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Metal-Oxide Semiconductor (MOS) Field-Effect Transistors (MOSFETs)

Metal-Oxide Semiconductor (MOS) Field-Effect Transistors (MOSFETs). 1. Introduction. - Transistors are three-terminal devices. - Voltage between two terminal controls the current flowing in the third terminal. - Amplifiers, or switches.

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Metal-Oxide Semiconductor (MOS) Field-Effect Transistors (MOSFETs)

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  1. Metal-Oxide Semiconductor (MOS) Field-Effect Transistors (MOSFETs) 1

  2. Introduction - Transistors are three-terminal devices. - Voltagebetween two terminalcontrolsthecurrentflowing in the third terminal. - Amplifiers, or switches. Compared to Bipolar Junction Transistors (BJT), MOSTFETs; - Can be made quite small (require small area). - Can be manufactured with simple fabrication process. - Can be operated with little power. - Can be integrated densely (>200 millions on a single IC chip, Very-large-scale-integrated circuit, VLSI). - Digital and analog functions can be implemented almost exclusively ( i.e., with very few or no resistors). - Digital and analog functions can be implemented on the same IC chip (mixed-signal design). 4.1 Device Structure and Physical Operation The enhancement-type MOSFET is the most widely used field-effect transistor. 4.1.1 Device Structure – (n-channel enhancement-type MOSFET = enhancement-type NMOS) 0.2~100 μm 2-~50 nm 0.1~3 μm MOS Microelectronic Circuits - Fifth Edition Sedra/Smith

  3. Insulated-gate FET, IGFET (almost no current through the gate : ~10-15 A) Another name for the MOSFET: 4.1.2 Operation with No gate Voltage Between drain and source : Back-to-back pn junction No current flow 4.1.3 Creating a Channel for current flow - An n-channel is formed in a p-type substrate – inversion layer. - If a voltage is applied between drain and source, current flows through this n-channel. (NMOS) - Threshold voltage Vt : a voltage of υGS at which a sufficient number of mobile electrons accumulate in the channel region to form a conducting channel.(+0.5 ~ 1 V) - The gate and the channel form a capacitor. - The positive charges on the gate and the electrons in the channel develop an electric field. - This electric field controls the current flow in the channel. Field-Effect Transistor (FET) ! Figure 4.2 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. Microelectronic Circuits - Fifth Edition Sedra/Smith

  4. 4.1.4 Applying a Small VDS(< 50 mV) The device acts as a resistance whose value is determined by υGS. Specifically, the channel conductance is proportional to υGS – Vt’ and thus iD is proportional to (υGS – Vt) υDS. (υGS – Vt) : excess gate voltage, effective voltage, overdrive voltage 4.1.5 Operation as VDSincreased. - As υDS is increased υGD =υGS – υDSdecreases and channel takes the tapered form, and resistance between the drain and gate increases. - At υGD =υGS – υDS = Vtor υDS = υGS- Vt,the channel depth at the drain end is almost zero! – The channel is pinched off. - Increasing υDS beyond this value has, theoretically, no effect on the channel shape and channel current.-Saturation! When υDS = small or 0 V Microelectronic Circuits - Fifth Edition Sedra/Smith

  5. Figure 4.6 The drain current iD versus the drain-to-source voltage vDS for an enhancement-type NMOS transistor operated with vGS > Vt. Figure 4.7 Increasing vDS causes the channel to acquire a tapered shape. Eventually, as vDS reaches vGS – Vt’ the channel is pinched off at the drain end. Increasing vDS above vGS – Vt has little effect (theoretically, no effect) on the channel’s shape. Microelectronic Circuits - Fifth Edition Sedra/Smith

  6. 4.1.6 Derivation of the iD-vDS Relationship At the beginning of saturation region, υDS= υGS-Vt Ex. 4.1 p245 Microelectronic Circuits - Fifth Edition Sedra/Smith

  7. 4.1.7 The p-Channel MOSFET p.247 - NMOS has virtually replaced because it is smaller, faster, and needs lower supply voltage. - The p-Channel MOSFET is fabricated on an n-type substrate with p+ regions for the drain and source. - But you have to be familiar with PMOS because: there are many discrete PMOSFETs and there are complementary MOS, CMOS!! - The p-Channel MOSFET has holes as charge carriers. - υGS, υDS, and Vt are negative. The current flows from the source to the drain. - PMOS technology originally dominated MOS manufacturing. - CMOS is the most widely used of all the IC technologies in analog and digital circuit design !! 4.1.8 Complementary MOS, or CMOS Figure 4.9 Cross-section of a CMOS integrated circuit. Note that the PMOS transistor is formed in a separate n-type region, known as an 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. Not shown are the connections made to the p-type body and to the n well; the latter functions as the body terminal for the p-channel device. Microelectronic Circuits - Fifth Edition Sedra/Smith

