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 | | موضوع: ترانزستور تأثير المجال الإثنين فبراير 22, 2010 11:18 am | |
| ترانزستورات تأثير المجال
Field Effect Transistors FET
المحتويات:
فكرة عامة باللغة العربية عن تاريخ و تصنيف FET
ملخص عام مختصر عن FET
مقارنة بين BJT و FET
JFET
MOSFET
MOSFET's as Switches
ترانزستورات تأثير المجال
Field Effect Transistors FET
تمكن في عام 1953م مهندسان من مختبرات بيل الأمريكية وهما أين روس (Ian
Ross) وجورج ديسي (George Dacey) من تصنيع ترانزستور يعمل بآلية تختلف عن
تلك المستخدمة في الترانزستور ثنائي القطبية وهو ترانزستور تأثير المجال
ذي الوصلة (Junction Field Effect Transistors (FET.
ويتكون هذا الترانزستور من شريحة من السيليكون مطعمة إما كنوع سالب (N) أو
كنوع موجب (P) ويوصل بطرفي هذه الشريحة قطبان معدنيان يسمى أحدهما المصدر
(source) وهو يناظر الباعث (emitter) ويسمى الآخر المصرف (drain) وهو يناظر
المجمع (collector).
ومن الواضح أنه عند تسليط جهد خارجي بين المصدر والمصرف فإن تيارا كهربائيا
سيسري بين القطبين بغض النظر عن اتجاه الجهد المسلط وذلك على العكس من
الترانزستور ثنائي القطبية. ولكي يتم التحكم بمرور التيار بين القطبين فإنه
يتم تطعيم الشريحة على جانبيها وعند وسطها بنوع تطعيم مخالف لنوع التطعيم
الأساسي للشريحة ليتكون بذلك وصلتين حول الشريحة ويتم ربط الوصلتين بقطب
معدني يسمى البوابة (gate) وهو يناظر القاعدة (****). ويطلق على منطقة
الشريحة المحصورة بين الوصلتين اسم القناة (channel) ويتحدد عرض القناة
الفعلي الذي يمكن للتيار أن يمر من خلاله من عرض القناة الحقيقي مطروحا منه
عرض المنطقتين المنضبتين في الوصلتين.
وعند تسليط جهد ذي انحياز عكسي بين البوابة وأحد القطبين الآخرين وغالبا
قطب المصدر فإنه يمكن التحكم بعرض البوابة وبالتالي كمية التيار الذي يمر
من خلالها. ومن الواضح أن عملية التحكم بالتيار المار بين المصدر والمصرف
يتم من خلال الجهد الكهربائي بدلا من التيار الكهربائي كما في الترانزستور
ثنائي القطبية. ولذلك فقد أطلق العلماء على هذا النوع من الترانزستورات
اسم ترانزستور تأثير المجال وذلك لأن المجال الكهربائي الناتج عن الجهد
المسلط على البوابة هو المسؤول عن عملية التحكم بمرور التيار في
الترانزستور. إن التيار الذي يسري في القناة مكون من نوع واحد فقط من
حاملات الشحنات وهي إما الإلكترونات في حالة القناة السالبة أو الفجوات في
حالة القناة الموجبة ولذا فقد تمت تسمية هذا الترانزستور بالترانزستور
أحادي القطبية (unipolar) وذلك عل عكس الترانستور ثنائي القطبية الذي
يستخدم النوعين من الحاملات في عمله.
وفي عام 1960م تمكن المهندسون في مختبرات بيل الأمريكية من تصنيع أحد أشهر
أنواع الترانزستورات أحادية القطبية وهو النوع المسمى ترانزستور تأثير
المجال من نوع معدن _ أكسيد _ شبه موصل (Metal-Oxide-Semiconductor
Field-Effect transistor (MOSFET)).
