field effect transistors and their applications. there are junction fets (jfet) and insulated gate...
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Field Effect Transistors and their applications
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
A silicon atom (Si)
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
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
One surplus for fitting in to the lattice
As
?
??
Arsenic doping (group 5) – N type
Si Si As Si
Si
Si
Si
As
Si
Si
Si
Si
-
-
A Gallium atom (Ga)
Has 3 outer electrons
One short for fitting in to the lattice
Ga
? ?
??
?
Gallium doping (group 3) – P type
Si Ga Si Si
Si
Si
Si
Si
Si
Ga
Si
Si
+
+
Holes are positive
A P-N Junction (N on left)
Si Si As Si
Si
Si
Si
As
Si
Si
Si
Si
-
-
Si Si As Si
Si
Si
Si
As
Si
Si
Si
Si
-
-
As Si
Si
Si
Si
As
As Si
Si Si
Si Si Si Ga Si Si
Si
Si
Si
Si
Si
Ga
Si
Si
+
Si Ga Si Si
Si Si
Si
Si
Ga
Si
Si
+
Si Ga
Si
Si
Si
Si
+
Si Ga
Si
Si
Si
Si
+
Ga
Si
Si
Si
As
Si
Si
Si
Si
-As Si
Si Si
Si Si
Si
Si
Ga
Si
Si
+
Si
Si
Si
SiGa
N Type P TypeDepleted
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 (+)
Base
(Original patent used point contact)
Electrons
NNP
Electrons are negative
NPN Transistor circuits
• Common base• Emitter follower (common collector)• Common emitter
Base
Collector
Emitter
A Planar NPN junction transistor
N NP
Collector Base Emitter
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
An Insulated Gate FET
Source Gate Drain
SubstrateN Type
P Type
Insulator
Metal
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)
Field Effect Transistors come in 4 forms
Transistor symbols
N type enhancement IGFET
Source
Drain
GateEmitter
Collector
Base
NPN junction transistorN type Junction FET
Source
Drain
Gate
P type enhancement IGFET
Source
Drain
Gate Emitter
Collector
Base
PNP junction transistorP type Junction FET
Source
Drain
Gate
In an IC the substrate can be separate
P type enhancement
P type enhancementexternal substrate
P type depletion
P type depletionexternal substrate
N type enhancement
N type enhancementexternal substrate
N type depletion
N type depletionexternal substrate
Insulated Gate Field Effect TransistorN Channel Enhancement mode
Source Gate Drain
SubstrateN Type
P Type
Insulator
Metal
N channel enhancement IGFET turned on
Source Gate + Drain
SubstrateN Type
P Type
Insulator
Metal
Channel
N enhance FET at Pinchoff
Source Gate + Drain +
SubstrateN Type
P Type
Insulator
Metal
Channel
N enhance FET beyond Pinchoff
Source Gate + Drain++
SubstrateN Type
P Type
Insulator
Metal
Channel
Curve L = 10v gateMostly resistive
Curve A = 2.5v gateMostly constant current (pinchoff)
BSS 145 N channel enhancement FET output curves
A
L
N channel Depletion mode FETON – No gate bias
Source Gate Drain
SubstrateN Type
P Type
Insulator
Metal
Channel
N channel Depletion mode FET off
Source Gate - Drain
SubstrateN Type
P Type
Insulator
Metal
No Channel
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 circuitsBy 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)
AND
NAND
A B
Simulated AND gate waveforms
AND
NAND
B
A
NAND gate IC Layout
(From Wikimedia commons)
SoftRock Receiver
SoftRock Receiver
LO
Mixers
RF Filter
Inphase and QuadratureGenerator
I
Q
Stereo OutputIF Filters(Balanced Output omitted)
LTSpice model of SoftRock Receiver
LTSpice output of SoftRock model
Questions?
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