  8. 4.2 Current-Voltage Characteristics Detailed analysis of equations 4.5 and 4.6 Figure 4.10 (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. 4.2.1 Circuit Symbol Normal direction of current flow. 4.2.2 The iD-vDS Characteristics Amplifier - Saturation region Switch – Cutoff and triode region • For the operation in the triode region, and keep υDS small enough so that the channel remains continuous. AtυGD =υGS – υDS = Vtor υDS = υGS- Vt,the channel depth at the drain end is almost zero! – The channel is pinched off. Microelectronic Circuits - Fifth Edition Sedra/Smith

  9. The operation of the MOS transistor as a linear resistance whose value is controlled by gate voltage ! • For the operation in the saturation region, just same as for the triode operation. (υGD =υGS – υDS) At the boundary between triode and saturation region, Eq.(4.20) shows that the saturation current is; (1) independent of the drain voltage. (2) determined by square of the gate voltage. Eq.(4.20) also shows that the saturated MOSFET behaves as an ideal current source. Figure 4.13Large-signal equivalent-circuit model of an n-channel MOSFET operating in the saturation region. Microelectronic Circuits - Fifth Edition Sedra/Smith

  10. At the boundary between triode and saturation region, Figure 4.14 The relative levels of the terminal voltages of the enhancement NMOS transistor for operation in the triode region and in the saturation region. Microelectronic Circuits - Fifth Edition Sedra/Smith

  11. 4.2.3 Finite Output Resistance in Saturation Eq.(4.20) shows that the saturation current is independent of the drain voltage. But, in practice, increasing υDS beyond υDSsat does affect the channel length. The phenomenon that the channel length is reduced form L to L-ΔL is known as channel-length modulation. Figure 4.15 Increasing vDS beyond vDSsat causes the channel pinch-off point to move slightly away from the drain, thus reducing the effective channel length (by ΔL). With extrapolation, Microelectronic Circuits - Fifth Edition Sedra/Smith

  12. 4.2.4 Characteristics of the p-Channel MOSFET Microelectronic Circuits - Fifth Edition Sedra/Smith

  13. 4.2.5 TheRole of the Substrate-The Body Effect - Usually, the source terminal is connected to the substrate (or body) terminal. - In integrated circuit, many MOS transistors are fabricated on a single substrate. - In order to maintain the cutoff condition for all the substrate-to-channel junctions, the substrate is usually connected to the most negative power supply in an NMOS circuit (the positive in a PMOS circuit). - The reverse bias will widen the depletion region. - The channel depth is reduced. - To return the channel to its former states, υGS has to be increased. The body effect can cause considerable degradation in circuit performance (Chap. 6) 4.2.6 Temperature Effect 4.2.7 Breakdown and Input Protection - The overall observed effect of a temperature increase is a decrease in drain current. - Weak avalanche : υDS (20~150 V) breakdown between drain and substrate. - This very interesting result is put to use in applying the MOSFET in power circuit (Chap. 11). - Punch-through : υDS (~20 V) breakdown between drain and source for short-channel devices. - υGS (>30 V) breakdown between gate and source. Microelectronic Circuits - Fifth Edition Sedra/Smith

  14. 4.3 MOSFET Circuits at DC (Bias Analysis) • For the operation in the triode region, - Neglect Channel-length modulation. (λ=0) - Overdrive voltage VOV=VGS-Vt(VOV, Vt > 0, for NMOS) - Overdrive voltage VSG=|VGS|=|Vt|+|VOV| for PMOS • For the operation in the saturation region, EXAMPLE4.2 Vt=0.7 V, μnCox =100 μA/V2, L=1 μm, W =32 μm Design the circuit so that the transistor operates at ID = 0.4 mA and VD = +0.5 V Since VD > VG, saturation region ! Thus source must be at -1.2 V. Microelectronic Circuits - Fifth Edition Sedra/Smith