ويتم تصنيع هذه الترانزستورات بالطريقة السطحية من خلال إنتاج منطقة مطعمة
تسمى القناة بأحد نوعي التطعيم السالب أو الموجب على سطح رقاقة من
السيليكون ثم توضع طبقة من أكسيد السيليكون العازل تعلوها طبقة أخرى من
المعدن كما يوحي بذلك أسمه. ويتم توصيل ثلاثة أقطاب معدنية أحدها إلى
الطبقة المعدنية ويسمى البوابة بينما يوصل الطرفان الآخران إلى المنطقة شبه
موصلة في مكانيين متقابلين حول البوابة يسميان المصدر والمصرف. ويسمى هذا
النوع من الترانزستورات بترانزستور الموصفت المنضب ((Depletion MOSFET)
حيث يلزم تسليط جهد بقطبية محددة على البوابة ليحول نوع المادة شبه
الموصلة التي تقع تحتها من موجب إلى سالب أو العكس لكي يتم التحكم بمرور
التيار بين المصدر والمصرف. وفي النوع المسمى الموصفت المعزز (Enhancement
MOSFET) يتم تطعيم رقاقة السيليكون بمنطقتين منفصلتين من النوع السالب أو
الموجب بينهما منطقة وسطى تطعم بنوع مغاير لتلك التي للمنطقتين المنفصلتين
ثم توضع طبقة من أكسيد السيليكون العازل تعلوها طبقة أخرى من المعدن لتغطي
المنطقة الوسطى ويتم توصيل ثلاثة أقطاب اثنان بالمنطقتين المنفصلتين وهما
المصدر والمصرف والثالث بالطبقة المعدنية وهو البوابة. ويلزم تسليط جهد
بقطبية محددة على البوابة ليحول نوع المادة شبه الموصلة التي تقع تحتها من
موجب إلى سالب أو العكس لكي يتم التحكم بمرور التيار بين المصدر والمصرف.
إن أهم ما يميز الترانزستور أحادي القطبية على ثنائي القطبية هو عدم حاجته
لدائرة كهربائية معقدة لتحديد نقطة تشغيله وكذلك قلة استهلاكه للطاقة وصغر
المساحة التي يحتلها على سطح البلورة الشبه موصلة ولكن عيبه الرئيسي هو أن
سرعة تبديله أقل منها في الترانزستور ثنائي القطبية بسبب أن البوابة تعمل
كمكثف يحتاج شحنها تفريغها زمن طويل نسبيا.
Summary of Field Effect
Transistors
• Field Effect Transistors, or FET's are "Vol***e Operated
Devices" and can be divided into two main types: Junction-gate devices
called JFET's and Insulated-gate devices called IGFET´s or more commonly
known as MOSFET's.
• Insulated-gate devices can also be sub-divided into Enhancement types
and Depletion types. All forms are available in both N-channel and
P-channel versions.
• FET's have very high input resistances so very little or no current
(MOSFET types) flows into the input terminal making them ideal for use
as electronic switches.
• The input impedance of the MOSFET is even higher than that of the JFET
due to the insulating oxide layer and therefore static electricity can
easily damage MOSFET devices so care needs to be taken when handling
them.
• FET's have very large current gain compared to junction transistors.
• They can be used as ideal switches due to their very high channel
"OFF" resistance, low "ON" resistance.
The Field Effect Transistor Family-tree
Field Effect Transistors can be used to replace normal Bipolar Junction
Transistors in electronic circuits and a simple comparison between FET's
and transistors stating both their advan***es and their disadvan***es
is given below.
.
The Field Effect Transistor
The Field Effect Transistor, or simply FET however, use the vol***e that
is applied to their input terminal to control the output current, since
their operation relies on the electric field (hence the name field
effect) generated by the input vol***e. This then makes the Field Effect
Transistor a VOL***E operated device.
The Field Effect Transistor is a unipolar device that has very similar
properties to those of the Bipolar Transistor ie, high efficiency,
instant operation, robust and cheap, and they can be used in most
circuit applications that use the *****alent Bipolar Junction
Transistors, (BJT). They can be made much smaller than an *****alent BJT
transistor and along with their low power consumption and dissipation
make them ideal for use in integrated circuits such as the CMOS range of
chips.
We remember from the previous tutorials that there are two basic types
of Bipolar Transistor construction, NPN and PNP, which basically
describes the physical arrangement of the P-type and N-type
semiconductor materials from which they are made. There are also two
basic types of Field Effect Transistor, N-channel and P-channel. As
their name implies, Bipolar Transistors are "Bipolar" devices because
they operate with both types of charge carriers, Holes and Electrons.
The Field Effect Transistor on the other hand is a "Unipolar" device
that depends only on the conduction of Electrons (N-channel) or Holes
(P-channel).
The Field Effect Transistor has one major advan***e over its standard
bipolar transistor cousins, in that their input impedance is very high,
(Thousands of Ohms) making them very sensitive to input signals, but
this high sensitivity also means that they can be easily damaged by
static electricity.
