# MOS Capacitor

Discussion in 'Electronic Basics' started by Confused Soul, Sep 30, 2004.

1. ### Confused SoulGuest

Hello Everyone,

I wanted to know is there any way we can find the Threshold Voltage
(Vt) and Flatband Voltage (Vfb) of a MOS capacitor from the CV
characteristics of the MOS capacitor?

Thanks
ConfusedSoul

2. ### MantraGuest

Vt only has meaning for a MOS*FET* but not a MOS*CAP*. It's a bit
frightening if you don't already know that.

You can measure Vfb using quasi-static (LF) CV and high-frequency CV
plots. Strictly there are more variables than unknowns even then
(flat band is a bit of a idealized theoretical concept rather than a
real measureable phenomena) but you can get a usable value for
engineering purposes. Again this is basic MOS theory 101 - first week
of class, no less.

Agilent, nee Hewlett-Packard, (e.g. 4140B/42XX) and Keithley make the
appropriate instruments for this.

MM

3. ### GuestGuest

:> Hello Everyone,
:>
:> I wanted to know is there any way we can find the Threshold Voltage
:> (Vt) and Flatband Voltage (Vfb) of a MOS capacitor from the CV
:> characteristics of the MOS capacitor?
:>
:> Thanks
:> ConfusedSoul

: Vt only has meaning for a MOS*FET* but not a MOS*CAP*. It's a bit
: frightening if you don't already know that.

Absolutely not true.

The capaciatance of a MOS CAP (MOSFET with its source and drain tied
together) when the voltage across is less than Vt (i.e. no inversion channel)
is very small (= Cgdo + Cgso where Cgdo and Ggso are small
overlap capacitances.)

When the voltage across the MOS CAP is larger than Vt, the channel
is inverted, the device is in the linear region, and the Capacitance is
approximately Cox * The Area of the Device (i.e. Much larger than Cgdo +
Cgso.) If you understand this, you should be able to answer your own
question now.