A few questions regarding Mosfets

Thread Starter

Alexandros81

Joined Feb 14, 2017
15
Hi.
I was reading the Microelectronic Circuits be Sedra/Smith and I have some questions on Mosfets.
I will ask the questions one at a time.

On
4.1.3 Creating a Channel for Current Flow
"Consider next the situation depicted in Fig. 4.2. Here we have grounded the source and the drain and applied a positive voltage
to the gate. Since the source is grounded. the gate voltage appears in effect between gate and source and thus is denoted v_GS"

Mosfet Tutorial 5.jpg

1st question: Does gate voltage also appears between gate and drain?
 
Last edited:

Papabravo

Joined Feb 24, 2006
16,832
Yes. Since the potential difference between the source and the drain is zero, we know that even though the rds(on) of the n-channel is very small no current can flow.
 

crutschow

Joined Mar 14, 2008
27,756
Does gate voltage appears between gate and drain also?
By definition, if there's a voltage between the gate and source, then there has to be a voltage between gate and drain also.
Since MOSFETs are symmetrical, which terminal acts as the source is determined by the relative voltages on the 3 terminals, so a MOSFET can conduct equally well in either direction when biased on.
But for typical 3-terminal MOSFETs, the source terminal is the one that is connected to the substrate diode (terminal B) so that it is normally reverse-biased.
 
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