charge from one capacitor plate to the opposite plate. We
read the state of the capacitor by monitoring the current
flow that erupts from the capacitor when a reference
voltage is applied to the capacitor. The stored datum
corresponds to the direction of that internal polarization.
The plot in Figure 1 shows two single-cycle hysteresis
loops in terms of absolute charge for a ferroelectric
capacitor starting from opposite dipole orientations. The
horizontal axis is the voltage applied across the
ferroelectric capacitor. The vertical axis is the count of the
charges that come from the capacitor during actuation. A
linear capacitor is also plotted for reference.
How much charge comes out of the capacitor when a
positive voltage is applied tells us in which direction the
ferroelectric capacitor pointed prior to the read operation.
A linear capacitor in series with a ferroelectric capacitor
can sense and report the amount of charge moving in
Figure 1. We attach this
capacitor stack to two I/O
pins of a microprocessor to
create a single bit of non-volatile memory.
The linear capacitor
must be sized so if the
ferroelectric capacitor does
not switch during a read
operation, the small amount
of charge that comes out of
the ferroelectric capacitor
into the linear capacitor
cannot generate enough
voltage on the common I/O
pin to be read as a 1. If the
ferroelectric capacitor does
switch, so much more
charge goes into the linear
capacitor that its voltage on the I/O pin is read as a 1 by
the microprocessor.
Figure 2 redraws the ferroelectric hysteresis loops of
Figure 1 in terms of charge delivered at the top of the
read voltage application.
Event
Detection
The Type AD
ferroelectric capacitor
used in the February
article to demonstrate
the memory circuit of
Figure 2 required six
nanocoulombs of
charge to be delivered
within 4. 2 volts. (Refer
to the vertical axis of
Figure 2.) Any sensor or generator that can source that
energy can fully switch that ferroelectric capacitor.
Attaching such a sensor to the ferroelectric capacitor in
Figure 2 creates an event detector. The microprocessor
presets the ferroelectric capacitor in one direction and
turns itself off. The sensor — if activated — moves the
ferroelectric capacitor to the opposite direction. Upon
waking up, the microprocessor executes a read operation.
Was the ferroelectric bit in its original state or did it
change?
There are a variety of readily-available sensors that
can set the state of the Type AD capacitor. A solar cell can
detect the presence of light in a room. A toy-scale electric
motor will generate energy when its axle is turned by
rolling motion. An antenna loop tuned to 13. 56 MHz can
be activated by the Near Field Communication (NFC)
smart tag antenna in your cell phone to provide enough
energy to switch a ferroelectric capacitor.
For this project, we will use the LTD0-028K
October 2015 49
FIGURE 1. UP and DOWN state full-cycle ferroelectric hysteresis
loops compared to an equivalent-sized
linear capacitor.
FIGURE 2. Microprocessor-based non-volatile ferroelectric memory and a plot of its UP/DOWN
memory states during a read operation.
Resources
Tester website
www.ferrodevices.com
Experimentalist website
www.ferromems.com
Vendor website for the
microprocessors
http://arduino.cc
Measurement Specialties, Inc.
http://meas-spec.com
