AN957 Datasheet, PDF (4/13 Pages) List of Unclassifed Manufacturers – Measuring HEXFETCharacteristics

2. Set the MAX PEAK VOLTS to 15V.
Index
AN-957 (v.Int)
3. Set the SERIES RESISTOR to 0.3 ohms.
4. Set POLARITY to PNP. This causes the drain (collector) terminal
to be negative with respect to the source (emitter) terminal.
5. Set the MODE CONTROL to NORM.
6. Set the VERTICAL CURRENT/DIV to 50 microA/div.
7. Set the HORIZONTAL VOLTS/DIV to 500 mV/div.
8. Set the CONNECTION SELECTOR to “SHORT” in the
“EMITTER GROUNDED” sector.
9. DISPLAY should be inverted.
10. Connect the device using the LEFT/ RIGHT switch. Increase the
VARIABLE COLLECTOR VOLTAGE until the drain current
reaches 250 microA as indicated by the trace on the screen. Read
the voltage on the horizontal center line (since this line corresponds
to I_D = 250 mA) (see Figure 4).
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500 PER
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Figure 4. Gate-source threshold voltage
5. I _GSS
This is the gate-source leakage current with the drain connected to the
source. An excessive amount of gate leakage current indicates gate oxide
damage.
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1. The device is connected as follows: gate to “C”, drain to “B”, source
to “E”. This is not the usual connection sequence, and a special test
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DIV
fixture will be required if bending of the leads is to be avoided.
2. Set MAX PEAK VOLTS to 75V.
3. Set the SERIES RESISTOR to a low value (for example, 6.5 ohms).
4. Set POLARITY to NPN.
5. Set the MODE switch to LEAKAGE.
6. Set the CONNECTION SELECTOR to the “SHORT” position in the
“EMITTER GROUNDED” sector.
Figure 5. Gate-source leakage current at +20 V
7. HORIZONTAL VOLTS/DIV should be set at 5V/div.
8. VERTICAL CURRENT/DIV should be set to an appropriately low range.
9. Connect the device using the LEFT/RIGHT switch. Increase the collector supply voltage using the VARIABLE
COLLECTOR SUPPLY control, but do not exceed 20V, the maximum allowable gate voltage. It may be necessary to adjust
the vertical sensitivity. Read the leakage current from the display (see Figure 5). In many cases, the leakage current will be in
the nanoamp range, in which case the trace will be dominated by currents which flow through the device capacitance as a
result of minute fluctuations in the collector supply voltage.
10. The above procedure is for determining gate leakage current with a positive gate voltage. To make the same measurement
using a negative voltage, reduce the VARIABLE COLLECTOR SUPPLY voltage to zero, change the POLARITY switch to
the PNP position, and reapply the voltage (see Figure 6). The trace will take time to settle because of the gate-source
capacitance.
6. g_fs
This is the forward transconductance of the device at a specified value of
I_D. g_fs represents the signal gain (drain current divided by gate voltage) in
the linear region. This parameter should be measured with a small ac
superimposed on a gate bias and the curve tracer is not the appropriate
tool for this measurement. Even with specific test equipment, as
indicated in Section 11, the dc bias tends to overheat the MOSFET very
rapidly and care should be exercised to insure that the pulse is suitably
short.
To Order
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Figure 6. Gate-source leakage current at -20 V