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FET
- 1. B .E Pres e nt a t oi n
- 3. Invention • In 1947,John Bardeen, Walter Brattain, and William Schockly, researchers at Bell Lab, invented Transistor. • They found Transistor Effect: “when electrical contacts were applied to a crystal of germanium, the output power was larger than the input.” • Awarded the Nobel Prize in physics (1956) • Revolutionized portability and efficiency of electronic devices John Bardeen, Walter Brattain, and William Schockly First model of Transistor, 1947
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- 5. FIELD-EFFECT TRANSISTORS Field-effect transistors (FETs)follow an other principle than bipolar junction transistors Meaning of “Field Effect” ,An electric field is develop by the charges , this electric field controls the conduction path of the output ckt . So there is an effect due to electric field and because of this reason we call the device field effect FETs controlled by voltage
- 6. Similarities: • Amplifiers • SwitchingDevice • Impedance Matching Circuits
- 7. DIFFERENCES • Voltage controlleddevices • Higher input impedance • Less sensitive to temp. variations • Unipolar device • Smaller/ Easily Integrated Chips • Current controlled devices • Lower impedance • Higher sensitive • Bipolar device • Bigger IC FET’s BJT’s
- 8. GENERAL OVERVIEW • BasicConcept : • The terminals of a FET refer to their function: Gate (G), Source (S), Drain (D) • FETs are voltage-controlled by the voltage between gate and source terminal • Voltage effects the electric field of the transistor which enlarges or diminishes the channel
- 9. CONSTRUCTION OF FET •Source: The source is the terminal through which majority carriers enter the Silicon Bar • Drain: Terminal through which Majoroty carriers leave the bar • Gate: controls Drain current and is always reverse biased
- 10. ANALOGY OF FETWITH WATER • The operation of FET can be compared to the water flow through a flexible pipe • When One end is pressed the cross sectional area decreases hence water flow decreases • In a FET drain is similar to outlet • Gate is similar to control in the figure below:
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- 14. THE CONCLUSION IS……. A PN Junctionwith reversed biased.. Establish “depletion region” There is no current through the junction As the increased in voltage , the wider the deption region. Depletion region has no free charges. Depletion region has fixed space.
- 16. JUNCTION FIELD-EFFECT TRANSISTORS (JFETS) •Simplest type of FET • Long channel semiconductor • Either p- or n-doped (p-type, n-type) • A contact at each ends at source and drain terminals • Gate terminal surrounds the channel and is doped opposite to the doping of the channel
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- 19. N PP Gate Sourc e Drain N channel I(ds) v(ds) Reversed bias V(gs) increased V(GS)=0 V(GS)=-0.5 V(GS)=-1 V(GS)=-1.5 V(GS)=-2 V(GS)=-2.5 I (ds)
- 20. N PP Gate Sourc e Drain N channel Reversed biasPinch off I (ds)
- 22. if VDD isincreased from 0 V, Id will increase proportionally, as shown in the graph of Figure .In this area, the channel resistance is essentially constant because the depletion region is not large enough to have significant effect. This is called the ohmic region because VDS and ID are related by Ohm’s law when ID begins to increase very rapidly with any further increase in VDS. Breakdown can result in irreversible damage to the device, so JFETs are always operated below breakdown and within the active
- 23. • VGS ControlsID : If VGS is set to increase by adjusting VGG . ID decreases as the magnitude of VGS is increased to larger negative values because of the narrowing of the channel Regions : Ohmic Region – linear region • JFET behaves like an ordinary resistor
- 24. • Saturation orAmplifier Region • JFET operates as a constant current device because Id is relatively independent of Vds Breakdown Region • If Vds is increased beyond its value corresponding to Va – avalanche breakdown voltage. • JFET enters the breakdown region where Id increases to an excessive value. Cut Off Region • As Vgs is made more and more negative, the gate reverse bias increases which increases the thickness of the depletion region. • As negative value of Vgs is increased, a stage comes when the 2 depletion regions touch each other. Vgs (off) = -Vp /Vp/ =
