7. Ferroelectric Non-volatile Memory Cells

Using ferroelectric materials for non-volatile memories is a very promising approach. Table 7.1 shows a comparison of ferroelectric memories, electrically erasable and programmable read-only memories (EEPROMs), and Flash memory. As the latter devices have been produced for several years now they represent a quite mature technology. This gives them some advantage in terms of integration density [PBOZ97]. Nevertheless, advanced state-of-the-art stacked capacitor-transistor designs show similar cell sizes as EEPROMs [ATM+98]. Already existing prototypes like the ferroelectric memory field effect transistors (FEMFET) are even expected to reach the density of Flash memory.

As can be seen from Table 7.1, non-ferroelectric devices require less power for read cycles. But in terms of overall power consumption and write speed ferroelectrics offer far superior properties [SG00][SMN+95]. They also do not require any voltage different from the supply voltage, which is a significant advantage.

Table 7.1: Comparison of different types of non-volatile memories, after Sheikoleslami [SG00]

Type	Area/Cell (normalized)	Read Access Time	Write Access Time	Energy per 32b Write	Energy per 32b Read
EEPROM	2	50ns	10$\mu$s	1$\mu$J	150pJ
Flash Memory	1	50ns	100ns	2$\mu$J	150pJ
Ferroelectric Memory	2-5	100ns	100ns	1nJ	1nJ

Basically, ferroelectric memory cells can be separated into two different design types, outlined in Fig. 7.1, depending on whether the ferroelectric material is integrated into the read/write transistor or not. In the latter case a capacitor is used for data storage. The capacitors can be placed either immediately in the vicinity of the read/write transistor or in a separated array of capacitors somewhere on the chip.

Figure 7.1: Overview of the different ferroelectric memory designs