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Field Effect Transistors and their applications

Field Effect Transistors and their applications. Types of Field Effect Transistors. There are Junction FETs (JFET) and Insulated gate FETs (IGFET) There are many types of IGFET. Most common is the MOSFET: Metal Oxide Semiconductor FET. A silicon atom (Si). Has 4 outer electrons

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Field Effect Transistors and their applications

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  1. Field Effect Transistors and their applications

  2. Types of Field Effect Transistors There are Junction FETs (JFET) andInsulated gate FETs (IGFET)There are many types of IGFET. Most common is the MOSFET: Metal Oxide Semiconductor FET.

  3. A silicon atom (Si) Has 4 outer electrons The outer electron shell needs 8 to be “full” (standing wave pattern) Silicon will try to lend or borrow 4

  4. Silicon (group 4) bonds

  5. A pure silicon crystal lattice

  6. An Arsenic atom (As) Has 5 outer electrons One surplus for fitting in to the lattice

  7. Arsenic doping (group 5) – N type

  8. A Gallium atom (Ga) Has 3 outer electrons One short for fitting in to the lattice

  9. Gallium doping (group 3) – P type Holes are positive

  10. A P-N Junction(N on left)

  11. What causes the depletion? • Electrons move from left to right to fill the + holes • Where electrons and holes combine the area is “depleted” of current carriers • This leaves the left (N Type) positive so eventually this prevents the depletion spreading any more. • Applying negative to N type replaces the depleted carriers and the current resumes (Forward biased diode) • Applying positive to the N type removes more electrons and increases the depletion. Almost no current flows. (Reverse biased diode)

  12. The junction transistor Emitter (-) Collector (+) Electrons Base Electrons are negative  (Original patent used point contact)

  13. NPN Transistor circuits • Common base • Emitter follower (common collector) • Common emitter Collector Base Emitter

  14. A Planar NPN junction transistor Collector Base Emitter

  15. Types of Field Effect Transistors There are Junction FETs (JFET) andInsulated gate FETs (IGFET)There are many types of IGFET. Most common is the MOSFET: Metal Oxide Semiconductor FET.

  16. An Insulated Gate FET

  17. Field Effect Transistors come in 4 forms N Channel (as an NPN transistor or Valve)P Channel (as a PNP transistor) N and P channel each come as:Enhancement mode (just IGFET)Depletion mode (IGFET or JFET)

  18. Transistor symbols

  19. In an IC the substrate can be separate

  20. Insulated Gate Field Effect TransistorN Channel Enhancement mode

  21. N channel enhancement IGFET turned on

  22. N enhance FET at Pinchoff

  23. N enhance FET beyond Pinchoff

  24. Curve L = 10v gateMostly resistiveCurve A = 2.5v gateMostly constant current (pinchoff) BSS 145 N channel enhancement FET output curves L A

  25. N channel Depletion mode FETON – No gate bias

  26. N channel Depletion mode FET off

  27. An application: The SoftRock Receiver

  28. FETs in the SoftRock Receiver The I & Q generator uses 4 D type Flip Flops 74HC74 The dual I/Q Mixer uses an FST 3253 dual 4 way multiplexer switch These all use logic circuits built with FETs Lets look at basic logic

  29. Logic circuits By combining together enough NAND gates you can build anything digital. So that includes computers. (However some other bits may make life easier) We will look at how to make the Flip Flops used in the I / Q generator another time. So lets look at a CMOS NAND gate. Complementary Metal Oxide Semiconductor FET (We will add an inverter to make an AND gate too)

  30. An AND gate (NAND + Inverter) NAND AND A B

  31. Simulated AND gate waveforms AND NAND B A

  32. NAND gate IC Layout (From Wikimedia commons)

  33. SoftRock Receiver

  34. SoftRock Receiver

  35. LTSpice model of SoftRock Receiver

  36. LTSpice output of SoftRock model

  37. Questions?

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