The Architecture of Electronic Data Carriers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 292

Bit line
Selected gate
Word line
Floating gate with
tunnelling oxidation layer
Pulse common plate line
Figure 10.33 The EEPROM cell consists of a modified field effect transistor with an additional
floating gate
voltage between the source and drain of the field effect transistor, meaning that this is
practically blocked. The current flow through the field effect transistor of an EEPROM
cell is evaluated by signal amplification of the memory chip, whereby the strength of
the current clearly indicates a '0' or '1'.
To write a '0' or '1' to an EEPROM cell, a high positive or negative voltage is
applied to the control gate, which activates the tunnel effect. The voltage required to
charge the EEPROM cell is around 17 V at the control gate which falls to 12 V at the
floating gate. However, RFID data carriers are supplied with 3V or 5V from the HF
interface (or a battery). Therefore a voltage of 25V is generated from the low supply
voltage of the chip using a cascaded charging pump integrated into the chip, which
provides the required 17 V after stabilisation.
It takes between 5 and 10ms to charge an EEPROM cell. The number of possible
write cycles is limited to between 10 000 and 100 000 for EEPROM cells. This is
because in every write operation electrons are captured by the tunnelling oxidation
layer and these are never released. These electrons influence the threshold voltage of
the field effect transistor, with the effect becoming greater with every write operation.
As soon as this parasitic effect of the tunnelling oxidation layer becomes greater than
the primary influence of the floating gate the EEPROM cell has reached its lifetime
(Rankl and Effing, 1996).
A charged floating gate loses its charge due to insulation losses and quantum
mechanical effects. However, according to the semiconductor manufacturer's figures,
EEPROMs still provide reliable data retention for 10 years. If the EEPROM cell is
nearing its lifetime, then information is only stored for short periods, which are deter-
mined by the parasitic influence of the oxide layer. For this reason, a plausibility test
should be carried out on stored data using checksums (e.g. CRC) in RFID data carriers
with EEPROM memories.
10.3.3 FRAM
High power consumption during writing and high write times of around 5-10 ms
have a detrimental effect on the performance of RFID systems that employ EEPROM