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I have a doubt. Consider an N-MOSFET: which is the voltage that can switch on it? The voltage between Gate and? Sometimes I read "between Gate and Bulk", sometimes "between Gate and Source", sometimes the ambiguous sentence "Gate voltage".

Then I have another question, related to the previous one. Consider the following pass transistor circuit:

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How can the signal at the Gate activate the MOSFET? It must be high with respect to what point (moreover in this circuit Drain and Source are not fixed points)?

Kinka-Byo
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1 Answers1

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A typical N-Channel MOSFET behaves as if a capacitor exists between the gate and source terminals. Usually, when referring to "gate voltage", one is usually talking about the gate-to-source voltage in the context of 3-pin MOSFETs. This "capacitor", when charged, turns on the transistor.

Your confusion lies probably in the fact that MOSFETs are actually symmetrical devices, meaning that source and drain can be interchanged. Discrete transistors have one of their terminals shorted to the bulk in order to deal with the many problems that can arise otherwise, but the same isn't quite true for MOSFETs on integrated circuits.

As I understand it the bulk is usually connected to ground (not shown in the circuit), hence a logic-high signal in the gate is enough to turn it on.

Chi
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  • Regarding your last statement: so is it the voltage between Gate and Bulk that activates the Mosfet? – Kinka-Byo Jun 09 '19 at 16:27
  • I believe it is so. In a n-enhancement mosfet, the bulk is a P-type substrate, and in order to create a channel between the N-type source and drain, a positive voltage between the gate and bulk must be applied in order to get a positive charge at the gate. This positive charge "repels the holes", effectively creating a "N channel" between the source and drain. As far as I understand it, this mechanism doesn't really require any specific voltages between gate and source in order to work. People just talk like that since that's the case for 3-pin mosfets, where the source is shorted to the bulk. – Chi Jun 09 '19 at 18:31
  • Your reasoning seems correct, from this point of view I understand it. Moreover, in Solid State books it is shown that in a MOS structure (so not necessary including Source and Drain) the channel is created by the voltage between Gate and Bulk, so it seems ok. But now I have another doubt. If you consider the passtransistor circuit, it is known that the voltage on the load will be VDD - Vt (if the input at left is VDD). The explanation that is usually given is that when the source reaches that values, Vgs becomes – Kinka-Byo Jun 09 '19 at 19:02