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FIELD EFFECT TRANSISTORS (FET). Celso José Faria de Araújo, Dr. FET X BJT (NPN). Drain. Collector. Gate. Base. Source. Emitter. FET 3 terminal device Channel e - current from source to drain controlled by the electric field generated by the gate
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FIELD EFFECT TRANSISTORS(FET) Celso José Faria de Araújo, Dr.
FET X BJT (NPN) Drain Collector Gate Base Source Emitter • FET • 3 terminal device • Channel e- current from source to drain controlled by the electric field generated by the gate • Extremely high input impedance for base • BJT • 3 terminal device emitter to collector e- current controlled by the current injected into the base Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Types of FETs npn npn Depletion mode junction FETs (JFET) pnp pnp Metal oxide semi-conductor FET (MOSFET) • In addition to carrier type (N or P channel), there are differences in • how the control element is constructed (Junction x Insulated) and • those devices must be used differently • depletion/enhancement mode • enhancement mode (insulated gate FETs, IGFETs, are the same as MOSFETs) Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Physical structure of the enhancement-type NMOS transistor: (a) perspective view; (b) cross section. Typically L = 1 to 10 m, W = 2 to 500 m, and the thickness of the oxide layer is in the range of 0.02 to 0.1 m. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Waterfalls analogy Water ( a ) MOSFET in the linear region ( b ) MOSFET with channel just pinched off at the drain. ( c ) Channel pinch off for vDS > vGS - VTN Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
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. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
An NMOS transistor with vGS > Vtand with a small vDS applied. The device acts as a conductance whose value is determined by vGS. Specifically, the channel conductance is proportional to vGS - Vt, and this iD is proportional to (vGS - Vt) vDS. Note that the depletion region is not shown (for simplicity). Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Linear Region (small vDS) vGS Vtn 0 vDS vDSSAT= vGS-Vtn VTN =1V Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Operation of the enhancement NMOS transistor as vDS is increased. The induced channel acquires a tapered shape and its resistance increases as vDS is increased. Here, vGS is kept constant at a value > Vt. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The drain current iD versus the drain-to-source voltage vDS for an enhancement-type NMOS transistor operated with vGS > Vt. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Output characteristicsfor an NMOS transistor withVtn = 1 V and k’n (W/L)= 25 x 10-6 A/V2 vD vS Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Derivation of the iD - vDS characteristic of the NMOS transistor. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The PMOS Transistor Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Output characteristics for a PMOS transistor withVtp = -1V and k’P (W/L)= 25 x 10-6 A/V2 vS vD Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
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. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
(a) An n-channel enhancement-type MOSFET with vGSand vDSapplied and with the normal directions of current flow indicated. (b)The iD - vDS characteristics for a device with Vt = 1 V and k’n(W/L) = 0.5 mA/V2 (d) The iD - vGS characteristic for an enhancement-type NMOS transistor in saturation. (d) Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Channel Length Modulation LM Increasing vDSbeyond vDSsat causes the channel pinch-off point to move slightly away from the drain, thus reducing the effective channel length (by L). Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Effect of vDS on iD in the saturation region. The MOSFET parameter VAis typically in the range of 30 to 200 V. Output Resistance: VAis directly proportional to L; thus two devices with the same process, and having channel lengths L1 e L2, will have Early voltage VA1 and VA2,. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
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 iDon vDS and is given by roVA/ID. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Summary of equations for the enhancement-mode MOSFET P-channel N-channel or or Vtp < 0 vDS 0 Vtn > 0 vDS 0 Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The current-voltage characteristics of a depletion-type n-channel MOSFET for which Vt = -4 V and k’n(W/L) = 2 mA/V2: (a) transistor with current and voltage polarities indicated; (b) the iD - vDS characteristics; (c) the iD - vGS characteristic in saturation. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Bias Circuits 1 • usually neglected ( 1/ =VA) • Q-point most often located in saturation for analog circuits ( assuming the MOSFET to be in the saturation region ) ( two solutions only one is possible ) Ex: VDD = 10 V R1 = 150 kR2 = 100 kRS = 10k RD = 50k k’n(W/L) = 50A/V2 Vt = 1 V Solution: ID = 100A VDS = 4 V Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Bias Circuits 2 V / R DD D load line curve V = V DS GS V GS1 Q V GS2 V GS3 V V = V DD DS GS2 Analysis Two power supplies are available A simple circuit utilizing a current source Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Basic MOSFET current mirror. Example: VDD = 5V ;IREF=10A; Q1and Q2are matched ; L=10 m and W=100 m; Vt = 1V ; k’n=20 A/V2 VA = 10L; Vo=+3V Q1: I V converter Q2: V I converter If Q2 in saturation then Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The operation of the MOSFET as an amplifier. