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On an n-type transistor, if a voltage above a certain threshold is applied to the gate, current can flow from source to drain.

On a p-type transistor, if a voltage below a certain threshold is applied to the gate, current can flow from source to drain.

But what about when the gate is disconnected? Every point on earth has some voltage -- 0V in a circuit diagram is merely a convenient reference rather than a point with zero voltage.

Thus, my disconnected gate much have some voltage -- it's just not readily known.

So is the answer that my disconnected gate might activate an n-type transistor, or it may activate a p-type transistor, but the answer is unknown and the behavior of the circuit is undefined?

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  • \$\begingroup\$ with the gate disconnected the gate pin responds to ambient electrostatic fields, commanding the transisor on or off (or partially on) as the fields change. \$\endgroup\$ Jul 24, 2016 at 1:19
  • \$\begingroup\$ I’ve seen a field-effect transister (lighting a LED) used to detect static. It would light if someone combed his hair 10 feet away! \$\endgroup\$
    – JDługosz
    Jul 24, 2016 at 9:42
  • \$\begingroup\$ Why would you want to look into this? We did this all of the time to make a noise source where we had a floating gate and no control input. \$\endgroup\$
    – b degnan
    Jul 24, 2016 at 14:49

3 Answers 3

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But what about when the gate is disconnected?

The disconneced gate pin acts as an antenna, and will pick up some electromagnetic noise from the environment - likely the 50 or 60 Hz from the nearest wall power lines.

The end result is largely random, and there are other effects like leakage currents to account for.

Thats why you want a pullup or pulldown resistor in cases where the gate of a MOSFet could otherwise be "open" - so your circuit is kept in a known state.

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    \$\begingroup\$ Is it possible to use this as a random value generator? \$\endgroup\$
    – Bradman175
    Jul 24, 2016 at 3:46
  • \$\begingroup\$ @Bradman175 A 50 Hz signal would be anything but random. However, in practice, it's likely that the gate would get stuck in one state. \$\endgroup\$
    – CL.
    Jul 24, 2016 at 7:17
  • \$\begingroup\$ @CL. Well I plan to make a question about how to create a true random number generator. \$\endgroup\$
    – Bradman175
    Jul 24, 2016 at 8:17
  • \$\begingroup\$ @Bradman175 random.org purports to use atmospheric noise to generate truly random numbers. \$\endgroup\$ Jul 24, 2016 at 16:49
  • \$\begingroup\$ @user1274193 Well I want numbers that I can generate in the matter of 0.05 seconds. And I don't want others to be able to guess it. If I get it from that website, I have to hope that the Internet connection is fast enough and that no one is also using that website, which is extremely unlikely. \$\endgroup\$
    – Bradman175
    Jul 25, 2016 at 2:46
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So is the answer that my disconnected gate might activate an n-type transistor, or it may activate a p-type transistor, but the answer is unknown and the behavior of the circuit is undefined?

Basically, yes. These (MOSFET) transistors may also be partly switched on (into their linear region).

This is the underlying cause of the problems which occur, when people leave inputs on CMOS ICs unconnected. These high-impedance inputs are influenced by electric fields in the area (e.g. people, charged objects etc.) in unpredictable ways. If the electric field voltage is high enough, and close enough to the transistor, then a discharge event could occur i.e. an ESD event, causing hidden partial (or immediately fatal) damage to the transistor - those gate oxides are thin!

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It dpeneds on the type of transistor.

For mosfets you are basically right the gate is essentially a small capacitor which is well-insulated from the other terminals of the transistor and can hold a charge. Small ammounts of charge can comple to this gate through stray capacitance leading to unpredictable behaviour.

BJTs will turn off when their base is disconnected.

Not sure what happens with jfets, i've never really worked with them.

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