SJTU Zhou Lingling1 Chapter 2 Field-Effect Transistors (FETs)

SJTU Zhou Lingling2 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

SJTU Zhou Lingling3 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.

SJTU Zhou Lingling4 Introduction Classification of MOSFET  MOSFET  P channel Enhancement type Depletion type  N channel Enhancement type Depletion type  JFET  P channel  N channel Widely used in IC circuits

SJTU Zhou Lingling5 Device Structure and Physical Operation Device structure of the enhancement NMOS Physical operation p channel device

SJTU Zhou Lingling6 Device Structure of the Enhancement-Type NMOS  Perspective view  Four terminals  Channel length and width

SJTU Zhou Lingling7 Device Structure of the Enhancement-Type NMOS  Cross-section view.  L = 0.1 to 3  m  W = 0.2 to 100  m  T ox = 2 to 50 nm

SJTU Zhou Lingling8 Physical Operation Creating an n channel Drain current controlled by v DS Drain current controlled by v GS

SJTU Zhou Lingling9 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

SJTU Zhou Lingling10 Drain Current Controlled by Small Voltage v DS  An NMOS transistor with v GS > V t and with a small v DS 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 ( v GS – V t ) v DS.

SJTU Zhou Lingling11 v DS Increased  Operation of the enhancement NMOS transistor as v DS is increased.  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.

SJTU Zhou Lingling12 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 v GS Triode region and saturation region Channel length modulation

SJTU Zhou Lingling13 Drain Current Controlled by v GS 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

SJTU Zhou Lingling14 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.

SJTU Zhou Lingling15 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.

SJTU Zhou Lingling16 Current-Voltage Characteristics Circuit symbol Output characteristic curves Channel length modulation Characteristics of p channel device Body effect Temperature effects and Breakdown Region

SJTU Zhou Lingling17 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.

SJTU Zhou Lingling18 Output Characteristic Curves (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.

SJTU Zhou Lingling19 Output Characteristic Curves Three distinct region  Cutoff region  Triode region  Saturation region Characteristic equations Circuit model

SJTU Zhou Lingling20 Cutoff Region Biased voltage The transistor is turned off. Operating in cutoff region as a switch.

SJTU Zhou Lingling21 Triode Region Biased voltage The channel depth from uniform to tapered shape. Drain current is controlled not only by v DS but also by v GS

SJTU Zhou Lingling22 Triode Region Assuming that the drain-t-source voltage is sufficiently small. The MOS operates as a linear resistance

SJTU Zhou Lingling23 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.

SJTU Zhou Lingling24 Saturation Region  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.

SJTU Zhou Lingling25 Relative Levels of the Terminal Voltages The relative levels of the terminal voltages of the enhancement NMOS transistor for operation in the triode region and in the saturation region.

SJTU Zhou Lingling26 Channel Length Modulation 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

SJTU Zhou Lingling27 Channel Length Modulation The MOSFET parameter V A depends on the process technology and, for a given process, is proportional to the channel length L.

SJTU Zhou Lingling28 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.

SJTU Zhou Lingling29 Large-Signal Equivalent Circuit Model 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

SJTU Zhou Lingling30 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.

SJTU Zhou Lingling31 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.

SJTU Zhou Lingling32 Characteristics of p Channel Device 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

SJTU Zhou Lingling33 The Body Effect 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.

SJTU Zhou Lingling34 Temperature Effects and Breakdown Region Drain current will decrease when the temperature increase. Breakdown  Avalanche breakdown  Punched-through  Gate oxide breakdown

SJTU Zhou Lingling35 MOSFET Circuit at DC a.Assuming device operates in saturation thus i D satisfies with i D ~v GS equation. b.According to biasing method, write voltage loop equation. c.Combining above two equations and solve these equations. d.Usually we can get two value of v GS, only the one of two has physical meaning. e.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.

SJTU Zhou Lingling36 MOSFET Circuit at DC The NMOS transistor is operating in the saturation region due to

SJTU Zhou Lingling37 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

SJTU Zhou Lingling38 The MOSFET As an Amplifier  Basic structure of the common-source amplifier.  Graphical construction to determine the transfer characteristic of the amplifier in (a).

