Transcript Document

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)
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Si
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Has 4 outer electrons
The outer electron shell
needs 8 to be “full”
(standing wave pattern)
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Silicon will try to lend
or borrow 4
Silicon (group 4) bonds
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
A pure silicon crystal lattice
An Arsenic atom (As)
Has 5 outer electrons
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One surplus for fitting
in to the lattice
As
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Arsenic doping (group 5) – N type
Si
Si
As
Si
Si
Si
Si
Si
Si
Si
Si
As
A Gallium atom (Ga)
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Has 3 outer electrons
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Ga
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One short for fitting in
to the lattice
Gallium doping (group 3) – P type
Si
Ga
Si
Si
Si
Si
Si
+
Si
+
Si
Si
Ga
Si
Holes are
positive
A P-N Junction (N on left)
N Type
Si
Si
Depleted
Si
Si
As
Si
Si
P Type
Si
Si
-
Si
Si
Si
+
Si
As
Si
Si
Si
Si
Ga
Si
Ga
Si
Si
Si
Si
Si
As
Si
Si
Si
Si
Ga
Si
Si
Si
Ga
Si
Si
Si
+
Si
As
Si
Si
Si
Si
-
Si
Si
Si
+
Si
As
Si
Si
Si
Si
Si
Si
Ga
Si
Si
Si
Si
Si
As
Si
Si
As
Si
Ga
Si
Si
Si
Ga
Si
Si
Si
+
Si
As
Si
Si
Si
Si
Si
As
Si
Si
Si
Si
Si
Si
+
Si
Si
Si
Si
Ga
Si
Ga
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)
The junction transistor
Emitter (-)
Collector (+)
Electrons
N
P
Base
(Original patent used point contact)
N
Electrons are
negative 
NPN Transistor circuits
• Common base
• Emitter follower (common collector)
• Common emitter
Collector
Base
Emitter
A Planar NPN junction transistor
Collector
N
Base
P
Emitter
N
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.
An Insulated Gate FET
Source
Substrate
Gate
Drain
NType
Insulator
P Type
Metal
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)
Transistor symbols
N type enhancement IGFET
N type Junction FET
Drain
Gate
Source
P type enhancement IGFET
Gate
Source
Drain
NPN junction transistor
Drain
Gate
Source
P type Junction FET
Gate
Collector
Base
Emitter
PNP junction transistor
Source
Drain
Emitter
Base
Collector
In an IC the substrate can be separate
P type enhancement
N type enhancement
P type enhancement
external substrate
N type enhancement
external substrate
P type depletion
N type depletion
P type depletion
external substrate
N type depletion
external substrate
Insulated Gate Field Effect Transistor
N Channel Enhancement mode
Source
Substrate
Gate
Drain
NType
Insulator
P Type
Metal
N channel enhancement IGFET turned on
Source
Gate +
Drain
Channel
Substrate
NType
Insulator
P Type
Metal
N enhance FET at Pinchoff
Source
Gate +
Drain +
Channel
Substrate
NType
Insulator
P Type
Metal
N enhance FET beyond Pinchoff
Source
Gate +
Drain++
Channel
Substrate
NType
Insulator
P Type
Metal
BSS 145 N channel
enhancement
FET output curves
L
Curve L = 10v gate
Mostly resistive
A
Curve A = 2.5v gate
Mostly constant
current (pinchoff)
N channel Depletion mode FET
ON – No gate bias
Source
Gate
Drain
Channel
Substrate
NType
Insulator
P Type
Metal
N channel Depletion mode FET off
Source
Gate -
Drain
NoChannel
Substrate
NType
Insulator
P Type
Metal
An application: The SoftRock Receiver
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
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)
An AND gate (NAND + Inverter)
NAND
A
B
AND
Simulated AND gate waveforms
AND
NAND
B
A
NAND gate IC Layout
(From Wikimedia commons)
SoftRock Receiver
SoftRock Receiver
I
LO
Inphase and Quadrature
Generator
Q
Mixers
RF Filter
(Balanced Output omitted)
IF Filters
Stereo Output
LTSpice model of SoftRock Receiver
LTSpice output of SoftRock model
Questions?