What is the meaning and difference between the symbols: \$V_{TN}\$ and \$V_T\$ commonly seen when talking about a MOSFET?

I'm guessing \$V_{TN}\$ is the threshold voltage, but what is \$V_T\$, and how do they relate to each other?

I'll give examples of equations where these symbols have been used in my course, and the context they were used in.

e.g.1) When looking at the transfer function of MOSFETS, the NMOSFET is said to be in the saturation region if : $$V_{DS} \ge V_{GS}-V_{TN}$$

e.g.2) When looking at the transistor as a switch, a PMOSFET is an open switch (off) if: $$|V_{GS}|>|V_{DD}-|V_{T}||$$ Assuming source \$V_S=V_{DD}\$

What is the difference between \$V_{TN}\$ and \$V_T\$, and why is that last part an assumption? Is it not a given that \$V_S=V_{DD}\$?

  • \$\begingroup\$ I think that \$V_{TN}\$ is the threshold voltage for N-MOS, while \$V_T\$ refers to a P-MOS. That would be strange, I'd call it \$V_{TP}\$. Also I believe your second equation is a bit mixed up, are you sure it's ok? \$\endgroup\$ Apr 30, 2014 at 13:05
  • \$\begingroup\$ Thanks, yeah that could be it, I didn't realise the \$N\$ in \$V_{TN}\$ meant nmos. So I guess that makes sense. And the second equation is quoted exactly as it is in the lecture slides. It confused me aswell though, it's the reason I asked this question. \$\endgroup\$
    – JessEE
    Apr 30, 2014 at 13:49
  • \$\begingroup\$ There is also an equation that says for a transistor modeled as a closed gate (on), it can be treated as a resistance and \$|V_{GS}|<|V_{DD}-|V_{T}||\$. \$\endgroup\$
    – JessEE
    Apr 30, 2014 at 13:52
  • \$\begingroup\$ Vs does not have to be the same as Vdd. Think about a CMOS NOR gate where one PMOS is above another PMOS. Another application would be a PMOS Wilson current mirror. Your main question, I'd have to dig open my books this evening if someone doesn't come up with an answer sooner. I remember having the same confusion when I took my electronics class. \$\endgroup\$
    – horta
    Apr 30, 2014 at 13:54
  • \$\begingroup\$ @horta ah yeah thanks, I didn't think about it like that :) Also I think Vladimir is right, and \$V_T\$ is the threshold voltage for a pmos. This makes the second equation I gave seem strange though... \$\endgroup\$
    – JessEE
    Apr 30, 2014 at 19:24

1 Answer 1


Sir, in case of NMOS, we take threshold voltage Vtn ( it is positive) and in case of PMOS, we take threshold voltage Vtp (it is negative). Transistor is said to be OFF if: NMOS OFF: VgsVtp In normal case, we always take NMOS as reference (its opposite is PMOS), so we can take Vtn as Vt. During calculations in linear and saturation region for PMOS, we can take |Vtp|.

These equations are in Book: CMOS DIGITAL INTEGRATED CIRCUITS (Analysis and Design) by *SUNG-MO (STEVE) KANG University of Illinois at Urbana- Champaign *YUSUF LEBLEBIGI Worcester Polytechnic Institute Swiss Federal Institute of Technology-Lausanne enter image description here

  • \$\begingroup\$ a very complicated way to write the same exact equations, apart \$\mu_{p/n}\$. \$\endgroup\$
    – not2qubit
    Feb 26, 2019 at 12:19

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