The Architecture of Electronic Data Carriers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 293

technology. A new, non-transient memory technology, which should improve this situ-
ation, has been under development for around 20 years: ferroelectrical RAM, or FRAM .
At the end of the 1980s the company Ramtron was established, which collaborated
with Hitachi on the development of this technology. The first RFID systems using
FRAM technology were produced by the Ramtron subsidiary Racom. However, the
development of FRAMs is still associated with many problems, and so RFID systems
using FRAMs are still not widespread.
The principle underlying the FRAM cell is the ferroelectric effect, i.e. the capability
of a material to retain an electrical polarisation even in the absence of an electric field.
The polarisation is based upon the alignment of an elementary dipole within a crystal
lattice in the ferroelectric material due to the effect of an electric field that is greater
than the coercive force of the material. An opposing electric field causes the opposite
alignment of the internal dipole. The alignment of the internal dipole takes on one of
two stable states, which are retained after the electric field has been removed.
Figure 10.34 shows a simplified model of the ferroelectric lattice. The central atom
moves into one of the two stable positions, depending upon the field direction of the
external electric field. Despite this, FRAM memories are completely insensitive to
foreign electric interference fields and magnetic fields.
To read the FRAM cell (Figure 10.35), an electric field ( U CC )isappliedtothe
ferroelectric capacitor via a switching transistor. If the stored information represents a
logic '1' then the cell is in position A on the hysteresis loop. If, on the other hand, it
represents a logic '0', the cell is in position C. By the application of the voltage U CC
we move to point B on the hysteresis loop, releasing electric charge, which is captured
and evaluated by the signal amplifiers on the memory chip. The magnitude of escaping
charge clearly indicates a '1' or '0', because a significantly greater charge escapes in
the transition from state A to B than in the transition from state C to B.
After the external (read) field U CC has been removed, the FRAM cell always returns
to state C, and thus a stored '1' is lost, because state C represents a '0'. For this reason,
as soon as a '1' is read, the memory chip's logic automatically performs a rewrite
operation. This involves applying an opposing electric field − U CC to the ferroelectric
capacitor, which changes the state of the FRAM cell, moving it to point D on the
Divalent or
monovalent metal
atoms
Oxygen atoms
Tetravalent or
pentavalent atom
Applied
electric
field
Figure 10.34 Basic configuration of a ferroelectric crystal lattice: an electric field steers the
inner atom between two stable states