+ 15v
‘S
b+IL -
I
I
1
J
PICOAMMETER
‘L = 15V = 16V =15nA
s;;ro4
2.6A. UNGUARDED CIRCUIT
+15v
)RL
I
I
1
I
I I
PICOAMMETER
2-66. GUARDED CIRCUIT
Figure 2-B. Guarding
CAUTION
When en external voltage source is con-
nected in series with the Model 220 output,
cere should be taken to limit the power
delivered to the Model 220. Refer to Figure
2-B for power limits.
Example 10 Operation as en Active Loed (Current
Sinkl-The Model 220 can be used as an active current sink
as shown in Figure 2-7. The output voltage V, is a function cf
E, I and R, where:
V,=E+IR,
E= External Voltage Source
I = Programmed Current on the Model 220
R,= Load Resistance
CAUTION
When the Model 220 is connected so es to
sink current (that is, power is delivered to
the Model 220 by en external power supplyl,
cere should be taken to limit the power
delivered to the Model 220. Figure 2-B shows
the power limits to the Model 220 used in
this oonfigumlon.
p$iiq~~~T
Figure 2-7. Fordeotions es 8 Current Sink with Resistive
For resistive loads ths Model 220 will deliver the programmed
current up to the compliance voltage iV,=VJ. The output
voltage V, must be within the power limits specified in Figure
2-B.
L
k- ,Oorn/\ .-__-_ ----!
Figure 2-B. Model 220 Recommended Operating Limits
2.5 APPLICATIONS
2.5.1 Calibration
Model 220, with its high accuracy, can be used as current
calibration source. The required current valuss and voltage
compliance limits can bs programmed into the memory loca-
tions. The three diismnt program modes allow the operator to
run through ths programmed values wether manually (step) or
automatically (single or continuous). In the single or con-
tinuous program mode different dwell times can be programm-
ed. This allows the operator to tailor the dwell times to his
needs. The step program modes may be the best way to run
through the calibration steps. In this mode the operator can
manually step through the memory locations and therefore,
output the current when it is required by the calibration pro-
cedure.
2.5.2 Resistivity Measurement
Certain semiconductor materials such as silicon have high
resistivities. The measurement of their reslstivity can be a dif-
ficult measurement. To aid in the measurement, special probes
of a hard metal such as tungsten are used. Because contact
resistance is so high, a four point probe is usually employed.
The outer two contact supply a constant current, the inner two
contacts measure the voltage drop across a portion of the
sample. Wti the geometry of the probe and wafer known,
resisdvity can then bs calculated.
The current source ussd must be stable and accurate. The
Model 220 is ideal for this application. The accurate and stable
current along with compliance voltage can bs easily progmm-
med before making the measurement.
The two voltmeters require a hiih impedance to ovemome
laad resistance problems. The Model 614 has the high input
impedance (greater than 5 x 1OpO in parallel with 2OpFl re-
2-9
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