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Fli;- HEWLETT
�� PACKARD
HP 4145B
Semiconductor Parameter
Analyzer
Technical Information
PREVENTING SMU OSCILLATION WHEN MAKING
DC PARAMETRIC MEASUREMENTS WITH THE HP 41458
1. INTRODUCTION.
The HP 41458 Semiconductor Parameter Analyzer features high speed, high resolution, and
high gain Source Monitor Units (SMUs) necessary for evaluating semiconductor processes
and devices. However, this combination of high gain and high speed creates OSCillation
problems under certain load conditions. The DC characterization of semiconductors is
adversely affected by these oscillations. Oscillation problems and the methods to prevent
and stop them are presented in this note.
2. CAUSES OF OSCILLATION
There are two modes of oscillation, one mode
caused by the SMU or the OUT, and the other
mode caused by connection leads.
A low
pass guard filter is included in the SMUs to
obtain stable operation when in the very low
current ranges, however, SMUs are high
speed, high-gain amplifiers, and will oscillate
under certain load conditions.
(1) SMU Induced Oscillation
Oscillations caused by the SMU occur
when an inductive load is connected to
the SMU's output. The SMU often has an
inductive load because the output im
pedance of the SMU is inductive during
V -mode operation and usually several
SMUs are connected together through
the OUT. For example, in Figure 1 the
SMUs connected to the MOSFET's gate
and drain are operating in the V-mode, so
these SMUs appear to be inductive, thus
making this configuration equivalent to a
Hartley oscillator.
Figure 2 shows a simplified drawing of an
SMU output circuit.
Cg is the capaci
tance between the center conductor and
the inner shield of the triaxial output
cable.
To prevent oscillation, the SMU
r---------------,
V Mode
C
V Mode
(Common)
Figure 1. Test Configuration
Buffer Amplifier
Rc I
CC
Figure 2. SMU Output Circuit
-
uses a network consisting of Cc and Rc
to compensate for the output inductive
reactance.
However, the value of Cc
cannot be increased enough to make the
output impedance capacitive because the
settling time would become too long.
Therefore, the SMU may oscillate if an
unusually large inductance is connected
to it. This could occur if the OUT is a
superbeta transistor (hFE > 3000) and the
SMU connected to the emitter is set to
one of the low current ranges.
(2)
Oscillation due to strays.
The test sample itself may also oscillate
due to the residual inductance and stray
capacitance of the connection cables,
test fixture or probe card.
Superbeta,
wide bandwidth bipolar transistors are
especially susceptible to OSCillation.
3. OSCILLATION COUNTERMEASURES
The type of devices which tend to cause oscil
lation are:
(1)
Superbeta (High hFE) bipolar transistors
(2)
High gm MOSFETs
(3)
Devices
which
have
large
stray
capacitance
(4)
Devices operated at very high frequency
(5)
Negative resistance devices
Some effective methods for eliminating oscilla
tions are:
For (1). (2). (3). and (4) above, position ferrite
beads as close as possible to the OUT to kill
high frequency oscillations, and to kill low
frequency oscillations, insert
a resistor as
shown in Figure 3. In some instances it may
be necessary to use more than one ferrite
bead.
Figure 4 shows two methods of stopping a
SMU from oscillating when measuring (5) of
negative resistance devices.
2
"Ferrite Beads or Resistor
( HP
PN 9170-0029)
Figure 3. Preventing Transistor Oscillation
G
R
Figure 4. Negative Resistance Measurements
Voltage-Controlled Negative Resistance Measurement.
Parallel conductance (G), effectively cancels the negative resistance of test sample V. If the
value of G is known, you can use the following equation to calculate the current through the
sample for any value of V. Use the User Function to display IV directly.
ly=I-GV
Current-Controlled Negative Resistance Measuremenl
Series resistance (R), effectively cancels the negative resistance of test sample Z. If the
value of R is known use the following equation to calculate the voltage across the sample
for any value of I using the following equation. Use the User Function to display VZ directly.
Vz=V-RI
Oscillation can also occur if you use cable other than that listed in the operation manual to
connect to the test fixture.
How To Use Ferrite Beads
1 . When making measurements using the 16058A test fixture, insert ferrite beads
on the connection leads of the Personality Board, or on the back side of the
Socket Board.
2. When making measurements using a manual prober, place ferrite beads at the
tip of the probe needles, but be sure to insulate the beads from the signal and
guard lines.
[]I] l.au·eoooa
PERSONALITY BOARO
o
�
0
CCl'lHECTIQN
SWITCH
1.7...;:&.=;;'�J
Ferrite Beads
( HP
PN 9170-0029)
3
