Field Effect Transistor

Hi friends..

Why FET is termed as ''FIELD EFFECT'' transistor..?

You should read some basics before asking questions. Knowing nothing you mAy be confused.

When the junction transistor was invented, the main collector current was controlled by the base CURRENT.

Before transistors people used vacuum tubes to control the current by an input(grid) VOLTAGE.

Therefore people were looking for a structure that would also use VOLTAGE to control the main CURRENT. This led to the Field Effect Transistor: there is a thin semiconductor channel the resistance of which is controlled by applying an electric field by an attached gate.
A FET controls the source-drain current by the gate-source VOLTAGE.

FET is short for Field Effect Transistor.The Field Effect Transistor is a device which enables us to use one electrical signal to control another. The name "transistor"is a shortened version of the original term, transfer resistor, which indicates how the device works. Field Effect Transistor exist in two major classifications. These are known as the junction FET (JFET) and the metal-oxide- semiconductor FET (MOSFET).

if you want to know its history or more information,maybe you could search it in the Wikipedia.

Fine friend (jiripolivka)...

I agree with your answer, but in BJT also output current is controlled by input current (So, it is called current controlled device. Of course, strictly speaking no current source is available). Here, why can't we call BJT as a "Field Effect' Device...?

jiripolivka said:

You should read some basics before asking questions. Knowing nothing you mAy be confused.
When the junction transistor was invented, the main collector current was controlled by the base CURRENT.

Click to expand...

Jiripolivka - are you sure that YOU are familiar with basics ? The question "how is the BJT controlled" was discussed extensively in this and in other forums.
It is clear - from theory and experiment - that the BJT is a voltage-controlled device (despite some contrary assertions, even in some textbooks) .

LvW said:

Jiripolivka - are you sure that YOU are familiar with basics ? The question "how is the BJT controlled" was discussed extensively in this and in other forums.
It is clear - from theory and experiment - that the BJT is a voltage-controlled device (despite some contrary assertions, even in some textbooks) .

Click to expand...

Yes, I am sure I know the basics!
Bipolar-junction transistor, BJT, needs CURRENT to control output current. Field-effect transistor needs VOLTAGE to control output current. The difference is that BJT loads the signal source as it consumes power to do it. FETs are controlled by input voltage only, so they do not consume power for control.

Both friends above: My first sentence was that before asking questions you should read the basics. You did not, so you are quite confused.

It is clear - from theory and experiment - that the BJT is a voltage-controlled device (despite some contrary assertions, even in some textbooks).

Click to expand...

Clear from theory - yes, as described by the Shockley equation and the Ebers-Moll model.
Experiments can be made with voltage as well as current control, I think. In an intuitive analysis of BJT circuits, it's sometimes reasonable to look at a BJT as current gain element, beyond theoretical implications.

So how do you describe the difference between FET and BJT without conflicting with theoretical insights? A BJT is controlled by applying a forward bias voltage to the base-emitter junction, injecting charge into junction which always implies a base current. A FET is controlled by a field voltage, ideally without involving a gate current.

jiripolivka said:

Both friends above: My first sentence was that before asking questions you should read the basics. You did not, so you are quite confused.

Click to expand...

Jiripolivka, perhaps you should be somewhat more careful with your remarks.

FvM said:

Clear from theory - yes, as described by the Shockley equation and the Ebers-Moll model.
Experiments can be made with voltage as well as current control, I think. In an intuitive analysis of BJT circuits, it's sometimes reasonable to look at a BJT as current gain element, beyond theoretical implications.

Click to expand...

I do not rely on formulas only. Very often, such formulas are good for practical use but do not tell, necessarily, the physical truth.
Here is a collection of arguments in favor of voltage control:

1.) Each design of a BJT amplifier starts with a suitable base-emitter voltage (0.6...0.7 V). The base current Ib is needed only because it exists and shouldn´t be neglected during calculation of the biasing network.
But for sufficient Ic stabilization (negative feedback) you even can forget Ib for designing the bias voltage divider. Where is the control function?

2.) Question: Undoubtly, it is the base-emitter voltage which „opens“ the transistor (allowing a certain amount of Ic and, unavoidable, also a base current Ib) - however, during operation as an amplifier suddenly the current Ib takes over the control? Is that logical? (Jiripolivka - what is your answer?)

3.) Regarding temperature dependence of Ic: There is only one single parameter which defines the temperature sensitivity of Ic: dVbe/dT=-2mV/K (for Ic=const.). And this is not a measured value - it was calculated based on voltage control mechanism! Is there anybody who knows about a similar figure derived from current control?

4.) How can you explain the negative feedback effect of an emitter resistor without assuming voltage control?

5.) Read the following statements:

Berkeley University::
IB is an undesirable but inevitable side effect of producing IC by forward biasing the BE junction.

Winfield Hill (Co-Author „Art of Electronics“):
BJTs are transconductance devices (voltage in, current out), just like tubes and FETs, which means they are first and foremost, voltage controlled.
Just because you can successfully bias a few BJT circuits with current doesn't meant they're current-controlled devices.
Is a long-tail pair differential-amplifier going to work with current control to the two base inputs? No way.
Is a cascode-connected stage going to work with current control to the base? No, of course not, it can't.
How about the operation of a current mirror? Hah!

I don't contradict any of the detail considerations. As you know, I agree with the principle classification of BJT as voltage controlled device anyway.

