difference between bipolar and mosfet

The most basic difference is that a bipolar transistor requires current at the control terminal (the base lead), whereas a mosfet requires none. However, there are advantages to both in different situations.

You generally cannot substitute a bipolar transistor for a fet, because the circuit will not be designed to supply the required base current.

MOSFETs have three leads, a source, a gate, and a drain. Bipolar transistors also have three leads, but they are called emitter, base, and collector. These leads roughly correspond to one another, ie, the emitter is like the source, the base is like the gate, and the collector is like the drain. Making the base (gate) more positive (for NPN and N-MOSFETs) or negative (for PNP or P-MOSFETs) with respect to the emitter (source) causes more current to flow from collector (drain) to emitter (source).

This terminology is totally confusing, and, sadly, you just have to get used to it if you want to talk about these things.

MOSFETs are used to construct CMOS devices, and are thus the main transistor component to microprocessors. They are also good for constructing huge power transistors, which are easier to control due to the lack of required gate current.

Bipolar transistors are generally more useful for analog design, where the lower noise, more easily predicted voltage requirements, and lower control voltages are useful.

For a FET, the electrostatic field of charges on the control terminal (the gate) is used to moderate the output. MOSFETs have a silicon oxide layer that insulates the gate from the charge. JFETs use a reverse-biased PN junction's depletion region to isolate the gate from the source and drain. For bipolar transistors, the movement of charges across PN junctions controls the output.

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Regards,
   Robert Monsen
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Robert Monsen
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Just wondering could someone explain fairly simply what the difference is between a bipolar and a fet ? Can I put a bipolar in place of a fet or vice versa ?

Regards, Chris

Reply to
Skeleton Man

Why?

The names have been specifically chosen to describe how the device actually functions.

Charge carriers are sourced or emitted from the source/emitter. These carriers are then drained off or collected by the drain/collector. The gate or base *voltage* controls the flow of carriers. I will give you that "base" is not on a par with "gate" in describing its function.

Once one understands the names, one will understand how mosfet and bipolar actually function. If you don't understand why the names are as they are, you wont understand how the devices function.

Here we go again... for bipolar transistors, it is the application of

*voltage* to the base emitter PN junction that controls the output current. The movement of charges is irrelevant.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

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Reply to
Kevin Aylward

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The application of a forward voltage to the base-emitter junction of a
bipolar transistor will, of course, cause charge to flow between the
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Reply to
John Fields

I'm not saything they are wrong, I'm just saying that it's a confusing blob of information until you memorize it. Once you figure it out, you can convince yourself that it makes sense, just like any terminology.

Right, you have to understand how the device works in order to understand the names of the terminals. Unfortunately, that is confusing for beginners, who often want to simply build something simple, and get confused by emitters, collectors, where which goes, whether PNP or NPN should be used, etc.

Who cares? The point was that with a bipolar transistor, one needs current into the base in order to pass current from collector to emitter. It doesn't work without the current. This is a useful fact which can often be exploited in circuits that simply want an on/off switch.

Whether it's 'right' is yet another matter. Newtonian physics is 'wrong', and based on incorrect physics, but for most things, it's OK to use. This is also true of design using beta. Lighten up.

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   Robert Monsen
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Robert Monsen

do

More or less. That is why you can't just replace MOSFETs with bipolar transistors. The bipolar transistor needs current into their base to operate, and mosfet circuits will not be designed to supply it.

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Regards,
   Robert Monsen
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Reply to
Robert Monsen

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Essentially, yes.  But, the voltage applied to the base must force
charge through the base-emitter junction before collector current can
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Reply to
John Fields

so if I'm to understand correctly.. a bi-polar will pass current between collector and emitter when a voltage is applied to the base ? and a fet will do a simmilar thing only doesn't require a current ? (at whichever terminal corresponds to a base on a bipolar)

Regards, Chris

Reply to
Skeleton Man

I actually liked your description on this point. It stated the facts without implying that base current controlled collector current. It was only your later statement that I had the issue with.

I wouldnt say that Newtonian physics is 'wrong'. Its more of an approximation. The essentials of Newtonian physics still as correct today as ever.

My point here is to avoid perpetuating common myths concerning the bipolar transistor that invariable leads to much confusion. Its better to nip some things in the bud.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

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Reply to
Kevin Aylward

No it isn't in the context of this question.

It is not relevent in terms of *control* of the collector current. It is an effect *caused* by Vbe. Whatever base current exist is besides the point and is all in the wash. Hint:

Ie = Is.(exp(Vbe/Vt) - 1)

Where does base current current appear in this first order description?

