Field Effect Transistors and their applications

heartlustElectronics - Devices

Nov 2, 2013 (3 years and 11 months ago)

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Field Effect Transistors and
their applications

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.

Types of Field Effect Transistors

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

Silicon (group 4) bonds

A pure silicon crystal lattice

An Arsenic atom (As)

Has 5 outer electrons


One surplus for fitting
in to the lattice

Arsenic doping (group 5)


N type

A Gallium atom (Ga)

Has 3 outer electrons

One short for fitting in
to the lattice

Gallium doping (group 3)


P type

Holes are
positive

A P
-
N Junction

(N on left)

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

Electrons are
negative


NPN Transistor circuits


Common base


Emitter follower (common collector)


Common emitter

Base

Collector

Emitter

A Planar NPN junction transistor

Collector

Base

Emitter

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.

Types of Field Effect Transistors

An Insulated Gate FET

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

In an IC the substrate can be separate

Insulated Gate Field Effect Transistor

N Channel Enhancement mode

N channel enhancement IGFET turned on

N enhance FET at Pinchoff

N enhance FET beyond Pinchoff

Curve L = 10v gate

Mostly resistive


Curve A = 2.5v gate

Mostly constant
current (pinchoff)

BSS 145 N channel
enhancement

FET output curves

A

L

N channel Depletion mode FET

ON


No gate bias

N channel Depletion mode FET off

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.


C
omplementary
M
etal
O
xide
S
emiconductor 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

LTSpice model of SoftRock Receiver

LTSpice output of SoftRock model

Questions?