SJTU Zhou Lingling39 The MOSFET As an Amplifier and as a Switch  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

SJTU Zhou Lingling40 Biasing in MOS Amplifier Circuits Voltage biasing scheme  Biasing by fixing voltage  Biasing with feedback resistor Current-source biasing scheme

SJTU Zhou Lingling41 Biasing in MOS Amplifier Circuits  The use of fixed bias (constant V GS ) can result in a large variability in the value of I D.  Devices 1 and 2 represent extremes among units of the same type.  Current becomes temperature dependent  Unsuitable biasing method

SJTU Zhou Lingling42 Biasing in MOS Amplifier Circuits  Biasing using a fixed voltage at the gate, and a resistance in the source lead  (a) basic arrangement;  (b) reduced variability in I D ;  (c) practical implementation using a single supply;

SJTU Zhou Lingling43 Biasing in MOS Amplifier Circuits  (d) coupling of a signal source to the gate using a capacitor C C1 ;  (e) practical implementation using two supplies.

SJTU Zhou Lingling44 Biasing in MOS Amplifier Circuits Biasing the MOSFET using a large drain-to-gate feedback resistance, R G.

SJTU Zhou Lingling45 Biasing in MOS Amplifier Circuits (a)Biasing the MOSFET using a constant-current source I. (b)Implementation of the constant-current source I using a current mirror.

SJTU Zhou Lingling46 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

SJTU Zhou Lingling47 The ac Characteristic  Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier.  Small signal condition

SJTU Zhou Lingling48 The ac Characteristics The definition of transconductance The definition of output resistance The definition of voltage gain

SJTU Zhou Lingling49 The Small-Signal Models (a)neglecting the the channel-length modulation effect (b)including the effect of channel-length modulation, modeled by output resistance r o = |V A | /I D.

SJTU Zhou Lingling50 The Small-Signal Models (a)The T model of the MOSFET augmented with the drain-to-source resistance r o. (b)An alternative representation of the T model.

SJTU Zhou Lingling51 Modeling the Body Effect Small-signal equivalent-circuit model of a MOSFET in which the source is not connected to the body.

SJTU Zhou Lingling52 Single-Stage MOS Amplifier Characteristic parameters Basic structure Three configurations  Common-source configuration  Common-drain configuration  Common-gate configuration

SJTU Zhou Lingling53 Characteristic Parameters of Amplifier  This is the two-port network of amplifier.  Voltage signal source.  Output signal is obtained from the load resistor.

SJTU Zhou Lingling54 Definitions Input resistance with no load Input resistance Open-circuit voltage gain Voltage gain

SJTU Zhou Lingling55 Definitions(cont’d) Short-circuit current gain Current gain Short-circuit transconductance gain

SJTU Zhou Lingling56 Definitions(cont’d) Open-circuit overall voltage gain Overall voltage gain

SJTU Zhou Lingling57 Definitions(cont’d) Output resistance of amplifier properOutput resistance

SJTU Zhou Lingling58 Definitions(cont’d) Voltage amplifier

SJTU Zhou Lingling59 Definitions(cont’d) Transconductance amplifier

SJTU Zhou Lingling60 Relationships Voltage divided coefficient

SJTU Zhou Lingling61 Basic Structure of the Circuit Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations.

SJTU Zhou Lingling62 The Common-Source Amplifier  Common-source amplifier based on the circuit of basic structure.  Biasing with constant- current source.  C C1 And C C2 are coupling capacitors.  C S is the bypass capacitor.

SJTU Zhou Lingling63 Equivalent Circuit of the CS Amplifier

SJTU Zhou Lingling64 Equivalent Circuit of the CS Amplifier Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized.

SJTU Zhou Lingling65 Characteristics of CS Amplifier Input resistance Voltage gain Overall voltage gain Output resistance

SJTU Zhou Lingling66 Summary of CS Amplifier Very high input resistance Moderately high voltage gain Relatively high output resistance

SJTU Zhou Lingling67 The Common-Source Amplifier with a Source Resistance

SJTU Zhou Lingling68 Small-signal Equivalent Circuit with r o Neglected

SJTU Zhou Lingling69 Characteristics of CS Amplifier with a Source Resistance Input resistance Voltage gain Overall voltage gain Output resistance

SJTU Zhou Lingling70 Summary of CS Amplifier with a Source Resistance Including R S results in a gain reduction by the factor (1+g m R S ) R S takes the effect of negative feedback.

