acq_ex11 : MOSFET Capacitance Measurement

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acq_ex11 : MOSFET Capacitance Measurement

Requires: Utmost IV, SmartSpice, SmartView

Minimum Versions: Utmost IV 1.11.0.R, SmartSpice 4.10.6.R, SmartView 2.28.2.R

This example describes how to perform MOSFET capacitance measurements.

The project file
acq_ex11.prj
for this example should be loaded into your database. When opened, the
project will look as shown in
acq_ex11_01.png
. In the project hardware, the DC Analyzer and LCR Analyzer are enabled for measurement as
shown in
acq_ex11_02.png
.

This project includes four capacitance measurement setups between Gate to other nodes
in a four terminal MOSFET device and are defined as

cgg Total gate capacitance
cgb Gate to body capacitance
cgd Gate to drain capacitance
cgc Gate to drain/source capacitance

There are two important connections when measuring capacitance, the high terminal
(Hi) where the ac bias is applied and the low terminal (Lo) where the measurement of the
ac impedance and thus the capacitance happens. When measuring capacitance it is
important to connect the device correctly.

To measure the total gate capacitance (cgg), the low terminal of the measurement
is connected to the drain, source and body nodes of the mosfet as shown in
acq_ex11_03.png
. As this is effectively a two terminal device measurement, this measurement can be
made using the LCR instrument alone. The zero volts on the drain/source/body can be
supplied by the LCR instrument itself. In this case, we set the connection for this
terminal to ‘manual ground’ as shown in
acq_ex11_04.png
to indicate that the bias is already taken care of.

For the remaining measurements, the connection for the low terminal should be set
to ‘manual ground’ and it is very important that the other terminals are connected
to a DC source. You should not allow the other terminals to ‘float’ as the measured
capacitance contributions will be incorrect.

Before measuring any capacitance, you should calibrate or zero the LCR instrument to remove
the effect of the test instrument and test setup from the measured result. If you have a dedicated
open calibration structure, this should be used when calibrating the LCR instrument.
To calibrate the LCR instrument, select the capacitance measurement setup in the measurement
sequence and then select
Measure->Calibrate
from the project menu.
Once calibration is done, capacitance measurement will be performed by selecting
Measure->Run
from the project menu.

For this example, we can now switch the project into its simulation mode by selecting
Measure->Simulation
from the project menu. This will allow us to view the typical characteristics which will
be generated when the measurement is performed.

When the measurement sequence is run, the measured data will be automatically stored in
the database and the measured results will be shown in the dataset selector area of the
acquisition project as shown in
acq_ex11_05.png
.

Select all of the data which has been measured and display in the viewer by selecting
Dataset->Plot
from the project menu. This will display all four measured characteristics as shown in
acq_ex11_06.png
.

In the viewer, the charts can be merged and displayed together in one single plot
as shown in
acq_ex11_07.png
. Here we have added labels and changed line colours for clarity.

There are three components to the mosfet gate capacitance, which together make up the
total gate capacitance. The gate to source capacitance, the gate to drain capacitance
and the gate to body capacitance. This can be represented by the following equations.

cgg = cgs + cgd + cgb
cgc = cgs + cgd
cgg = cgc + cgb

When performing mosfet capacitance measurements you should make sure that the above holds
true for your measurements.

The cgc measurement is often used to increase the measurement value when performing mosfet
overlap capacitance measurements for symmetrical mosfet transistors.