Next: 4 Description of the Simulator
Up: 3 Measurement and Parameter Extraction
Previous: 3.2 Small Signal Equivalent Circuit
Parameter Extraction
3.3 Determination of Capacitances
by Quasi Static Approximation
The capacitances shown in Figure
3.1 are considered to be frequency independent. Thus, they can be determined
by a quasi static approximation. The voltages on the device are defined
in Figure
3.3.
Figure 3.3 Definition of voltages and capacitances
of a FET.
The charge on the gate is a function of two voltages, therefore the
capacitance can be written in the general form
. 
(13)

If the transfer characteristics of a FET is considered V_{DS}
is kept constant and the first part in (13)
is zero. In this case V_{GS} and therefore also V_{GD}
is changed. This way the total gate capacitance C_{G} = (C_{GS}+C_{GD})
is determined by
. 
(14)

Considering the output characteristics of a FET V_{DS}
is changed and V_{GS} is kept constant, therefore the second
part in (13) is zero and
only C_{GD} is given by
. 
(15)

Thus, C_{GS} is determined by C_{GS} = C_{G}
 C_{GD}.
Next: 4 Description of the Simulator
Up: 3 Measurement and Parameter Extraction
Previous: 3.2 Small Signal Equivalent Circuit
Parameter Extraction
Helmut Brech
19980311