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What is FET?

Lecture on Field Effect Transistor (FET) by :- Uttampreet Singh Lecturer-Electrical Engg. Govt. Polytechnic College, G.T.B.garh, Moga. What is FET?.

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What is FET?

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  1. Lecture onField Effect Transistor (FET)by:-Uttampreet SinghLecturer-Electrical Engg.Govt. Polytechnic College, G.T.B.garh, Moga

  2. What is FET? FET is uni-polar device i.e. operation depends on only one type of charge carriers (h or e) . It is a Voltage controlled Device (gate voltage controls drain current)

  3. ADVANTAGES OF FET • Very high input impedance (109-1012 ) • Source and drain are interchangeable • Low Voltage Low Current Operation is possible (Low-power consumption) • Less Noisy • No minority carrier storage (Turn off is faster) • Very small in size, occupies very small space in ICs

  4. Current Controlled vs Voltage Controlled Devices

  5. Types of Field Effect Transistors (The Classification) • JFET MOSFET(IGFET) • JFET MOSFET(IGFET) • JFET MOSFET(IGFET) n-Channel JFET p-Channel JFET FET FET FET FET FET Enhancement MOSFET Depletion MOSFET n-Channel DMOSFET p-Channel DMOSFET n-Channel EMOSFET p-Channel EMOSFET

  6. JFET Construction There are two types of JFET’s: n-channel and p-channel. The n-channel is more widely used. There are three terminals: Drain (D) and Source (S) are connected to n-channel Gate (G) is connected to the p-type material

  7. N-Channel JFET Operation The nonconductive depletion region becomes thicker with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)

  8. Drain Drain Gate Gate Source Source SYMBOLS p-channel JFET n-channel JFET

  9. CHARATERISTICS • At the pinch-off point: • • any further increase in VGS does not produce any increase in ID. • VGS at pinch-off is denoted as Vp. • • ID is at saturation or maximum. It is referred to as IDSS.

  10. ID IDSS • As VGS becomes more negative: • • the JFET will pinch-off at a lower voltage (Vp). • • ID decreases (ID < IDSS) even though VDS is increased. • • Eventually ID will reach 0A. VGS at this point is called Vp or VGS(off). • • Also note that at high levels of VDS the JFET reaches a breakdown situation. ID will increases uncontrollably if VDS > VDSmax.

  11. Transfer Characteristics The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear): As a result, FET’s are often referred to a square law devices

  12. Transfer (Transconductance) Curve From this graph it is easy to determine the value of ID for a given value of VGS It is also possible to determine IDSS and VP by looking at the knee where VGS is 0

  13. Case Construction and Terminal Identification

  14. p-Channel JFET p-Channel JFET operates in a similar manner as the n-channel JFET except the voltage polarities and current directions are reversed

  15. P-Channel JFET Characteristics • As VGS increases more positively • the depletion zone increases • ID decreases (ID < IDSS) • eventually ID = 0A • Also note that at high levels of VDS the JFET reaches a breakdown situation. ID increases uncontrollably if VDS > VDSmax.

  16. MOSFET(Metal Oxide Semiconductor FET)

  17. MOSFET • There are two types of MOSFET’s: • Depletion mode MOSFET (D-MOSFET) • Operates in Depletion mode the same way as a JFET when VGS 0 • Operates in Enhancement mode like E-MOSFET when VGS > 0 • Enhancement Mode MOSFET (E-MOSFET) • Operates in Enhancement mode • IDSS = 0 until VGS > VT (threshold voltage)

  18. Depletion Mode MOSFET Construction The Drain (D) and Source (S) leads connect to the to n-doped regions These N-doped regions are connected via an n-channel This n-channel is connected to the Gate (G) via a thin insulating layer of SiO2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS

  19. D-MOSFET Symbols

  20. Basic Operation A D-MOSFET may be biased to operate in two modes: the Depletion mode or the Enhancement mode

  21. p-Channel Depletion Mode MOSFET The p-channel Depletion mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed

  22. Enhancement ModeMOSFET’s

  23. Enhancement Mode MOSFET Construction The Drain (D) and Source (S) connect to the to n-doped regions These n-doped regions are not connected via an n-channel without an external voltage The Gate (G) connects to the p-doped substrate via a thin insulating layer of SiO2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS

  24. E-MOSFET Symbols

  25. Basic Operation The Enhancement mode MOSFET only operates in the enhancement mode. VGS is always positive IDSS = 0 when VGS < VT As VGS increases above VT, ID increases If VGS is kept constant and VDS is increased, then ID saturates (IDSS) The saturation level, VDSsat is reached.

  26. p-Channel Enhancement Mode MOSFETs The p-channel Enhancement mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed.

  27. Summary Table JFET D-MOSFET E-MOSFET

  28. THANKS

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