  15. EXAMPLE4.3 Design the circuit to obtain ID of 0.08 mA. R=? VD = ?, μnCox=200 μA/V2, L=0.8 μm. VDS = VGS, Saturation region! Let’s find VGS! Figure 4.21 Circuit for Example 4.3. Microelectronic Circuits - Fifth Edition Sedra/Smith

  16. EXAMPLE4.5 EXAMPLE4.4 Vt=1 V, k’(W/L) 1 mA/V2 Design the circuit so that VD = +0.1 V. What is the effective resistance between drain and source? Assume saturation region operation. Since VD < VG, and Vt=1 V, triode region ! In practice, 12 kΩ, 5% Saturation region operation! Microelectronic Circuits - Fifth Edition Sedra/Smith

  17. EXAMPLE4.6 Vt= -1 V, k’(W/L) 1 mA/V2 - Design the circuit so that the transistor operate in saturation region at ID = 0.5 mA and VD = +3 V. - What is the largest value that RD can have while maintaining saturation region operation? For this, a possible selection is RG1=2 MΩ, RG2= 3MΩ - Overdrive voltage VSG=|VGS|=|Vt|+|VOV| for PMOS Microelectronic Circuits - Fifth Edition Sedra/Smith

  18. EXAMPLE4.7 For υI =+2.5 V Vt= ±1 V, k’(W/L) 1 mA/V2 for NMOS and PMOS - for QP, VGS = 0 V, cutoff ! - Find iDN, iDP, υO, for υI =0 V, +2.5 V, and -2.5 V. υO should be negative for IDN. υGD will be greater than Vt. for QN, triode ! For υI = -2.5 V For υI =0 V, - Exact complement of +2.5 V - QN and QP are perfectly matched. - QN will be off. - Equal |VGS| (2.5 V) • The circuit is symmetrical. • (upper and lower part) for Qp, triode ! - Thus |VDG| = 0 V. - Thus in saturation region ! Microelectronic Circuits - Fifth Edition Sedra/Smith

  19. 4.4 The MOSFET as an Amplifier and as Switch The MOSFET acts as a Voltage-Controlled Current Source ! Transconductance Amplifier ! Saturation Region !!! υGS iD Nonlinear ! For linear amplification, we need dc-bias voltage VGS and require small input signal υgs. 4.4.1 Large-Signal Operation-The Transfer Characteristics 4.4.2 Graphical Derivation of The Transfer Characteristics Load-line equation For a given input υI(υGS), We can find output υO (υDS). Basic structure of the Common-Source (CS) (ground-source) amplifier. Microelectronic Circuits - Fifth Edition Sedra/Smith

  20. 4.4.2 Graphical Derivation of The Transfer Characteristics υO υI = υGS 4.4.3 Operation as a Switch Turn off : υI < Vt, somewhere on XA Turn on : υI close to VDD, close to C Digital Logic Inverter ! 4.4.4 Operation as a Linear Amplifier Between A and B Microelectronic Circuits - Fifth Edition Sedra/Smith

  21. υI < Vt, υO = VDD υI ≥ Vt, υI ≥ Vt, υO≥ υI - Vt υO≤υI - Vt 4.4.5 Analytical Expression for the Transfer Characteristics Cutoff-region, XA: Saturation-regionAQB: At Q, For dc bias point Q, υI = VIQ, υO= VOQ , End point of the saturation region Triode-regionBC: small Taylor expansion, (1+x)-1 =1- x+ x2/2-…. EXAMPLE4.8, p.277 Microelectronic Circuits - Fifth Edition Sedra/Smith

  22. 4.5 Biasing in MOS Amplifier Circuits 4.5.1 Biasing by Fixing VGS - Large ΔID!, Not useful ! The spread in the values of parameters (e.g. W/L) is large among the same type of MOSFET. 4.5.2 Biasing by Fixing VGS and Resistor in the Source - Good for discrete MOSFET 4.5.3 Biasing Using a Drain-to-Gate Feedback Resistor - Good for discrete MOSFET 4.5.4 Biasing Using a Constant-Current Source Figure 4.29 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. - Good for IC 4.5.2 Biasing by Fixing VGS and Resistor in the Source Excellent Biasing Technique for Discrete MOSFET Circuits - For given VGand RS, } ID, VGS can be determined. - For two FETs of the same type, Smaller ΔID than that of Fig. 4.39 - For one FET, ID increases. –> (4.46) VGS decreases. –> (a) ID decreases.–> ID is stabilizes. Rs provides negative feedback resulting in stabilized ID. Degeneration resistance Microelectronic Circuits - Fifth Edition Sedra/Smith