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- 26. METAL OXIDE SEMICONDUCTORFET: MOSFET • As Compared to BJT, MOS transistor can be made quiet small and their manufacturing process is relatively simple. • MOSFET also known as insulated-gate field-effect transistors (IGFET) is preferred in power electronics due to its ability of fast switching especially in timing circuits. • MOSFET has a "Metal Oxide" gate(silicon dioxide-
- 27. •This isolation ofthe controlling gate makes the input resistance of the MOSFET extremely high in the Mega-ohms region (infinite), thus switching loss at input side can controlled and stabilized. •As the gate terminal is isolated from the main c urrent carrying channel "No current flows into the gate” so MOSFET acts as a voltage controlled resistor (like JFET). •MOSFET is specially used in digital
- 28. N-CHANNEL E-MOSFET SHOWINGCHANNEL LENGTH LAND CHANNEL WIDTH W
- 29. Substrate Channel Drain Insulator Gate OPERATION OFA TRANSISTOR VSG > 0 n type operation Positive gate bias attracts electrons into channel Channel now becomes more conductive More electrons Source VSD VSG VSD
- 30. • Gate isinsulated from the body of FET so it is called insulated gate FET(IGFET) • Structurally there exits no channel between source and drain . • Because a thin layer of P- type substrate touching the metal oxide film provides channel for electrons and hence acts like N-type material. N CHANNEL ENHANCEMENT MOSFET MOSFET ARCHITECTURE
- 31. • It consistsof a lightly doped p type substrate in to which two heavily Doped n type material are diffused. • The surface is coated with a layer of silicon dioxide(Sio2 ) • Holes are cut through the Sio2 to make contact with n-type blocks. N CHANNEL ENHANCEMENT MOSFET CONSTRUCTIO N…….
- 32. WORKING OF THEENHANCEMENT MOSFET • Drain is made positive with respect to the source and no potential is applied to the gate as shown in figure. • The two n-blocks and p-type substrate form back to back pn junctions connected by the Resistance of the p-type material. • Both the junctions cannot be forwarded at the Same time so N CHANNEL ENHANCEMENT MOSFET
- 33. WORKING OF THEENHANCEMENT MOSFET • So MOSFET is cut off when gate source voltage Is zero • The gate is made positive with respect to source substrate as shown in figure • A channel of electrons (n- channel) is formed in between the source and drain regions. N CHANNEL ENHANCEMENT MOSFET
- 34. • Consequently positive chargesappears on the gate and negative charges appears in the substrate between the drain and source. • As VGS no. electrons in the channel ID . N CHANNEL ENHANCEMENT MOSFET
- 35. E-MOSFET TRANSFER CHARACTERISTI C • E-MOSFET doesnot have a significant IDss parameter • t there is ideally no drain current until VGS reaches a certain nonzero value called the threshold voltage, VGS(th). • the curve starts at VGS(th) rather than VGS(off) • The equation for the E- MOSFET transfer characteristic curve is
- 36. OF THE E-TYPE MOSFET •𝑉𝐺𝑆 is always positive • As 𝑉𝐺𝑆 increases, 𝐼𝐷 increases • As 𝑉𝐺𝑆 is kept constant and 𝑉𝐷𝑆 is increased, then 𝐼𝐷 saturates (𝐼𝐷𝑆𝑆 ) and the saturation level, 𝑉𝐷𝑆𝑠𝑎𝑡 is reached. • 𝑉𝐷𝑆𝑠𝑎𝑡 can be calculated by • 𝑉𝐷𝑠𝑎𝑡 = 𝑉𝐺𝑆 − 𝑉T
- 37. TRIODE REGION A VOLTAGE-CONTROLLEDRESISTOR @SMALL VDS G p n+n+ metal S DB oxide +- +++ +++ - - - - VGS1>Vt p n+n+ metal S DB oxide +- +++ +++ +++ - - - - - - VGS2>VGS1 p n+n+ metal S DB oxide +- +++ +++ +++ - - - - - - - - - VGS3>VGS2 +++ ID VDS 0.1 v increasing VGS Increasing VGS puts more charge in the channel, allowing more drain current to flow cut-off
- 38. SATURATION REGION OCCURS ATLARGE VDS p n+n+ metal source S gate G drain D body B oxide + - +++ +++ +++ VDS large As the drain voltage increases, the difference in voltage between the drain and the gate becomes smaller. At some point, the difference is too small to maintain the channel near the drain pinch-off
- 39. Saturation Region occurs atlarge VDS p n+n+ metal source S gate G drain D body B oxide + - +++ +++ +++ VDS large The saturation region is when the MOSFET experiences pinch-off. Pinch-off occurs when VG - VD is less than VT.
