Test and Measurement
The physics of RTS noise
RTS noise in MOSFETs can be explained as a threshold
voltage shift caused by the random capture and emission of
thermally excited electrons at a trap existing in the boundary
between the gate dielectric and the substrate (Figure 2).
The Vth shift caused by a single electron captured at the
trap is approximated by the following equation .
Here, q is electron charge, L is gate length, W is gate width
and Cox is gate capacitance. This equation clearly shows that Figure 1
the Vth shift becomes larger as the device shrinks. Since the
Issue III 2009
time constants for the capture or emission of electrons from
square4
traps can vary from microseconds to seconds, some of the
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pixel defects caused by RTS noise in the amplifier may not be
perceptible by the human eye. The B1500A’s WGFMU module
has a low voltage noise floor of less than 0.1 mV (rms), and its
current measurement capability supports sampling rates from
1 S/sec to 200 MS/sec and a bandwidth extending from dc to
16 MHz. These features, combined with a deep measurement
memory capable of storing up to 4 million points per channel,
oasiasemiconductor
enable the B1500A’s WGFMU module to measure RTS noise
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over a frequency range that extends from less than 1 Hz to
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many Mega-Hz.
The B1500A’s WGFMU solution consists of the WGFMU
27
module as well as two remote-sense and switch units (RSUs). Figure 2
The WGFMU module contained in the mainframe generates
the arbitrarily waveforms, and these waveforms are then
transmitted through a cable to the RSU. The RSU, which
performs the actual current or voltage measurement, is
separate from the WGFMU module so that it can be placed
near the device under test (DUT) to minimize cable lengths
and guarantee accurate high-speed measurement. Since each
WGFMU module supports two RSUs, RTS noise on a MOSFET
can be measured with a single WGFMU module by
connecting one RSU to the gate and one RSU to the drain. In
this case, the substrate (or bulk) and source terminals should
be connected to the common (ground) level of the outer
shield of the coaxial cable (Figure 3). Up to five WGFMU
modules can be installed in a single B1500A, for a total of ten
channels maximum. Figure 3
Sample software to measure and analyze RTS noise are
bundled with the B1500A’s WGFMU module (Figure 4). Using
this sample software, users can start RTS noise evaluation
immediately using the WGFMU module.
Figure 5 shows a simplified circuit diagram of the
WGFMU and RSU. The WGFMU has arbitrary linear waveform
generator (ALWG) voltage generation capability, with the
waveform generated by the ALWG output through the RSU.
The RSU is where the actual current or voltage measurement
is made. The WGFMU has two operation modes: PG mode
and Fast IV mode. The PG mode combines a very fast voltage
measurement capability with 50 Ohm output impedance to
minimize waveform reflections. The Fast IV mode has a
slightly slower measurement speed and slower waveform Figure 4
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