Field Effect Transistors and their applications

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Field Effect Transistors and their applications
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There are Junction FETs (JFET) andInsulated gate
FETs (IGFET)There are many types of IGFET.
Most common is the MOSFET Metal Oxide
Semiconductor FET.
Types of Field Effect Transistors
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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
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Silicon (group 4) bonds
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A pure silicon crystal lattice
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An Arsenic atom (As)
Has 5 outer electrons One surplus for fitting in
to the lattice
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Arsenic doping (group 5) N type
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A Gallium atom (Ga)
Has 3 outer electrons One short for fitting in to
the lattice
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Gallium doping (group 3) P type
Holes are positive
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A P-N Junction (N on left)
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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)

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The junction transistor
Emitter (-)
Collector ()
Electrons
Base
Electrons are negative ?
(Original patent used point contact)
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NPN Transistor circuits

• Common base
• Emitter follower (common collector)
• Common emitter

Collector
Base
Emitter
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A Planar NPN junction transistor
Collector
Base
Emitter
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There are Junction FETs (JFET) andInsulated gate
FETs (IGFET)There are many types of IGFET.
Most common is the MOSFET Metal Oxide
Semiconductor FET.
Types of Field Effect Transistors
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An Insulated Gate FET
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N Channel (as an NPN transistor or Valve)P
Channel (as a PNP transistor) N and P channel
each come asEnhancement mode (just
IGFET)Depletion mode (IGFET or JFET)
Field Effect Transistors come in 4 forms
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Transistor symbols
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In an IC the substrate can be separate
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Insulated Gate Field Effect TransistorN Channel
Enhancement mode
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N channel enhancement IGFET turned on
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N enhance FET at Pinchoff
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N enhance FET beyond Pinchoff
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Curve L 10v gateMostly resistiveCurve A
2.5v gateMostly constant current (pinchoff)
BSS 145 N channel enhancement FET output curves
L
A
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N channel Depletion mode FETON No gate bias
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N channel Depletion mode FET off
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An application The SoftRock Receiver
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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
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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)
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An AND gate (NAND Inverter)
NAND
AND
A
B
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Simulated AND gate waveforms
AND
NAND
B
A
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NAND gate IC Layout
(From Wikimedia commons)
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SoftRock Receiver
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SoftRock Receiver
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LTSpice model of SoftRock Receiver
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LTSpice output of SoftRock model
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Questions?