- 40. Saturation Region occurs atlarge VDS p n+n+ metal source S gate G drain D body B oxide + - +++ +++ +++ VD>>Vs VGS - VDS < VT or VGD < VDS > VGS - VT VT
- 41. SATURATION REGION ONCE PINCH-OFFOCCURS, THERE IS NO FURTHER INCREASE IN DRAIN CURRENT ID VDS 0.1 v increasing VGS triode saturation VDS>VGS-VT VDS<VGS-VT
- 42. CONSTRUCTION OF N CHANNELDEPLETION MOSFET • An n-type channel is obtained by diffusion between N+ type source and drain in an n-channel MOSFET. • In depletion MOSFET a lightly doped n-type channel has been introduced between to heavily doped source& drain blocks,. • •In depletion MOSFET a lightly doped n-type channel has been introduced between to heavily doped source& drain blocks.
- 43. CONSTRUCTION OF P CHANNELDEPLETION MOSFET • An p-type channel is obtained by diffusion between p+ type source and drain in an p channel MOSFET. • In p-channel depletion MOSFETs are made by using n-type substrate and diffusing a lightly doped p-type channel between two heavily doped P-type source & drain blocks
- 44. WORKING • Negative gate •When Vgs =0 electrons can flow freely from source to drain through the conducting channel. since a channel exists between drain & source, Id flows even when Vgs =0 • With negative voltage a depletion MOSFET behave like JFET. • The action of negative voltage on gate is to deplete the channel of free n-type charge carriers so named as depletion MOSFET. • The negative potential at the gate pressure electrons toward the p -type substrate and attract the holes for the p-type substrate • When 𝑉𝐺𝑆 is reduced to 𝑉𝑃 (pinch off voltage), then N CHANNEL DEPLETION MOSFE
- 45. DRAIN CHARACTERISTICS • Whenthe gate source voltage is zero considerable drain current flows. • When the gate is applied with negative voltage, positive charge are induced in the n channel through the SiO2 layer of the gate capacitor. • The conduction in n channel FET is due to electrons i.e., the majority carriers • Therefore the induced positive charges make the n-channel less conductive. • The voltage drop due to the drain current causes the channel region nearer to the drain to be more depleted than the region due to the
- 46. The transfer characteristicsare similar to the JFET In Depletion Mode operation: When VGS = 0V, ID = IDSS When VGS < 0V, ID < IDSS When VGS > 0V, ID > IDSS The formula used to plot the Transfer Curve, is: The transfer characteristics are similar to the JFET In Depletion Mode operation: When VGS = 0V, ID = IDSS When VGS < 0V, ID < IDSS When VGS > 0V, ID > IDSS The formula used to plot the Transfer Curve, is: 2 GS D DSS P V I = I 1- V
- 47. The depletion MOSFETcan also be operated in enhancement mode simply by applying a positive voltage to the gate
- 48. DUAL-GATE MOSFETS • Thedual-gate MOSFET can be either a depletion or an enhancement type. The only difference is that it has two gates Advantage of the dual-gate: •Capacitance is reduced • Used for Automatic gain control (AGC)
- 50. COMPARISON OF MOSFETAND JFET • JFET Gate is not insulated from the channel • Channel and gate forms two pn junctions • There are only 3 leads • Can be operated in depletion mode only • Input impedance is high • MOSFET or IGFET is insulated from the channel • Channel and gate forms parrallel plate capacitor. • There are 4 leads • Can be operated in both depletion and enhancement mode • Input impedance is very high JFET MOSFET
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