FETx

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2 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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CONTENTS



1. History

2. FET Definition

3. FET operation

4. Types of field
-
effect transistors

5. MOSFET development

6. MOSFET composition

6.1 metal
-
oxide
-
semicoductor

structure

6.2 MOSFET structure and channel formation

6.3 modes of operation

7. MOSFET scaling

8. Other MOSFET types



The principle of field
-
effect transistors was first patented by
Julius Edgar
Lilienfeld

in 1925 and by
Oskar
Heil

in 1934, but practical semi
-
conducting
devices were only developed much later after the
transistor

effect was
observed and explained by the team of
William Shockley

at
Bell Labs

in
1947
.


History


The field
-
effect transistor (FET) is a generic term for
a device that controls
current through a circuit via an applied voltage
, i.e. it behaves like a voltage
-
controlled resistor.


A FET has
three terminals
:


gate: as in the “gate” keeper of the current


source: the source of the current


drain: the destination of the current


FETs
can be made in
NPN

or
PNP

variety.


FETs are “
Unipolar
” (
conduct either electrons or holes, not both
)


Definition of a FET


The FET controls the
flow
of

electrons

(or

electron holes
)
from the source to drain.




The FET operation is as follows:


apply a
voltage
to the gate


this voltage sets up an
electric
field in the “body
” of the device


electric field
inhibits/supports the
flow of charge from source to
drain


FET operation


in an
n
-
channel

depletion
-
mode

device,
a negative gate
-
to
-
source voltage

causes
a

depletion region

to expand in width
and encroach on the channel from the
sides, narrowing the channel.



If the depletion region expands to
completely close the channel, the
resistance of the channel from source to
drain becomes large
, and the FET is
effectively turned off like a
switch.

Depletion mode device



in an
n
-
channel

enhancement
-
mode

device
,
a positive gate
-
to
-
source voltage
is
necessary to create a
conductive
channel
.The

positive voltage attracts free
-
floating
electrons within the body towards the gate,
forming a conductive channel
.

Further
gate
-
to
-
source voltage increase
will
attract even more electrons
towards the gate
which are able to create a conductive channel from source to drain; this process is
called

inversion
.

Enhancement
-
mode

device


Field
-
effect transistors are also distinguished by the method of insulation
between channel and gate. Types of FETs are:


The
DEPFET

is a FET formed in a
fully
-
depleted substrate
and acts as a
sensor.


he
DNAFET

is a specialized FET that acts as a
biosensor
, by using
a gate
made of single
-
strand DNA molecules

to detect matching DNA strands.


The
HEMT

(High Electron Mobility Transistor), also called an HFET .


The
IGBT

(Insulated
-
Gate Bipolar Transistor) is a device for power control.


The
ISFET

is an
Ion
-
Sensitive Field Effect
Transistor used to measure ion
concentrations in a solution.




Types of field
-
effect transistors


he
JFET

(Junction Field
-
Effect Transistor) uses a reverse biased p
-
n
junction to separate the gate from the body.


The
MODFET

(Modulation
-
Doped Field Effect Transistor).


The
MOSFET

(Metal

Oxide

Semiconductor Field
-
Effect Transistor) utilizes
an
insulator

(typically
SiO
2
) between the gate and the body.


The
DGMOSFET

is a MOSFET with
dual gates
.


The
NOMFET

is a
Nanoparticle

Organic Memory
Field
-
Effect Transistor.


The
OFET

is an
Organic Field
-
Effect Transistor
using an organic
semiconductor in its channel.



The most commonly used FET is the
MOSFET

MOSFET development


Usually the
semiconductor

of choice is
silicon
, but some chip
manufacturers, most notably
IBM

and
Intel
, recently started using a
chemical compound

of silicon and germanium (
SiGe
) in MOSFET.


many semiconductors with better electrical properties than silicon, such as
gallium arsenide
, do not form good semiconductor
-
to
-
insulator interfaces,
thus are not suitable for MOSFETs.


The gate is separated from the channel by a thin insulating layer,
traditionally of silicon dioxide and later of
silicon
oxynitride
.


When a voltage is applied between the gate and body terminals, the electric
field generated penetrates through the oxide and creates an "inversion
layer" or "channel" at
the semiconductor
-
insulator interface
. The inversion
channel is of the same type, P
-
type or N
-
type, as the source and drain, thus
it provides a channel through which current can pass.

MOSFET composition


in MOSFET the terminals ,
(
source

and
drain
), each connected to individual
highly doped regions that are separated by the body region
. These regions
can be either p or n type, but they must both be of the same type, and of
opposite type to the body region. The source and drain (unlike the body) are
highly doped as signified by a '+' sign after the type of doping.

MOSFET structure and channel formation


If the MOSFET is an
n
-
channel or
nMOS

FET
, then the source and drain are
'n+' regions and the body is a 'p' region.
with sufficient gate voltage
, holes
from the body are driven away from the gate, forming an
inversion layer or
n
-
channel

at the interface between the p region and the oxide.


This conducting channel extends between the source and the drain, and
current is conducted through it when a voltage is applied between source
and drain.
Increasing the voltage on the gate
leads to a
higher electron
density

in the inversion layer and therefore
increases the current flow
between the source and drain
.

N
-

and P
-

Channel MOSFET and channel formation



If The P and N regions are reversed
from the
P
-
Channel device
.


There are two basic modes of operation of FET’s

depletion and
enhancement.



Modes of operation

Depletion mode,

refers to the decrease of
carriers in the channel

due to variation in gate voltage.




Enhancement mode
refers

to the increase of carriers in the channel
due to application

of gate voltage.


Over the past decades, the MOSFET has continually been scaled down in size;
typical
MOSFET channel lengths were once several
micrometres
, but modern
integrated circuits are incorporating MOSFETs with channel lengths
of tens of
nanometers
. Intel began production of a process featuring a
32

nm feature size
(with the channel being even shorter) in late
2009
.





Historically, the difficulties with decreasing the size of the MOSFET have been
associated with the
semiconductor device fabrication process
, the need to use
very low voltages
, and
with poorer electrical performance
necessitating circuit
redesign and innovation (
small MOSFETs exhibit higher leakage currents, and
lower output resistance
,).

MOSFET scaling

Dual gate MOSFET


in dual gate MOSFET both gates control the current in the device.


It is commonly
used for small signal devices
in
radio frequency

applications
where the second gate is normally used for gain control or mixing and
frequency conversion.





FinFET

The
Finfet
, is a double gate device, one
of a number of geometries being
introduced to mitigate the effects of
short channels and reduce drain
-
induced barrier lowering.

Other MOSFET types

Power MOSFET



Power MOSFETs

have a
different structure
than the one presented above.
the structure is
vertical and not planar
.




it is possible for the transistor
to sustain both high blocking voltage and high current.