voltage gain l Output resistance 2022/1/25 SJTU J. Chen
59 Relationships l Voltage divided coefficient l Hence the appropriate configuration should be chosen according to the signal source and load properties, such as source resistance, load resistance, etc 2022/1/25 SJTU J. Chen
Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations. 2022/1/25 SJTU J. Chen 60
The common-source amplifier Ø The simplest common-source amplifier biased with constantcurrent source. Ø CC 1 And CC 2 are coupling capacitors. Ø CS is the bypass capacitor. 2022/1/25 SJTU J. Chen 61
Equivalent circuit of the CS amplifier 2022/1/25 SJTU J. Chen 62
Equivalent circuit of the CS amplifier Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized. 2022/1/25 SJTU J. Chen 63
Characteristics of CS amplifier l Input resistance l Voltage gain l Overall voltage gain l Output resistance Ø Summary of CS amplifier - 2022/1/25 Very high input resistance Moderately high voltage gain Relatively high output resistance SJTU J. Chen 64
The CS amplifier with a source resistance 2022/1/25 SJTU J. Chen 65
66 Small-signal equivalent circuit with ro neglected Ø Voltage gain Ø Overall voltage gain RS takes the effect of negative feedback l Gain is reduction by (1+gm. RS) l 2022/1/25 SJTU J. Chen
67 The Common-Gate amplifier ØBiasing with constant current source I ØInput signal vsig is applied to the source ØOutput is taken at the drain ØGate is signal grounded ØCC 1 and CC 2 are coupling capacitors 2022/1/25 SJTU J. Chen
68 The CG amplifier Ø A small-signal equivalent circuit Ø T model is used in preference to the π model Ø Ro is neglecting 2022/1/25 SJTU J. Chen
69 The CG amplifier fed with a current-signal input l Voltage gain l Overall voltage gain 2022/1/25 SJTU J. Chen
Summary of CG amplifier l Noninverting amplifier l Low input resistance l Relatively high output resistance l Current follower l Superior high-frequency performance 2022/1/25 SJTU J. Chen 70
The common-drain or source-follower amplifier ØBiasing with current source ØInput signal is applied to gate, output signal is taken at the source 2022/1/25 SJTU J. Chen 71
The CD or source-follower amplifier Ø Small-signal equivalentcircuit model Ø T model makes analysis simpler Ø Drain is signal grounded Overall voltage gain 2022/1/25 SJTU J. Chen 72
Circuit for determining the output resistance 2022/1/25 SJTU J. Chen 73
Summary of CD or source-follow amplifier l Very high input resistance l Voltage gain is less than but close to unity l Relatively low output resistance l Voltage buffer amplifier l Power amplifier 2022/1/25 SJTU J. Chen 74
Summary and comparisons l The CS amplifier is the best suited for obtaining the bulk of gain required in an amplifier. l Including resistance RS in the source lead of CS amplifier provides a number of improvements in its performance. l The low input resistance of CG amplifier makes it useful only in specific application. It has excellent highfrequency response. It can be used as a current buffer. l Source follower finds application as a voltage buffer and as the output stage in a multistage amplifier. 2022/1/25 SJTU J. Chen 75
The internal capacitance and high-frequency model l Internal Ø The gate capacitive effect § § Ø capacitances Triode region Saturation region Cutoff region Overlap capacitance The junction capacitances § § Source-body depletion-layer capacitance drain-body depletion-layer capacitance l High-frequency 2022/1/25 model SJTU J. Chen 76
The gate capacitive effect l MOSFET operates at triode region l MOSFET operates at saturation region l MOSFET operates at cutoff region 2022/1/25 SJTU J. Chen 77
78 Overlap capacitance results from the fact that the source and drain diffusions extend slightly under the gate oxide. l The expression for overlap capacitance l Typical value l This additional component should be added to Cgs and Cgd in all preceding formulas l 2022/1/25 SJTU J. Chen
The junction capacitances • Source-body depletion-layer capacitance • drain-body depletion-layer capacitance 2022/1/25 SJTU J. Chen 79
High-frequency model 2022/1/25 SJTU J. Chen 80
81 High-frequency model The equivalent circuit for the case in which the source is connected to the substrate (body) The equivalent circuit model with Cdb neglected (to simplify analysis) 2022/1/25 SJTU J. Chen
The MOSFET unity-gain frequency l Current gain l Unity-gain 2022/1/25 frequency SJTU J. Chen 82
The depletion-type MOSFET l. 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 Ø There is no need to induce a channel Ø The depletion MOSFET can be operated at both enhancement mode and depletion mode Ø 2022/1/25 SJTU J. Chen 83
Circuit symbol for the n-channel depletion-MOS Simplified circuit symbol applicable for the case the substrate (B) is connected to the source (S). Circuit symbol for the nchannel depletion-type MOSFET 2022/1/25 SJTU J. Chen 84
85 Characteristic curves ØExpression of characteristic equation ØDrain current with the i. D–v. GS characteristic in saturation 2022/1/25 SJTU J. Chen
The i. D–v. GS characteristic in saturation ØSketches of the i. D–v. GS characteristics for MOSFETs of enhancement and depletion types ØThe characteristic curves intersect the v. GS axis at Vt. 2022/1/25 SJTU J. Chen 86
The output characteristic curves 2022/1/25 SJTU J. Chen 87
88 The junction FET D Depletion layer P+ N-channel G P+ n-type Semiconductor S 2022/1/25 SJTU J. Chen D G S
89 Physical operation under v. DS=0 D P+ P+ G UGS = 0 P+ P+ G S S 2022/1/25 D D S UGS < 0 SJTU J. Chen UGS = UGS(off)
90 The effect of UDS on ID for UGS(off) <UGS < 0 动画 2022/1/25 SJTU J. Chen
Summary of semiconductor devices 2022/1/25 SJTU J. Chen 91
92 l Diode, BJT and FET are nonlinear devices made of semiconductor, mostly silicon l Diode A diode allows current to flow in forward direction and hence can perform functions such as rectification, demodulation/detection, switch etc. Ø The reverse current may become dramatically large at breakdown, such phenomena can be used as voltage regulator Ø 2022/1/25 SJTU J. Chen
93 l Bipolar Junction Transistor Ø Ø Ø 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) 2022/1/25 SJTU J. Chen
94 l Field Effect Transistor A FET has three terminals: gate, source and drain Ø The drain current is controlled by gate voltage and is almost independent on drain voltage. Ø It can perform functions such as amplification, logic calculation and switch, etc. Ø A FET should be properly biased for normal operation Ø There are three basic configurations, each has different performance (input/output resistance, gain, high frequency response, etc) Ø 2022/1/25 SJTU J. Chen
95 l As the microelectronics develops, more and more functions are fulfilled by IC chips l The discrete devices and circuits, however, are still very important not only for practical applications, but also for better understanding and design of LSICs l Quantitative calculation is sometimes complicated but not difficult l As long as you know the parameter definitions clearly, results can be derived KCL, KVL, etc 2022/1/25 SJTU J. Chen
96 Homework l April 6, 2010: Ø 5. 25; 5. 40; 5. 47; 5. 63; 5. 116 2022/1/25 SJTU J. Chen
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