Chapter 2 Field-Effect Transistors(FETs)

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-Stage 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 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 structure 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 100 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 is 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 conductance is proportional to effective voltage. • Drain current is proportional to (vGS – Vt) vDS. SJTU Zhou Lingling

11. vDSIncreased • 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 current 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 isn’t disappeared • Drain current is saturated and only controlled by the vGS • Triode region and saturation region • Channel length modulation SJTU Zhou Lingling

13. Drain Current Controlled by vGS • vGScreates 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 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 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 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 Lingling

16. 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 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 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 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 iD–vDS characteristics for a device with k’n(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 voltage • The transistor is turned off. • Operating in cutoff region as a switch. SJTU Zhou Lingling

21. Triode Region • Biased voltage • The channel depth from uniform to tapered shape. • Drain current is controlled not only by vDSbut also byvGS SJTU Zhou Lingling

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

23. Saturation Region • Biased voltage • The channel is pinched off. • Drain current is controlled only byvGS • Drain current is independent of vDS and behaves as an ideal current source. SJTU Zhou Lingling

24. Saturation Region • The iD–vGS characteristic for an enhancement-type NMOS transistor in saturation • Vt = 1 V, k’nW/L = 1.0 mA/V2 • Square law of iD–vGS 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 and 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 increased. • 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 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 Lingling

29. Large-Signal Equivalent Circuit Model 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 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 circuit 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 operation 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 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 effect---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 temperature 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 iD~vGS equation. • According to biasing method, write voltage loop equation. • Combining above two equations and solve these equations. • Usually we can get two value of vGS, only the one of two has physical meaning. • Checking the value of vDS • if vDS≥vGS-Vt, the assuming is correct. • if vDS≤vGS-Vt, the assuming is not correct. We shall use triode region equation to solve the problem again. SJTU Zhou Lingling

36. MOSFET 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. 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 point 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

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 Amplifier 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 using 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 source 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 π model • T model • Modeling the 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 transconductance • The definition of output resistance • The definition 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 drain-to-source resistance ro. • An alternative representation of the T model. SJTU Zhou Lingling