  23. (a) basic arrangement (d) coupling of a signal source to the gate using a capacitor CC1 (c) practical implementation using a single supply For Fig. c and d - RG : ~ MΩ for large input impedance to the signal source (Fig. d) • CC1 : large capacitance, coupling, signal dc block not to disturb bias. • suitable only in discrete circuit design (Sect. 4.7). • RD : large enough to obtain high gain, • small enough to allow for swing and operation in saturation. (Ex4.6) For Fig. e - RG : for a dc ground at the gate for a high input impedance to a signal source. (e) practical implementation using two supplies. Microelectronic Circuits - Fifth Edition Sedra/Smith

  24. EXAMPLE4.9 Design! Vt=1 V, k’(W/L) 1 mA/V2 As a rule of thumb for design, Sol) One-third of the power supply voltage as a drop across each of RD, MOSFET, RS. Figure 4.31 Circuit for Example 4.9. Microelectronic Circuits - Fifth Edition Sedra/Smith

  25. 4.5.3 Biasing Using a Drain-to-Gate Feedback Resistor - Good for Discrete MOSFET - RG : Feedback resistor ~ MΩ - Feedback mechanism – negative feedback or degeneration ID increases. – (4.49) VGS decreases. – (a) ID decreases. ID is stabilizes ! What about shorting gate and drain instead of RG? • Bias for Common source amplifier • Drawback of a limited output voltage swing. 4.5.4 Biasing Using a Constant-Current Source - Good for IC - RG : ~ MΩ for large input impedance to the signal source, for a dc ground at the gate. - RD : for dc voltage at the drain, for output signal swing, for operation in saturation. - Constant-current source : Q1 is the heart of the circuit Drain is shorted to the gate – saturation ! } ID1, VGS can be determined. ID2 can be determined. What about design ? Implementation of the constant-current source using a current mirror. D4.22, p286 Microelectronic Circuits - Fifth Edition Sedra/Smith

  26. 4.6 Small-Signal Operation and Models 4.6.2 The signal current in the drain terminal dc bias current Current proportional to input Nonlinear distortion To reduce the nonlinear distortion, Figure 4.34Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier. If this small-signal condition is satisfied, Microelectronic Circuits - Fifth Edition Sedra/Smith

  27. 4.6.3 The Voltage Gain For small-signal condition For out of cutoff For saturation Figure 4.36 Total instantaneous voltages vGSand vD for the circuit in Fig. 4.34. Microelectronic Circuits - Fifth Edition Sedra/Smith

  28. 4.6.4 Separating the DC Analysis and the Signal Analysis For the signal analysis, Ideal constant voltage sources are replaced by short circuits. Ideal constant current sources are replaced by open circuits. 4.6.5 Small-signal Equivalent-Circuit Model FET behaves as a voltage-controlled current source. The input impedance is very, very high. The output impedance is also high. Small-signal models for the MOSFET (a) neglecting the dependence of iDon vDS in saturation (the channel-length modulation effect (b) including the effect of channel-length modulation, modeled by output resistance ro = |VA| /ID. To include the channel-length modulation, For PMOS, Microelectronic Circuits - Fifth Edition Sedra/Smith

  29. 4.6.6 The Transconductance gm - For large gm, we need large (W/L) and (VGS - Vt). - However, large VG has disadvantage of reducing the allowable voltage signal swing at the drain. cf.) Transconductance of BJT is proportional to the bias current and independent of physical size and geometry of the device. Practical example - gm = 0.35 mA/V for W/L =1 - gm = 3.5 mA/V for W/L =100 - gm = 20 mA/V for BJT with IC= 0.5 mA. Three Design Parameters W/L, VOV, ID Two the above can be chosen independently. Microelectronic Circuits - Fifth Edition Sedra/Smith

  30. EXAMPLE4.10 Sol) } Vt=1.5 V, kn’ (W/L) =0.25 mA/V2, VA = 50 V. Small-signal gain=?, input resistance=?, maximum input signal =? Microelectronic Circuits - Fifth Edition Sedra/Smith

  31. 4.6.6 The T Equivalent-Circuit Model T Model Hybrid-π Model Microelectronic Circuits - Fifth Edition Sedra/Smith