What did I mean with "sometimes"? There's a number of BJT circuits where the base current is forced by a current source so that it's completely or mostly independent of the base-emitter voltage. In this and only in this case, I consider "to look at a BJT as current gain element".

Both friends above: My first sentence was that before asking questions you should read the basics. You did not, so you are quite confused.

Click to expand...

It's not clear to me which posts the statement is exactly referring to. At least it sounds a bit patronizing. I suggest to avoid this tone.

FvM said:

I don't contradict any of the detail considerations. As you know, I agree with the principle classification of BJT as voltage controlled device anyway.
What did I mean with "sometimes"? There's a number of BJT circuits where the base current is forced by a current source so that it's completely or mostly independent of the base-emitter voltage. In this and only in this case, I consider "to look at a BJT as current gain element".

Click to expand...

Hi FvM - I know what you mean, and I also know that sometimes we use the term "base current injection".
However, what is a current source? Do we really "produce" a current? Of course, we can - sometimes - treat the device AS IF it would be current-controlled.
But, for my opinion, a sharper look reveals the following:
A current source feeding the base of a BJT is nothing else than a voltage source driving a current through a voltage divider circuit which consists of a (large) ohmic resistor Rb and the resistive path between base and emitter, where the voltage Vbe is developed.
And if we approximately know the amount of base current we can calculate the corresponding value of Rb.
However, don´t forget: Also this calculation always starts with fixation of a suitable value for the voltage Vbe.

:razz: Just to annoy LvW.....

In my copy of Horowitz and Hill, the first section in the "Transistors" chapter is titled "First model: current amplifier" and describes transistor action as follows:

A small current flowing into the base controls a much larger current flowing into the collector."

Click to expand...

Similarly, the first chapter of the other text book I have handy ("A practical introduction to electronic circuits" by Martin Hartley Jones) describes transistor action like this:

The transistor is a current-controlled amplifying device: if a small current flows between the base and emitter, it gives rise to a much larger current between collector and emitter.

Click to expand...

Hey godfreyl - no problem at all, you didn´t "annoy" me. Of course, I know that several textbooks describe the BJT as being controlled.
Don´t ask me - why. I really don´t know. And, of course, I also know the part of Horowitz/Hill as cited by you. .
But - do you also know what they say some pages later (2.10 Improved transistor model)?
Here is an excerpt:
The important change is in property 4 (section 2.01) ,where we said earlier that Ic=hfe*Ib. We thought of the transistor as a current amplifier... That´s roughly correct and for some applications it´s good enough.
But to understand differential amplifiers, log. converters, temperature compensation and other important applications you must think of the transistor as a transconductance device - collector current is determined by base-to-emitter voltage.

I repeat: To understand.....you must...

At this point I like to remind you that Horowitz/Hill is concentrated primarily on applications rather than device physics. This may be the reason for mentioning the "first model" as quoted by you.
As I have mentioned already, to me it is a (funny?) phenomenon that textbooks - as cited by you - really believe that a small quantity (in this case: a current) could directly control a larger quantity of the same kind.
This is impossible from the energy point of view!
More than that, in all of these books it is shown why a pn junction (diode) has an exponential voltage-current relationship. Fine.
But I cannot understand why some authors think that in a transistor the pn junction suddenly change its behaviour and does not react anymore to the applied voltage.
Just because the relation Ic=B*Ib looks so nice and simple?

LvW

I agree with all of that, but the thing is a lot of books start out with the simple "current control" model, so that's the first thing that get's glued into the reader's head.

My view is that if a student thinks transistors are current controlled devices, it's probably best to leave well enough alone, rather than trying to correct them. After all, if their teacher says it's current controlled and their text book (as far as they've read it) says it's current controlled, then if you say otherwise, they either won't believe you or it will just cause confusion. Hopeully in a year or two they will have gained a fuller understanding.

godfreyl said:

My view is that if a student thinks transistors are current controlled devices, it's probably best to leave well enough alone, rather than trying to correct them. After all, if their teacher says it's current controlled and their text book (as far as they've read it) says it's current controlled, then if you say otherwise, they either won't believe you or it will just cause confusion. Hopeully in a year or two they will have gained a fuller understanding.

Click to expand...

Hi Godfreyl, thank you for your response.
Fortunately, there are some resources which correctly explain the transistor function as voltage -controlled.
I know, of course, what you mean - however, my „educational approach“ is different from yours:
For my opinion, it can only be a benefit to both - teacher and student - if they start a discussion about different explanations to be found in various information sources.

As another example: In some contributions/submissions - also in this forum - the gain for a common emitter stage with emitter resistor Re is given as -Rc/Re.
Certainly, this is an approximation which in some cases is appropriate and exact enough.
But the student should KNOW that this is only an approximation - and he should know under which special conditions it cannot be used.

The same applies to the transistors transconductance gm which appears in each gain formula (for FET and BJT).
Correction: It should appear.
However, some people believe it is better to use only the inverse value (1/gm) and call it „internal emitter resistance“ re=1/gm (in some cases, even with a capital letter: Re).
For which purpose? I don´t know.
For my opinion, this is (a) misleading and (b) simply false because re is not a resistive property of the emitter path.

In general, it is my opinion that students should start learning to be critical against simple equations and simple explanations.
I think, they shouldn´t simply believe something and - as often as possible - they should ask „why?“. This is the first step to really understand how an electronic circuit works.

(PS: Sometimes I´ve got the impression that I fight - like Don Quixote - against the windmills)