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Why are JFET/FET called voltage controlled devices while BJTs are called current controlled devices. Both require a voltage to operate properly. In both an electric field is produced so what is the difference?

Now the question is that current is also generated due to voltage and still BJT is current controlled and FET voltage controlled.

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Because the base-emitter voltage of a BJT in its operating region will be affected by base-emitter current, and vice versa, changes to the base-emitter voltage of a given transistor will affect the collector-emitter current. On the other hand, the amount of base-emitter voltage change required to affect a given collector-emitter current change is often huge and unpredictable; it will vary enormously with temperature, aging, the phase of the moon, etc. By contrast, within a transistor's "linear" operating region, doubling the base-emitter current will roughly double the collector-emitter current. Not absolutely-precisely double it, but pretty close. Such behavior is far more predictable than the relationship between base-emitter voltage and base-collector current.

A FET or MOSFET by contrast doesn't have any gate current except for currents resulting from leakage or stray capacitance. Those currents aren't exactly zero, but manufacturers generally try to minimize them. As such, it's not really possible to characterize the transistor's response to different levels of gate current. The relationship between the gate-source voltage and the drain-source current isn't nearly as predictable as the relationship between base-emitter current and collector-emitter current in a BJT, but it's a still apt to be the most predictable way to characterize the device's operation (it's a lot more predictable and consistent than the comparable relationship on a BJT).

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  • \$\begingroup\$ current is also generated due to Voltage and still BJT is current controlled and FET voltage controlled. \$\endgroup\$
    – Ali Khan
    Mar 28 '13 at 12:47
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The statement is wrong. Both devices are voltage-controlled. The accurate model for a bipolar transistor is the Ebers-Moll equation.

Note how the independent variable in the Ebers-Moll equation is: \$V_{BE}\$. Not base current!

A BJT is a transconductance device, just like a JFET.

The idea that a BJT amplifies current is a feature of the approximate model which can be used to design the majority of simple BJT circuits.

A particular simplification/idealization of the BJT is a current device, not the real BJT. It just does not work that way. There simply isn't a little demon in the base which counts electrons moving through the base, and dispenses \$\beta\$ times that many electrons into the collector.

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    \$\begingroup\$ But in most text books it is stated that Bjt is current contolled device and Fet is voltage controlled device, why? How can we figure out it is not the correct statement? \$\endgroup\$
    – Ali Khan
    Mar 29 '13 at 7:24
  • \$\begingroup\$ can you elaborate on the above comment of mine? \$\endgroup\$
    – Ali Khan
    Mar 29 '13 at 12:18
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    \$\begingroup\$ How you can figure it out is by getting better textbooks. Or maybe by reading through to the more advanced chapters where they introduce the more accurate model of the BJT. Take a look at The Art of Electronics by Horowitz and Hill, for instance. \$\endgroup\$
    – Kaz
    Mar 29 '13 at 16:32
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    \$\begingroup\$ For a fixed base voltage the collector current will change a lot with the change in base resistance (Assume a common emitter fixed biased configuration ) . But in case of FET, for fixed gate voltage the drain current won't change with the change in gate input resistance. How do you explain it? @kaz \$\endgroup\$
    – Sadat Rafi
    May 28 '20 at 11:31
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In BJTs, the emitter current is proportional of the base current (this is the hFE value of the BJT), while in FETs the source/drain current is proportional to the gate voltage. What this means in practice, is that if you have a voltage source can source very little current (such as your finger, after you've dragged your feet across a carpeted room to build up some static electricity), you can use this signal to control the FET, but not a BJT.

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    \$\begingroup\$ I wouldn't say that a FET's current is "proportional" to the gate voltage, but it the pinch-off current on a FET is a somewhat consistent function of gate voltage. Are there are some FETs that where current is really proportional to voltage? I've not heard of such things. \$\endgroup\$
    – supercat
    Mar 28 '13 at 14:23
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    \$\begingroup\$ WHat I meant by "proportional" is "monotonically increasing w.r.t. to gate voltage." You are correct in pointing out that proportional is not the best term I could have used. \$\endgroup\$ Mar 28 '13 at 18:13
  • \$\begingroup\$ I wish there was a nice short term to express the general concept of a monotonic increasing relationship. With a BJT, the maximum collector-emitter current is pretty close to being a quasi-constant multiple of the base-emitter current, so with a BJT the strict meaning of the term "proportional" would apply. It's worth mentioning, though, that depending upon the type of FET, current may be a decreasing function of voltage even when the voltage is positive. \$\endgroup\$
    – supercat
    Mar 28 '13 at 20:56
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In very simple words and no maths involved:

FETs in general, source-drain current is controlled with a very little current from the gate.

BJTs require more current, because of how it works, collector-emitter current is proportional to the current from the base.

Both use tension and current, but the impedance of FET are so high, and the current that they need to control is so low, that you only need a tension over the gate to do the control. Thats why FETs are considered tension controlled devices.

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