  32. Microelectronic Circuits - Fifth Edition Sedra/Smith

  33. Table 4.2 Microelectronic Circuits - Fifth Edition Sedra/Smith

  34. 4.7 Single-Stage MOS Amplifiers (Discrete circuits) Microelectronic Circuits - Fifth Edition Sedra/Smith

  35. Microelectronic Circuits - Fifth Edition Sedra/Smith

  36. 4.7 Single-Stage MOS Amplifiers (Discrete circuits) IC MOS amplifiers : Chap. 6 4.7 is useful to understand IC amplifier. 4.7.1 The Basic Structure 4.7.2 Characterizing Amplifiers The material of Sect. 1.5 was limited to unilateral amplifiers. Now, let’s include non-unilateral amplifiers. • Source : υsig + Rsig. Real signal source or previous amplifier. • Load : RL. Real load or previous amplifier. 2. Ri, Ro, Aυo, Ais, Gm do not depend on the value of Rsig and RL. Rin, Rout, Aυ, Ai, Gυo, Gυ may depend on the value of Rsig and RL. 3. For non-unilateral amplifiers, Rin may depends on RL, Routmay depends on Rsig. For unilateral amplifiers, Rin = Ri, Rout= Ro. Figure 4.42 Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations. 4. The loading of the amplifier on the signal is determined by the input resistance Rin. 5. When evaluating the gain Aυfrom the open-circuit gain Aυo, Ro is the output to use. Chap. 4 : unilateral only Chap. 6 : non-unilateral also When evaluating the overall voltage gain Gυfrom its open-circuit value Gυo, Rout is the output to use. Microelectronic Circuits - Fifth Edition Sedra/Smith

  37. EXAMPLE4.11, p304 υsig = 10 mV, Rsig= 100 kΩ. RL. = 10 kΩ Find all the amplifier parameters. To determine υi, we need to know the value of Rin obtained with RL=0. Microelectronic Circuits - Fifth Edition Sedra/Smith

  38. 4.7.3 The Common-Source (CS) Amplifiers - The most widely used of all MOSFET amplifier circuits. Bypass capacitor (~μF) Bypass capacitor (~μF) For signal ground (b) Equivalent circuit of the amplifier for small-signal analysis. Figure 4.43(a) Common-source amplifier based on the circuit of Fig. 4.42. } (c) Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized. Microelectronic Circuits - Fifth Edition Sedra/Smith

  39. RSac RSdc 4.7.4 The Common-Source (CS) Amplifier with a Source Resistance Figure 4.44(a) Common-source amplifier with a resistance RS in the source lead. (b) Small-signal T-equivalent circuit with ro neglected. - The effect rois not important (SPICE) in discrete-circuit amp. - The effect roplays major role and must be taken into account in IC amp. } - Rs increases dc bias stability. (Sect. 4.5) • Rs decreasesυgs to reduce nonlinear distortion. • (4.86), (4.58) Need trade-off ! - Rs increases the bandwidth(Sect. 4.12). - Rs decreases gain. (4.90) Split RS !! Microelectronic Circuits - Fifth Edition Sedra/Smith

  40. 4.7.5 The Common-Gate (CG) Amplifier (b) A small-signal equivalent circuit of the amplifier in (a). Figure 4.45 (a) A common-gate amplifier based on the circuit of Fig. 4.42. (c) The common-gate amplifier fed with a current-signal input. Microelectronic Circuits - Fifth Edition Sedra/Smith

  41. Unity-gain current amplifier, current follower ! CG amplifier is applied to the cascode circuit. 1. Unlike the CS amplifier (inverting), CG amp is non-inverting. 2. While the CS amplifier has a very high input impedance, that of the CG amp is low. 3. Overall voltage gain of the CG amp is smaller than that of CS amp by the factor of 1+ gmRsig. Microelectronic Circuits - Fifth Edition Sedra/Smith

  42. 4.7.5 The Common-Drain (CD) or Source-Follower Amplifier ac ground Figure 4.46 (a) A common-drain or source-follower amplifier. (b) Small-signal equivalent-circuit model. (c) Small-signal analysis performed directly on the circuit. (d) Circuit for determining the output resistance Rout of the source follower. Microelectronic Circuits - Fifth Edition Sedra/Smith

  43. The source follower has a very high input impedance, a relatively low output impedance, a gain less than but close to unity. Unity-gain buffer amplifier ! (Sect. 1.5) Output stage of multi-stage amplifier ! means the voltage at the source follows that at the gate. Source follower ! Microelectronic Circuits - Fifth Edition Sedra/Smith

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