SJTU Zhou Lingling71 The Common-Gate Amplifier  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

SJTU Zhou Lingling72 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 r o

SJTU Zhou Lingling73 The Common-Gate Amplifier Fed with a Current-Signal Input

SJTU Zhou Lingling74 Characteristics of CG Amplifier Input resistance Voltage gain Overall voltage gain Output resistance

SJTU Zhou Lingling75 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+g m R sig ） Relatively high output resistance Current follower Superior high-frequency performance

SJTU Zhou Lingling76 The Common-Drain or Source- Follower Amplifier Biasing with current source Input signal is applied to gate, output signal is taken at the source.

SJTU Zhou Lingling77 The Common-Drain or Source- Follower Amplifier  Small-signal equivalent- circuit model  T model makes analysis simpler  Drain is signal grounded

SJTU Zhou Lingling78 Small-Signal Analysis Performed Directly on the Circuit

SJTU Zhou Lingling79 Circuit for Determining the Output Resistance of CD Amplifier

SJTU Zhou Lingling80 Characteristics of CD Amplifier Input resistance Voltage gain Overall voltage gain Output resistance

SJTU Zhou Lingling81 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

SJTU Zhou Lingling82 Summary and Comparisons 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. Can be used as a current buffer. Source follower finds application as a voltage buffer and as the output stage in a multistage amplifier.

SJTU Zhou Lingling83 The MOSFET Internal Capacitance and High-Frequency Model 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

SJTU Zhou Lingling84 The Gate Capacitive Effect MOSFET operates at triode region MOSFET operates at saturation region MOSFET operates at cutoff region

SJTU Zhou Lingling85 Overlap Capacitance 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 This additional component should be added to C gs and C gd in all preceding formulas.

SJTU Zhou Lingling86 The Junction Capacitances Source-body depletion-layer capacitance drain-body depletion-layer capacitance

SJTU Zhou Lingling87 High-Frequency Model

SJTU Zhou Lingling88 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 C db neglected (to simplify analysis).

SJTU Zhou Lingling89 The MOSFET Unity-Gain Frequency Current gain Unity-gain frequency

SJTU Zhou Lingling90 Frequency Response of the CS Amplifier Capacitively coupled common-source amplifier.

SJTU Zhou Lingling91 Frequency Response of the CS Amplifier A sketch of the frequency response of the amplifier in (a) delineating the three frequency bands of interest.

SJTU Zhou Lingling92 The High-frequency Response Equivalent Circuit

SJTU Zhou Lingling93 The High-frequency Response The circuit of (a) simplified at the input and the output

SJTU Zhou Lingling94 The High-frequency Response The equivalent circuit with C gd replaced at the input side with the equivalent capacitance C eq ;

SJTU Zhou Lingling95 The High-frequency Response The frequency response plot, which is that of a low-pass single-time-constant circuit.

SJTU Zhou Lingling96 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.

SJTU Zhou Lingling97 The Depletion-Type MOSFET Circuits symbol Structure Characteristic curves

SJTU Zhou Lingling98 Circuit Symbol for the n-Channel Depletion-Type MOSFET (a)Circuit symbol for the n-channel depletion-type MOSFET. (b)Simplified circuit symbol applicable for the case the substrate (B) is connected to the source (S).

SJTU Zhou Lingling99 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.

SJTU Zhou Lingling100 Characteristic Curves  Transistor with current and voltage polarities indicated.  Typical value for discrete transistor: V t = –4 V and k n (W/L) = 2 mA/V 2

SJTU Zhou Lingling101 The Output Characteristic Curves

SJTU Zhou Lingling102 The i D – v GS Characteristic in Saturation  the i D – v GS characteristic in saturation.  Expression of characteristic equation  Drain current with

SJTU Zhou Lingling103 The i D – v GS Characteristic in Saturation  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.