There are two main types of field effect transistor, the Junction Field
Effect Transistor or JFET and the Insulated-gate Field Effect Transistor
or IGFET), which is more commonly known as the standard Metal Oxide
Semiconductor Field Effect Transistor or MOSFET for short.
The Junction Field Effect Transistor
We saw previously that a bipolar junction transistor is constructed
using two PN junctions in the main current path between the Emitter and
the Collector terminals. The Field Effect Transistor has no junctions
but instead has a narrow "Channel" of N-type or P-type silicon with
electrical connections at either end commonly called the DRAIN and the
SOURCE respectively.
Both P-channel and N-channel FET's are available. Within this channel
there is a third connection which is called the GATE and this can also
be a P or N-type material forming a PN junction and these connections
are compared below.
The semiconductor "Channel" of the Junction Field Effect Transistor is a
resistive path through which a vol***e Vds causes a current Id to flow.
A vol***e gradient is thus formed down the length of the channel with
this vol***e becoming less positive as we go from the drain terminal to
the source terminal.
The PN junction therefore has a high reverse bias at the drain terminal
and a lower reverse bias at the source terminal. This bias causes a
"depletion layer" to be formed within the channel and whose width
increases with the bias. FET's control the current flow through them
between the drain and source terminals by controlling the vol***e
applied to the gate terminal.
In an N-channel JFET this gate vol***e is negative while for a
P-channel JFET the gate vol***e is positive.
Bias arrangement for an N-channel JFET and corresponding circuit
symbols.
The cross sectional diagram above shows an N-type semiconductor channel
with a P-type region called the gate diffused into the N-type channel
forming a reverse biased PN junction and its this junction which forms
the depletion layer around the gate area. This depletion layer restricts
the current flow through the channel by reducing its effective width
and thus increasing the overall resistance of the channel.
When the gate vol***e Vg is equal to 0V and a small external vol***e
(Vds) is applied between the drain and the source maximum current (Id)
will flow through the channel slightly restricted by the small depletion
layer. If a negative vol***e (Vgs) is now applied to the gate the size
of the depletion layer begins to increase reducing the overall effective
area of the channel and thus reducing the current flowing through it, a
sort of "squeezing" effect. As the gate vol***e (Vgs) is made more
negative, the width of the channel decreases until no more current flows
between the drain and the source and the FET is said to be
"pinched-off".
In this pinch-off region the gate vol***e, Vgs controls the channel
current and Vds has little or no effect. The result is that the FET acts
more like a vol***e controlled resistor which has zero resistance when
Vgs = 0 and maximum "ON" resistance (Rds) when the gate vol***e is very
negative.
Output characteristic vol***e-current curves of a
typical junction FET.
The vol***e Vgs applied to the gate controls the current flowing between
the drain and the source terminals. Vgs refers to the vol***e applied
between the gate and the source while Vds refers to the vol***e applied
between the drain and the source. Because a Field Effect Transistor is a
VOL***E controlled device, "NO current flows into the gate!" then the
source current (Is) flowing out of the device equals the drain current
flowing into it and therefore (Id = Is).
The characteristics curves example shown above, shows the four different
regions of operation for a JFET and these are given as:
• Ohmic Region - The depletion layer of the
channel is very small and the JFET acts like a variable resistor.
• Cut-off Region - The gate vol***e is
sufficient to cause the JFET to act as an open circuit as the channel
resistance is at maximum.
• Saturation or Active Region - The JFET
becomes a good conductor and is controlled by the gate-source vol***e,
(Vgs) while the drain-source vol***e, (Vds) has little or no effect.
• Breakdown Region - The vol***e between the
drain and source, (Vds) is high enough to causes the JFET's resistive
channel to break down and pass current.
The control of the drain current by a negative gate potential makes the
Junction Field Effect Transistor useful as a switch and it is essential
that the gate vol***e is never positive for an N-channel JFET as the
channel current will flow to the gate and not the drain resulting in
damage to the JFET. The principals of operation for a P-channel JFET are
the same as for the N-channel JFET, except that the polarity of the
vol***es need to be reversed.
The MOSFET
As well as the Junction Field Effect Transistor, there is another type
of Field Effect Transistor available whose Gate input is electrically
insulated from the main current carrying channel and is therefore called
an Insulated Gate Field Effect Transistor.
The most common type of insulated gate FET or IGFET as it is sometimes
called, is the Metal Oxide
Semiconductor Field Effect Transistor or MOSFET
for short.
