difference between bipolar and mosfet

[Skeleton Man]

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

 

[Robert Monsen]

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

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.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[Kevin Aylward]

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,

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.

and, sadly, you just have to
get used to it if you want to talk about these things.

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.

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.

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
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[John Fields]

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.

---
The application of a forward voltage to the base-emitter junction of a
bipolar transistor will, of course, cause charge to flow between the
collector and emitter, but the movement of charges across the
base-emitter junction _is_ relevant, since without that movement there
can be no collector current.

In a MOSFET, however, the only movement of charge required to control
the drain-source current is that required to charge and discharge the
gate capacitance.

 

[Robert Monsen]

Why?

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.

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.

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.

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.

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.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[Skeleton Man]

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

 

[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 ?

 

[Robert Monsen]

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

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.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[Kevin Aylward]

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.

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.

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.

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

This is also true of design using beta. Lighten up.

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
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

---
The application of a forward voltage to the base-emitter junction of a
bipolar transistor will, of course, cause charge to flow between the
collector and emitter, but the movement of charges across the
base-emitter junction _is_ relevant,

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

since without that movement there
can be no collector current.

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
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Ban]

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.

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).

 

[Miles Harris]

Why?

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

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!

 

[Miles Harris]

The application of a forward voltage to the base-emitter junction of a
bipolar transistor will, of course, cause charge to flow between the
collector and emitter, but the movement of charges across the
base-emitter junction _is_ relevant, since without that movement there
can be no collector current.

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.

In a MOSFET, however, the only movement of charge required to control
the drain-source current is that required to charge and discharge the
gate capacitance.

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

 

[Miles Harris]

---
Yes, but it still requires current to charge the gate capacitance.
However, once that capacitor is charged up, current can flow through
the drain-to-source channel with no further current required into the
gate.

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.

 

[Miles Harris]

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.

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.

 

[Kevin Aylward]

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.

Well, its a bit more subtle

Depletion and enhancement both work exactly the same way, which is what
you actually said, but not so obviously. Increasing the gate voltage
will increase the current in both type of devices. The essential
difference is that at OV an enhancement device is off, where as a
depletion device needs a negative voltage to get it off. That is, its
*only* Vto that is different.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[John Popelish]

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 ?

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.

...and a fet will do
a simmilar thing only doesn't require a current ? (at whichever terminal
corresponds to a base on a bipolar)

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).

 

[Jamie]

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

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..

 

[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).

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.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[John Fields]

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.

---
It's only a transconductance device because of the voltage required to
force charge through the base-to-emitter diode, that charge changing
the electrical properties of the base material to more closely
approximate those of the collector and emitter. That is, when charge
is injected into the base-to-emitter diode of a PNP transistor, the
"N" type base material becomes more and more "P" like as more and more
current is forced through it, with the result that the transistor
starts looking more and more like a single piece of low-resistance "P"
type material as more and more current flows through the
base-to-emitter junction. That being the case, collector current will
flow when base current does, and will increase with increasing base
current until the transistor goes into saturation. Of course it's the
base-to-emitter voltage which makes the whole thing happen, but what
_I_ think is misleading is to burden an inquirer with too much detail
too soon. Hence, initially describing the BJT in terms of beta and
leaving out the transconductance part alleviates the confusion which
will inevitably arise if the BJT and the FET are both described in
terms of transconductance. After all, the question wasn't "How are
the BJT and the FET alike?" it was "How are they different?".
---