Sencore TF151 - Zero Bias Drain Current (IDSS) Testing; IDSS Impact and Example

Leakage in an FET is extremely low, and any indication of leakage on the TFISI can
be considered bad, indicating, a defective FET . The actual maximum leakage rating
can be found in the Sencore FET and Transistor Reference Book supplied with the
TFlSl.
If the Field Effect transistor is a dual gate type, the leakage of the second gate can
be easily checked by simply setting the Function switch to the LEAKAGE (lG2SS)
position and reading the meter on the leakage scale fOT leakage of the second gate.
The gate leakage of an FET is extremely small and may not appear to move the point-
er of the me ter at all. If the meter moves any amount on an FET, the FET can be
considered defective and should be replaced. To check to see if the meter has moved
you may disconnect the gate lead from the FETwhile watching the meter needle when
checking for leakage. Any movement of the meter when connecting or disconnecting
the gate lead indicates a leakage and a defective FET.
ZERO BIAS DRAIN CURRENT (lOSS)
Zero bias drain current or lOSS is a very important paramet er to the FET. This
test is used to match and "CUL L" FETs for various circuits and on the production
line. An FET may have a very wide range of lOSS and will have to be selected to
work in the various circuits designed by the different manufacturers . In a balanced
bridge circuit or differential amplifier, the two FETs must be matched for Gm and
lOSS for the circuit to be balanced and work properly. In a cascode amplifier circuit,
the upper FET should have a zero bias drain current of two to four times that of the
lower FET for best gain and stability of the circuit. Thus, the manufacturers will
select the FETs to match the circuit and will generally color code them. When or-
dering replacement FETs from the manufacturers. be sure to specify the color code
of the FET for best operation of the circuit.
With the TFlS1, the lOSS check will give you the actual zero bias drain current for
depl e tion type FET's. Enhancement type FET's are forward biased and lOSS is not
measured. However, the drain test current that is indicated when the lOSS button
is pushed is useful for matching, culling, etc.
The following example shows how the zero bias drain curr e nt can affect the circuit
and if the wrong value is chosen, the possibility of failure in a very short period of
time is great. The example is with the depletion type FET, but the readings ob-
tained with the enhancement mode FET can be used for similar calculations as well
as FET matching.
If a replacement type FET is put into a circuit, the zero bias drain current should
be checked with the TF1S1 and the power dissipated across the FET calcula ted, to be
sure that you do l]Ot exceed the maximum rating of the device. In the example of
figure 7, three FET' s with different lOSS are considered for replacement in the cir-
cuit. If the FET has a maximum dissipation of 100 milliwatts, all three could be
used. However, FET B would be running at its maximum dissipation and would have
a good chance of failure in a short period of time as compared to FET A. To com-
pute the dissipation, follow these simple steps:
+20V
FEr
FET
FET
A
B
C
MAX DISSIPATION
loss
SmA
lOrnA
7mA
OF FH = lOOmw
V
Rl
SV
IOV 7V
Vos
ISV IOV
13V
01551-
7Smw
IOOmw
9lmw
PATION
Figure 7. Field Effect Transistor Replacement In A Circuit
15