Chapter 2 Field-Effect Transistors(FETs).ppt

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1、SJTU Zhou Lingling,1,Chapter 2 Field-Effect Transistors(FETs),SJTU Zhou Lingling,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-S

2、tage MOS amplifier The MOSFET Internal Capacitance and High-Frequency Model The depletion-type MOSFET,SJTU Zhou Lingling,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

3、manufacturing process,SJTU Zhou Lingling,4,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 Lingling,5,Device Structure and Physical Operation,Device structur

4、e of the enhancement NMOS Physical operation p channel device,SJTU Zhou Lingling,6,Device Structure of the Enhancement-Type NMOS,Perspective view Four terminals Channel length and width,SJTU Zhou Lingling,7,Device Structure of the Enhancement-Type NMOS,Cross-section view.L = 0.1 to 3 mm W = 0.2 to 1

5、00 mm Tox= 2 to 50 nm,SJTU Zhou Lingling,8,Physical Operation,Creating an n channel Drain current controlled by vDS Drain current controlled by vGS,SJTU Zhou Lingling,9,Creating a Channel for Current Flow,The enhancement-type NMOS transistor with a positive voltage applied to the gate.An n channel i

6、s induced at the top of the substrate beneath the gate. Inversion layer Threshold voltage,SJTU Zhou Lingling,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 conduc

7、tance is proportional to effective voltage. Drain current is proportional to (vGS Vt) vDS.,SJTU Zhou Lingling,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 curre

8、nt is controlled by both of the two voltages.,SJTU Zhou Lingling,12,Channel Pinched Off,Channel is pinched off Inversion layer disappeared at the drain point Drain current isnt disappeared Drain current is saturated and only controlled by the vGS Triode region and saturation region Channel length mo

9、dulation,SJTU Zhou Lingling,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,SJTU Zhou

10、 Lingling,14,p Channel Device,Two reasons for readers to be familiar with p channel device Existence in discrete-circuit. More important 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

11、 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 Lingling,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 u

12、sed 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 Lingling,16,Current-Voltage Characteristics,Circuit symbol Output characteristic curves Channel length modulation Characteristics of p channel device Body effect Temperature

13、effects and Breakdown Region,SJTU Zhou Lingling,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 symb

14、ol 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 Lingling,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 iDv

15、DS characteristics for a device with kn (W/L) = 1.0 mA/V2.,SJTU Zhou Lingling,19,Output Characteristic Curves,Three distinct region Cutoff region Triode region Saturation region Characteristic equations Circuit model,SJTU Zhou Lingling,20,Cutoff Region,Biased voltageThe transistor is turned off.Oper

16、ating in cutoff region as a switch.,SJTU Zhou Lingling,21,Triode Region,Biased voltageThe channel depth from uniform to tapered shape. Drain current is controlled not only by vDS but also by vGS,SJTU Zhou Lingling,22,Triode Region,Assuming that the drain-t-source voltage is sufficiently small.The MO

17、S operates as a linear resistance,SJTU Zhou Lingling,23,Saturation Region,Biased voltageThe channel is pinched off. Drain current is controlled only by vGSDrain current is independent of vDS and behaves as an ideal current source.,SJTU Zhou Lingling,24,Saturation Region,The iDvGS characteristic for

18、an enhancement-type NMOS transistor in saturation Vt = 1 V, kn W/L = 1.0 mA/V2 Square law of iDvGS characteristic curve.,SJTU Zhou Lingling,25,Relative Levels of the Terminal Voltages,The relative levels of the terminal voltages of the enhancement NMOS transistor for operation in the triode region a

19、nd in the saturation region.,SJTU Zhou Lingling,26,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 inc

20、reased. Current drain is modified by the channel length modulation,SJTU Zhou Lingling,27,Channel Length Modulation,The MOSFET parameter VA depends on the process technology and, for a given process, is proportional to the channel length L.,SJTU Zhou Lingling,28,Channel Length Modulation,MOS transist

21、ors dont 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 Lingling,29,Large-Signal Equivalent Circuit Model,Large-signal equivalent circuit model of the n-channel MOSFET in

22、 saturation, incorporating the output resistance ro. The output resistance models the linear dependence of iD on vDS,SJTU Zhou Lingling,30,Characteristics of p Channel Device,Circuit symbol for the p-channel enhancement-type MOSFET. Modified symbol with an arrowhead on the source lead. Simplified ci

23、rcuit symbol for the case where the source is connected to the body.,SJTU Zhou Lingling,31,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 opera

24、tion in the triode region and in the saturation region.,SJTU Zhou Lingling,32,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 iD on vDS,SJTU

25、Zhou Lingling,33,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 effe

26、ct-the body voltage can control iD 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 Lingling,34,Temperature Effects and Breakdown Region,Drain current will decrease when the tem

27、perature increase. Breakdown Avalanche breakdown Punched-through Gate oxide breakdown,SJTU Zhou Lingling,35,MOSFET Circuit at DC,Assuming device operates in saturation thus iD satisfies with iDvGS equation. According to biasing method, write voltage loop equation. Combining above two equations and s

28、olve these equations. Usually we can get two value of vGS, only the one of two has physical meaning. Checking the value of vDS if vDSvGS-Vt, the assuming is correct. if vDSvGS-Vt, the assuming is not correct. We shall use triode region equation to solve the problem again.,SJTU Zhou Lingling,36,MOSFE

29、T Circuit at DC,The NMOS transistor is operating in the saturation region due to,SJTU Zhou Lingling,37,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 Lingling,38

30、,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 Lingling,39,The MOSFET As an Amplifier and as a Switch,Transfer characteristic showing operation as an amplifier biased at po