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Small-signal models for the MOSFET: (a) neglecting the dependence of iD on vDS in saturation (channel-length modulation effect); and (b) including the effect of channel-length modulation modeled by output resistance ro = |VA|/ID. (b.1) -Model (a.1) -Model (a.2) T-Model (b.2) T-Model Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Example: Vt = 1.5V ; k’n(W/L)=0.25 mA/V2 VA = 50V Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Small-signal equivalent-circuit model of a MOSFET in which the body is not connected to the source. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Basic amplifier configurations with current sources Source follower or common-drain amplifier Common-source amplifier Common-gate amplifier Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The CMOS common-source amplifier: (a) circuit; (b)i-v characteristic of the active-load Q2; (c) graphical construction to determine the transfer characteristic; and transfer characteristic. ID2 ID2 Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Example: VDD=10V;Vtn = | Vtp| = 1V ; k’n= 2 k’p= 20 A/V2(W=100 m; L =10 m |VA|= 100V for both the n and p device)IREF=100 A. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The CMOS common-gate amplifier: (a) circuit; (b) small-signal equivalent circuit; and (c) simplified version of the circuit in (b). Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The source follower: (a) circuit; (b) small-signal equivalent circuit; and (c) simplified version of the equivalent circuit. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
(a) NMOS amplifier with enhancement load; (b) graphical determination of the transfer characteristic; (c) transfer characteristic. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The NMOS amplifier with depletion load: (a) circuit; (b) graphical construction to determine the transfer characteristic; and (c) transfer characteristic. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Operation of the CMOS inverter when v1 is high: (a) circuit with v1 = VDD (logic-1 level, or VOH); (b) graphical construction to determine the operating point; and (c) equivalent circuit. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Operation of the CMOS inverter when v1 is low: (a) circuit with v1 = 0V (logic-0 level, or VOL); (b) graphical construction to determine the operating point; and (c) equivalent circuit. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The voltage transfer characteristic of the CMOS inverter. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Dynamic operation of a capacitively loaded CMOS inverter: (a) circuit; (b) input and output waveforms; (c) trajectory of the operating point as the input goes high and C discharges through the QN; (d) equivalent circuit during the capacitor discharge. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
(a) High-frequency equivalent circuit model for the MOSFET; (b) the equivalent circuit for the case the source is connected to the substrate (body); (c) the equivalent circuit model of (b) with Cdb neglected (to simplify analysis). Strong inversion and saturation: VGS – Vt > 200 mV Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Determining the short-circuit current gain Io/Ii. For physical frequencies s = j, it cam be seen that the magnitude of the current gain becomes unity at the frequency: Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The Junction Field-Effect Transistor ( a ) JFET with zero gate-source bias ( b ) JFET with negative gate-source voltage that is less negative than the pinch-off voltage VP. Note W’ < W ( c ) JFET at pinch-off with VGS = VP Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
The Junction Field-Effect Transistor ( a ) JFET with small drain-source ( b ) JFET with channel just at pinch-off with vDS = vDSP ( a ) JFET with small drain-source ( b ) JFET with channel just at pinch-off with vDS = vDSP Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Summary of equations for the JFET P-channel N-channel Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Output characteristics for a JFET with IDSS = 200 A and VP = -4V Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
Transfer characteristic for a saturated JFET with IDSS = 1 mA and VP = -3.5V Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
When to use JFETs • JFET have much higher input impedances and much lower input currents than BJTs. • BJTs are more linear than JFETs. • The gain of a BJT is much higher than that of a JFET. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
JFET (Example) The JFET in the circuit below has VP=-3V, IDSS=9mA, and =0. Find the values of all resistors so that VG=5V, ID=4mA, and VD=11V. Design for 0.05mA in the voltage divider. For the JFET circuit designed in the former exercise, let an input signal vi be capacitively coupled to the gate, a large bypass capacitor be connected between the source and ground, and the output signal vo be taken from the drain through a large coupling capacitor. The resulting common-source amplifier is shown below. Calculate gm and ro (assuming VA=100V). Also find Ri and Ro. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.
References • SEDRA. Adel S. e SMITH, Kenneth C. Microelectronic Circuits. Oxford University Press. • BOYLESTAD, Robert e NASHELSKY, Louis. Dispositivos Eletrônicos e Teoria de Circuitos. Prentice Hall do Brasil. • SZE, S. M. Physics of Semiconductor Devices. John Wiley & Sons. • SCHILLING, Donald L. e BELOVE, Charles. Circuitos Eletrônicos Discretos e Integrados. Guanabara Dois. • COMER, David J. Introduction to Semiconductor Circuits Design. Addison Wesley Publishing Company. • MALVINO, Albert P. Eletrônica. Volume I. McGraw-Hill. Field Effect Transistors Electronics – Celso José Faria de Araújo, M.Sc.