The MOSFET type of field effect transistor has a "Metal Oxide" gate
(usually silicon dioxide commonly known as glass), which is electrically
insulated from the main semiconductor N-channel or P-channel.
This isolation of the controlling gate makes the input resistance of
the MOSFET extremely high in the Mega-ohms region and almost infinite.
As the gate terminal is isolated from the main current carrying channel
""NO current flows into the gate"" and like the JFET, the MOSFET also
acts like a vol***e controlled resistor. Also like the JFET, this very
high input resistance can easily accumulate large static charges
resulting in the MOSFET becoming easily damaged unless carefully handled
or protected.
Basic MOSFET Structure and Symbol
We also saw previously that the gate of a JFET must be biased in such a
way as to forward-bias the PN junction but in a MOSFET device no such
limitations applies so it is possible to bias the gate in either
polarity.
This makes MOSFET's specially valuable as electronic switches or to make
logic gates because with no bias they are normally non-conducting and
the high gate resistance means that very little control current is
needed. Both the P-channel and the N-channel MOSFET is available in two
basic forms, the Enhancement type and the Depletion type.
Depletion-mode MOSFET
The Depletion-mode MOSFET, which is less common than the
enhancement types is normally switched "ON" without a gate bias vol***e
but requires a gate to source vol***e (Vgs) to switch the device "OFF".
Similar to the JFET types. For N-channel MOSFET's a "Positive" gate
vol***e widens the channel, increasing the flow of the drain current and
decreasing the drain current as the gate vol***e goes more negative.
The opposite is also true for the P-channel types. The depletion mode
MOSFET is *****alent to a "Normally Closed" switch.
Depletion-mode N-Channel MOSFET and circuit Symbols
Depletion-mode MOSFET's are constructed similar to their JFET transistor
counterparts where the drain-source channel is inherently conductive
with electrons and holes already present within the N-type or P-type
channel. This doping of the channel produces a conducting path of low
resistance between the drain and source with zero gate bias.
Enhancement-mode MOSFET
The more common Enhancement-mode MOSFET is the reverse of the
depletion-mode type. Here the conducting channel is lightly doped or
even undoped making it non-conductive. This results in the device being
normally "OFF" when the gate bias vol***e is equal to zero.
A drain current will only flow when a gate vol***e (Vgs) is applied to
the gate terminal. This positive vol***e creates an electrical field
within the channel attracting electrons towards the oxide layer and
thereby reducing the overall resistance of the channel allowing current
to flow. Increasing this positive gate vol***e will cause an increase in
the drain current, Id through the channel. Then, the Enhancement-mode
device is *****alent to a "Normally Open" switch.
Enhancement-mode N-Channel MOSFET and circuit Symbols
Enhancement-mode MOSFET's make excellent electronics switches due to
their low "ON" resistance and extremely high "OFF" resistance and
extremely high gate resistance.
Enhancement-mode MOSFET's are used in integrated circuits to produce
CMOS type Logic Gates and power switching circuits as they can be driven
by digital logic levels.
MOSFET Summary
The MOSFET has an extremely high input gate resistance and as such a
easily damaged by static electricity if not carefully protected.
MOSFET's are ideal for use as electronic switches or common-source
amplifiers as their power consumption is very small.
Typical applications for MOSFET's are in Microprocessors, Memories,
Calculators and Logic Gates etc. Also, notice that the broken lines
within the symbol indicates a normally "OFF" Enhancement type showing
that "NO" current can flow through the channel when zero gate vol***e is
applied and a continuous line within the symbol indicates a normally
"ON" Depletion type showing that current "CAN" flow through the channel
with zero gate vol***e.
For P-Channel types the symbols are exactly the same for both types
except that the arrow points outwards.
This can be summarised in the following switching table.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة][b]
[size=16][b][size=12]The MOSFET as a Switch
We saw previously, that the N-channel, Enhancement-mode MOSFET operates
using a positive input vol***e and has an extremely high input
resistance (almost infinite) making it possible to interface with nearly
any logic gate or driver capable of producing a positive output. Also,
due to this very high input (Gate) resistance we can parallel together
many different MOSFET's until we achieve the current handling limit
required.
While connecting together various MOSFET's may enable us to switch high
current or high vol***e loads, doing so becomes expensive and
impractical in both components and circuit board space. To overcome this
problem Power Field Effect Transistors or Power FET's where developed.
We now know that there are two main differences between FET's,
Depletion-mode for JFET's and Enhancement-mode for MOSFET's and on this
page we will look at using the Enhancement-mode MOSFET as a Switch.