If base current was relevant to *control* of the emitter/collector current, it would surly appear in the first order description.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

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Kevin Aylward

do

to break it down in a simple manner. Bi-polar requires a minimum voltage to over come the the cut off effects of the Be (Base-Emitter) just like a diode would do. this on the average around 0.6 and varies on different voltage and styles of bi-polar. once you reach the break over point current starts the flow in the Be, that is if you have the emitter connected to an end point to cause current to build other wise all you get is the voltage past through the Be. if you were to put a voltmeter on the E and Current meter (I) in series with C (collector), with no load on the E, you can see the measured voltage that is being applied to B-theBreakdownPoint of BE, this is the same effect as passing lets say 12.0 through a diode and resulting in 11.4 on the average. you will notice that very little to no current will show in the meter. as soon as you apply a load on the E, current will develop and this acts like a current bridge allowing the C (collector) to flow over it. the end results of current is the ratio between base current and Collector current which is many times referred to as Hfe. which means in short for example, 10 Ma Be, will cause 100 ma Ce if the Hfe is 10 keep in mind that Bi-polar are not linear devices, temp and current windows in the BE will effect the range. they make nice simple thermo devices to be used in a temp gauge :)

----- FET's are more like static bridges. the Drain+Source are like a field resistor that required a field of electrons to create a conductive path much like the tubes of yester years. the gate applies this field of voltage and the only current you may see is the initial charge of capacitance that exist in that gate section. once charged, the a mount of current is very low to maintain the set point. just think of charging a cap. that is why high freq FET's are tricky to design, must keep the Cap low while still trying to get the effect. FETS are good for Bi-switches, good linear range, has much less effects with ambient temps and very populer where Hi-Z is required to convert Very low voltage and current gerating devices to a use able bi-polar conversion. for example a Type J thermo couple where the generated current is so low that using Bi-polar is not very good but the FET is perfect. using a ceramic mic where capactance veraition is used. etc..

with out getting into to much biasing details etc, i think i may have explain it well enough..

Reply to
Jamie

The important detail you are missing in this description is that the base voltage must be with respect to the emitter. Not all uses of bipolar junction transistors hold the emitter at a fixed voltage, so you have to keep the emitter voltage in mind when you are thinking about whether a particular base voltage change will turn the emitter to collector current up or down. The base to emitter path is also a diode junction, so the applied voltage will also have to deal with forward biased diode current.

Yes. The gate corresponds to the BJT's base. But it is insulated either by a reverse biased junction (in junction fets) or by an insulating layer (usually silicon dioxide in Metal (gate) insulated by Oxide on Silicon fets otherwise known as mosfets ). Again, it is the gate to source voltage that controls the conductivity of the drain to source path. Even though the gate is insulated, it forms a plate of a capacitor, so if you want to turn a fet on or off very quickly, you may have to deal with a considerable capacitive current during the voltage swing. In general, fets take a larger gate to source voltage change (several to more than 10) to make the channel conductivity swing from non conducting to full conduction than BJTs do (less than 1 volt).

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John Popelish
Reply to
John Popelish

You are right. It is possible, with circuit modification, to go between enhancement devices.

Just to summarize:

For going from NPN to N-MOSFET (or PNP to P-MOSFET), you need a) more gate voltage to turn it on. This varies much more with mosfets than with NPNs. Given a particular circuit, it may not be possible to fully turn on a mosfet because of this. b) a way to pull the gate to ground to turn it off, since the NPN automatically turns off when the base no longer is getting current, whereas the n-channel mosfet may float.

For going from N-MOSFET to NPN (or P-MOSFET to PNP), you need a) A way to limit base current (which may be as simple as a resistor) b) Far less base voltage (which the resistor may take care of) c) Possibly much more current than the driving circuit can provide.

However, replacing a bipolar with a JFET or depletion mosfet is much more difficult.

Also, the characteristics of these devices is completely different. A BJT has a current gain which is exponential in voltage (or somewhat linear in current), whereas current through a FET has a quadratic relationship to gate voltage in the active region.

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   Robert Monsen
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Reply to
Robert Monsen

You are wrong here, the base voltage of a bipolar will be below 1V whereas the gate voltage of a Mosfet should be much higher, usually 10 to 15V. Logic level FETs need at least +4V gate drive. A bipolar drive needs to be current limited, not so a Mosfet. So usually you can replace a FET by a bipolar transistor if you put a resistor in series with the base (if you are driving low current apps like relays or LEDs).

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ciao Ban
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Reply to
Ban

True. The *really* confusing differences lie in the labels applied to the regions of operation of the various devices. The terms linear, saturation, ohmic, cut-off and so forth mean different things according to the device under discussion. Beginners beware!

Reply to
Miles Harris

Now the OP will be confused by another over-simplification. It depends on whether the FET is of the enhancement or depletion mode type. Your statement is correct for enhancement mode FETs, but wrong for depletion mode ones. Depletion mode FETs are 'normally on' and will conduct fully with *no* applied gate voltage. You have to apply a

*negative* voltage to the gate to moderate the drain current. Enough negative voltage will cut-off the drain current altogether. No doubt *you* know this, but it should be pointed out to the OP.
Reply to
Miles Harris

Yes, but that's misleading. It's essential to concentrate on the relationship between the applied voltage to the base/emitter junction and the resultant collector current. The BJT is a transconductance device and should be viewed as such.

Correct. The time it takes to perform this charge/discharge cycle dictates the maximum useable frequency of the FET.

Reply to
Miles Harris

Yet another oversimplification. The bias requirements are *totally* different and should not be studied by means of comparison with BJTs. As Kevin Aylward said, it's better to nip these misconceptions in the bud before they become entrenched views.

Reply to
Miles Harris

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It's only a transconductance device because of the voltage required to
force charge through the base-to-emitter diode, that charge changing
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Reply to
John Fields

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Have you paid any attention at all to the subject line?  It reads: 

"difference between bipolar and mosfet", not "How many different
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John Fields

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