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?