By applying a suitable drive vol***e to the Gate of an FET the
resistance of the Drain-Source channel can be varied from an
"OFF-resistance" of many hundreds of kΩ's, effectively an open circuit,
to an "ON-resistance" of less than 1Ω, effectively a short circuit. We
can also drive the MOSFET to turn "ON" fast or slow, or to pass high
currents or low currents. This ability to turn the power MOSFET "ON" and
"OFF" allows the device to be used as a very efficient switch with
switching speeds much faster than standard bipolar junction transistors.
An example of using the MOSFET as a switch
[center]
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
In this circuit arrangement an Enhancement-mode N-channel MOSFET is
being used to switch a simple lamp "ON" and "OFF" (could also be an
LED). The gate input vol***e VGS is taken to an appropriate positive
vol***e level to turn the device and the lamp either fully "ON", (VGS =
+ve) or a zero vol***e level to turn the device fully "OFF", (VGS = 0).
If the resistive load of the lamp was to be replaced by an inductive
load such as a coil or solenoid, a "Flywheel" diode would be required in
parallel with the load to protect the MOSFET from any back-emf.
Above shows a very simple circuit for switching a resistive load such as
a lamp or LED. But when using power MOSFET's to switch either inductive
or capacitive loads some form of protection is required to prevent the
MOSFET device from becoming damaged. Driving an inductive load has the
opposite effect from driving a capacitive load. For example, a capacitor
without an electrical charge is a short circuit, resulting in a high
"inrush" of current and when we remove the vol***e from an inductive
load we have a large reverse vol***e build up as the magnetic field
collapses, resulting in an induced back-emf in the windings of the
inductor.
For the power MOSFET to operate as an analogue switching device, it
needs to be switched between its "Cut-off Region" where VGS = 0 and its
"Saturation Region" where VGS(on) = +ve. The power dissipated in the
MOSFET (PD) depends upon the current flowing through the channel ID at
saturation and also the "ON-resistance" of the channel given as RDS(on).
For example.
Example No1
Lets assume that the lamp is rated at 6v, 24W and is fully "ON" and the
standard MOSFET has a channel "ON-resistance" ( RDS(on) ) value of
0.1ohms. Calculate the power dissipated in the MOSFET switch.
The current flowing through the lamp is calculated as:
Then the power dissipated in the MOSFET will be given as:
You may think, well so what!, but when using the MOSFET as a switch to
control DC motors or high inrush current devices the "ON" channel
resistance ( RDS(on) ) is very important. For example, MOSFET's that
control DC motors, are subjected to a high in-rush current as the motor
first begins to rotate. Then a high RDS(on) channel resistance value
would simply result in large amounts of power being dissipated within
the MOSFET itself resulting in an excessive temperature rise, and which
in turn could result in the MOSFET becoming very hot and damaged due to a
thermal overload. But a low RDS(on) value on the other hand is also
desirable to help reduce the effective saturation vol***e ( VDS(sat) =
ID x RDS(on) ) across the MOSFET.
When using MOSFET´s or any type of Field Effect Transistor for that
matter as a switching device, it is always advisable to select ones that
have a very low RDS(on) value or at least mount them onto a suitable
heatsink to help reduce any thermal runaway and damage.
Power MOSFET Motor Control
Because of the extremely high input or Gate resistance that the MOSFET
has, its very fast switching speeds and the ease at which they can be
driven makes them ideal to interface with op-amps or standard logic
gates. However, care must be taken to ensure that the gate-source input
vol***e is correctly chosen because when using the MOSFET as a switch
the device must obtain a low RDS(on) channel resistance in proportion to
this input gate vol***e. For example, do not apply a 12v signal if a 5v
signal vol***e is required. Power MOSFET´s can be used to control the
movement of DC motors or brushless stepper motors directly from computer
logic or Pulse-width Modulation (PWM) type controllers.
As a DC motor offers high starting torque and which is also
proportional to the armature current, MOSFET switches along with a PWM
can be used as a very good speed controller that would provide smooth
and quiet motor operation.
Simple Power MOSFET Motor Controller
As the motor load is inductive, a simple "Free-wheeling" diode is
connected across the load to dissipate any back emf generated by the
motor when the MOSFET turns it "OFF".
The Zener diode is used to prevent excessive gate-source input vol***es.
[/size][/b] [/size][/b] [/center] | |
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