31、int Q. Three segments: XA-the cutoff region segment AQB-the saturation region segment BC-the triode region segment,SJTU Zhou Lingling,40,Biasing in MOS Amplifier Circuits,Voltage biasing scheme Biasing by fixing voltage Biasing with feedback resistor Current-source biasing scheme,SJTU Zhou Lingling,

32、41,Biasing in MOS Amplifier Circuits,The use of fixed bias (constant VGS) can result in a large variability in the value of ID.Devices 1 and 2 represent extremes among units of the same type. Current becomes temperature dependent Unsuitable biasing method,SJTU Zhou Lingling,42,Biasing in MOS Amplifi

33、er Circuits,Biasing using a fixed voltage at the gate, and a resistance in the source lead (a) basic arrangement; (b) reduced variability in ID; (c) practical implementation using a single supply;,SJTU Zhou Lingling,43,Biasing in MOS Amplifier Circuits,(d) coupling of a signal source to the gate usi

34、ng a capacitor CC1; (e) practical implementation using two supplies.,SJTU Zhou Lingling,44,Biasing in MOS Amplifier Circuits,Biasing the MOSFET using a large drain-to-gate feedback resistance, RG.,SJTU Zhou Lingling,45,Biasing in MOS Amplifier Circuits,Biasing the MOSFET using a constant-current sou

35、rce I. Implementation of the constant-current source I using a current mirror.,SJTU Zhou Lingling,46,Small-Signal Operation and Models,The ac characteristic Definition of transconductance Definition of output resistance Definition of voltage gain Small-signal model Hybrid model T model Modeling the

36、body effect,SJTU Zhou Lingling,47,The ac Characteristic,Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier.Small signal condition,SJTU Zhou Lingling,48,The ac Characteristics,The definition of transconductanceThe definition of output resistanceThe definition

37、 of voltage gain,SJTU Zhou Lingling,49,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.,SJTU Zhou Lingling,50,The Small-Signal Models,The T model of the MOSFET augmented with the

38、 drain-to-source resistance ro. An alternative representation of the T model.,SJTU Zhou Lingling,51,Modeling the Body Effect,Small-signal equivalent-circuit model of a MOSFET in which the source is not connected to the body.,SJTU Zhou Lingling,52,Single-Stage MOS Amplifier,Characteristic parameters

39、Basic structure Three configurations Common-source configuration Common-drain configuration Common-gate configuration,SJTU Zhou Lingling,53,Characteristic Parameters of Amplifier,This is the two-port network of amplifier. Voltage signal source. Output signal is obtained from the load resistor.,SJTU

40、Zhou Lingling,54,Definitions,Input resistance with no loadInput resistanceOpen-circuit voltage gainVoltage gain,SJTU Zhou Lingling,55,Definitions(contd),Short-circuit current gainCurrent gainShort-circuit transconductance gain,SJTU Zhou Lingling,56,Definitions(contd),Open-circuit overall voltage gai

41、nOverall voltage gain,SJTU Zhou Lingling,57,Definitions(contd),Output resistance of amplifier proper,Output resistance,SJTU Zhou Lingling,58,Definitions(contd),Voltage amplifier,SJTU Zhou Lingling,59,Definitions(contd),Voltage amplifier,SJTU Zhou Lingling,60,Definitions(contd),Transconductance ampli

42、fier,SJTU Zhou Lingling,61,Relationships,Voltage divided coefficient,SJTU Zhou Lingling,62,Basic Structure of the Circuit,Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations.,SJTU Zhou Lingling,63,The Common-Source Amplifier,Common-source amplifi

43、er based on the circuit of basic structure. Biasing with constant-current source. CC1 And CC2 are coupling capacitors. CS is the bypass capacitor.,SJTU Zhou Lingling,64,Equivalent Circuit of the CS Amplifier,SJTU Zhou Lingling,65,Equivalent Circuit of the CS Amplifier,Small-signal analysis performed

44、 directly on the amplifier circuit with the MOSFET model implicitly utilized.,SJTU Zhou Lingling,66,Characteristics of CS Amplifier,Input resistance Voltage gainOverall voltage gainOutput resistance,SJTU Zhou Lingling,67,Summary of CS Amplifier,Very high input resistance Moderately high voltage gain

45、 Relatively high output resistance,SJTU Zhou Lingling,68,The Common-Source Amplifier with a Source Resistance,SJTU Zhou Lingling,69,Small-signal Equivalent Circuit with ro Neglected,SJTU Zhou Lingling,70,Characteristics of CS Amplifier with a Source Resistance,Input resistance Voltage gainOverall vo

46、ltage gainOutput resistance,SJTU Zhou Lingling,71,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.,SJTU Zhou Lingling,72,The Common-Gate Amplifier,Biasing with constant current source I Input si

47、gnal vsig is applied to the source Output is taken at the drain Gate is signal grounded CC1 and CC2 are coupling capacitors,SJTU Zhou Lingling,73,The Common-Gate Amplifier,A small-signal equivalent circuit of the amplifier in fig. (a). T model is used in preference to the model Neglecting ro,SJTU Zh

48、ou Lingling,74,The Common-Gate Amplifier Fed with a Current-Signal Input,SJTU Zhou Lingling,75,Characteristics of CG Amplifier,Input resistance Voltage gainOverall voltage gainOutput resistance,SJTU Zhou Lingling,76,Summary of CG Amplifier,Noninverting amplifier Low input resistance Has nearly ident

49、ical 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,SJTU Zhou Lingling,77,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 Lingling,78,The Common-Drain or Source-Follower Amplifier,