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difference between bipolar and mosfet

Discussion in 'Electronic Basics' started by Skeleton Man, Jan 8, 2005.

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  1. Skeleton Man

    Skeleton Man Guest

    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 ?

  2. 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.

    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.
  3. 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
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  4. John Fields

    John Fields Guest

    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.
  5. 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.

    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.
  6. Skeleton Man

    Skeleton Man Guest

    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)

  7. John Fields

    John Fields Guest

  8. 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.

    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.
  9. 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
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  10. 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
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  11. Ban

    Ban Guest

    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).
  12. Miles Harris

    Miles Harris Guest

    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!
  13. Miles Harris

    Miles Harris Guest

    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.
  14. Miles Harris

    Miles Harris Guest

    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.
  15. Miles Harris

    Miles Harris Guest

    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.
  16. 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
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  17. 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
  18. Jamie

    Jamie Guest

    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 :)

    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.

    with out getting into to much biasing details etc, i think i may have
    explain it well enough..
  19. 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.

    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.
  20. John Fields

    John Fields Guest

    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?".
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