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I know this might be a stupid question, but what is the difference between MOSFET transistor and normal transistor? There must be a difference but no one says it, people always talk about them as they are the same thing, but there shapes is different, even in circuits' schemes they are represented with different symbols, so there must be a difference.

Image of the transistors shapes Image of the transistors symbols

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closed as too broad by Elliot Alderson, Bimpelrekkie, Leon Heller, RoyC, Dwayne Reid Jan 20 at 5:53

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ What do you mean by normal transistor? bipolar junction transistor? \$\endgroup\$ – nidhin Jan 19 at 11:26
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    \$\begingroup\$ And what do you mean "their shapes are different"? \$\endgroup\$ – marcelm Jan 19 at 11:28
  • \$\begingroup\$ I added pictures for the symbols and the components, left one is "normal", and right is MOSFET \$\endgroup\$ – Azzam Alsharafi Jan 19 at 11:32
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    \$\begingroup\$ The left package in your first picture is TO-92 and the right one is TO-220. These are standard packages, and both are used for many different components, including MOSFETs, BJTs (your "normal transistors" I think), TRIACs, voltage regulators. Conversely, there are many more packages that both MOSFETs and BJTs may come in; common ones include DPAK, D2PAK, SOT23, SOT223, and TO-247. Moral of the story: the shape doesn't tell you anything, really. \$\endgroup\$ – marcelm Jan 19 at 12:15
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    \$\begingroup\$ I have here in my drawers components that look identical yet none of them are transistors. The actual device is inside, what you see is the package that is needed to make it usable and protect the transistor. You cannot and should not draw conclusions on the looks of an electronic device. \$\endgroup\$ – Bimpelrekkie Jan 19 at 12:29
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[From Question]: MOSFET transistor and normal transistor

A MOSFET is a normal transistor - it's a Metal-Oxide-Semiconductor Field-Effect Transistor

A BJT is another normal transistor - a Bipolar Junction Transistor

The question itself doesn't make much sense in that regard. But lets assume for now you mean BJT when you say "normal".

[From Question]: but there shapes is different

Are they? Transistors come in packages of many shapes and sizes

[From Comment]: left one is "normal", and right is MOSFET

The picture shows two different packages, however this is no indication of type.

Equally the left one could be a MOSFET (e.g. BS170 comes in a TO92 package). The right one could be a BJT (e.g. TIP41 comes in TO220 package).

[From Comment]: pictures for the symbols

This is where things start to differ.

In your picture, the symbols are misleadingly labelled.

  • The top symbol, labelled "transistor" is the symbol for a BJT. This is indeed a "transistor".
  • The bottom symbol is indeed a MOSFET. However this is also a "transistor".

There are also more than two types - e.g. JFET (Junction FET), MESFET (Metal-Semiconductor FET).


The reason for the difference in symbol and behaviour between a BJT and MOSFET is due to the way that the two types of transistor work differently, and exhibit different behaviour.

In simplest terms, we can say:

  • A BJT transistor is a current controlled device. That is to say the current flowing into the base of the transistor controls the current flowing into the collector.

  • A MOSFET is a voltage controlled device. The voltage you apply across the gate controls how much current flows into the drain.

In other words their behaviour is different. Both are transistors, but are used in different ways. I won't go into any more details in this answer, there are plenty of resources into the behaviour of MOSFETs vs BJTs.

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  • \$\begingroup\$ Thank you for the information, I think this is all what I was looking for \$\endgroup\$ – Azzam Alsharafi Jan 19 at 12:53
  • \$\begingroup\$ The pictures is from a book \$\endgroup\$ – Azzam Alsharafi Jan 19 at 12:55
  • \$\begingroup\$ How does the MOSFET works? I mean what is the relation between the voltage applied to the gate and the current that flows into the drain? \$\endgroup\$ – Azzam Alsharafi Jan 19 at 12:58
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    \$\begingroup\$ I upvoted, but I think you're a bit harsh: In pre-history when I was taught electronics, transistors were nearly all bjts, and you'd expect someone to mean a BJT when they said "transistor". Mosfets and jfets were considered rare and specialized beasts, so I can believe there are parts of the world where that mentality still holds. \$\endgroup\$ – james Jan 19 at 17:17
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    \$\begingroup\$ @AzzamAlsharafi - "How does the MOSFET works? I mean what is the relation between the voltage applied to the gate and the current that flows into the drain?" What did the Wikipedia article say? If you did not do any work to try to find out on your own, why not? \$\endgroup\$ – WhatRoughBeast Jan 20 at 0:37
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Bipolars operate by injecting small currents (electrons) into the base pin, and the emitter injects currents that attempt to cancel out the (opposite polarity) base currents. Because the BASE region is very thin, most of the emitter-injected charges miss their targets and end up gathered up in the Collector region of that transistor. The ratio of miss-to-hit is very important, having the special name BETA and is given ranges of values (often 50 to 300) on datasheets.

Bipolars have the very useful property of needing 0.058 volts more voltage across the base-emitter, for every 10:1 increase in current thru the transistor. This predictability can make the bipolar very easy to design complex amplifiers or oscillators, using almost any bipolar. For example, the "BARS" signal strength of your phone is likely to exploit this 0.058v per 10:1, which is a logarithmic behavior, for indication on your phone display, and to provide a value that is transmitted to the CellSite for use in managing the antennas' beam forming and the Transmitter power allocated to YOUR phone.

On the other hand, FETS need no input current to be controlled; FETs use electric fields just like the electric fields that make your hair stand on end in the winter; the charged particles on the controlling element (named the GATE) end up being opposed in the path (the channel) between Source and Drain; that opposing charge ends up being very favorably used inside the channel, used to complete a path between Source and Drain, and to vary the resistance of that path and/or the almost-constant-current allowed thru..

FETS have the very useful property of not needing input current flow, thus are very useful to handle sensors that cannot provide much output current (we call these HIGH IMPEDANCE sensors).

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    \$\begingroup\$ That 58mV metric is temperature-dependent, isn't it? I guess that's beyond the scope of this overview, though. \$\endgroup\$ – Hearth Jan 19 at 17:22
  • \$\begingroup\$ And the FETs have a similar need for that 0.058 volts (temperature dependent, yes) parameter, in the subthreshold region. Companies such as Medtronic design their pacemakers using FETs operating in the very low current regions of subthreshold. \$\endgroup\$ – analogsystemsrf Jan 20 at 12:16

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