AN2014 datasheet(9/65 Pages) STMICROELECTRONICS

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AN2014

EEPROM cell and memory array architecture

1.1.1

Reading the value stored in a memory cell

Figure 5 puts the three curves together, by way of comparison. It shows that for a given

Control Gate voltage, Vg, the current that flows through the drain, Id, will be detectably

higher or lower than that of the neutral device, depending on whether the reservoir of

electrons on the floating gate has been filled up, or emptied. This, then, is the basis of how

the memory cell can be read.

Figure 5. Using the voltage on the Control Gate to determine the charge

on the floating gate

1. A written cell draws a current I µA (where IµA > Id.ref); an erased cell does not draw any current (0 µA).

In most of ST EEPROM products, a predetermined biasing condition on the Control Gate

and the drain makes it possible to compare the current absorbed by the FLOTOX transistor

with a reference. Basically, with the predetermined biasing condition an erased FLOTOX

cell is not able to sink as much current as the reference (ideally the transistor is off). On the

other hand, a written FLOTOX cell sinks a current that is superior to the reference (the

transistor is on). By comparing to a reference current, the device is able to retrieve the

stored information as a digital signal on the output pins of the memory device.

1.1.2

Writing a new value to the memory cell

The next question, of course, is how the charge can be changed on the floating gate, given

that it is so well insulated by oxide, and keeps its charge even when there is no power

supply. The answer is that the Tunnel Oxide, shown in Figure 1, is very thin, and can be

used to transfer charge, when much higher voltages are applied than those normally used

during Read operations.

Filling the floating gate reservoir with negative charges (electrons) is called erase. After

erase, the FLOTOX transistor is in the Erased State (see Figure 3). Pulling out negative

charges from the floating gate is called Program. After Program, the FLOTOX transistor is in

the Written State (see Figure 4). One state is used to represent logic-0, and the other logic-

1, but the exact choice is manufacturer and product-type dependent).

Both operations use the Fowler-Nordheim tunneling effect. For this, a high electric field

(1 million V/mm, or more) is needed to make electrons pass through the thin Tunnel Oxide.

For a Tunnel Oxide thickness of 100Å, the high voltage needs to be at least 10V.

AI10234

Vg.ref

Id.ref

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