Flash Memory Operation & Technology

- overview of Flash memory operation and technology

Flash memory is an effective form of semiconductor memory.

Flash memory operation is very similar to that of the older EPROM and EEPROM technologies.

Flash memory operation is based around methods including Fowler-Nordheim tunnelling and hot electron injection.

Flash memory operation basics

Flash memory is a high density form of semiconductor memory. This is brought about by the fact that each Flash memory cell is made up from a single field effect transistor and it is very similar in structure to the ordinary EPROM.

Each Flash memory cell consists of the basic channel with the source and drain electrodes separated by the channel about 1 µm long. Above the channel in the Flash memory cell there is a floating gate which is separated from the channel by an exceedingly thin oxide layer which is typically only 100 Å thick. It is the quality of this layer which is crucial to the reliable operation of the memory.

Diagram of the typical structure of a flash memory cell showing the different semiconductor layers and areas
Flash memory cell structure

Above the floating gate there is the control gate. This is used to charge up the gate capacitance during the write cycle.

In the case of traditional EPROMs, these memory chips are erased by the application of UV light. To accommodate this these memory devices have a translucent window which can be exposed to the UV light. However this process takes upwards of twenty minutes. It also requires the memory chip to be removed from its circuit and placed in a special eraser where the UV light can be contained.

The Flash memory cell functions by storing charge on the floating gate. The presence of charge will then determine whether the channel will conduct or not. During the read cycle a "1" at the output corresponds to the channel being in its low resistance or ON state.

Programming the Flash memory cell is a little more complicated, and involves a process known as hot-electron injection. When programming the control gate is connected to a "programming voltage". The drain will then see a voltage of around half this value while the source is at ground. The voltage on the control gate is coupled to the floating gate through the dielectric, raising the floating gate to the programming voltage and inverting the channel underneath. This results in the channel electrons having a higher drift velocity and increased kinetic energy.

Collisions between the energetic electrons and the crystal lattice dissipate heat which raises the temperature of the silicon. At the programming voltage it is found that the electrons cannot transfer their kinetic energy to the surrounding atoms fast enough and they become "hotter" and scatter further afield, many towards the oxide layer. These electrons overcome the 3.1 eV (electron volts) needed to overcome the barrier and they accumulate on the floating gate. As there is no way of escape they remain there until they are removed by an erase cycle.

The erase cycle for Flash memory uses a process called Fowler-Nordheim tunnelling. The process is initiated by routing the programming voltage to the source, grounding the control gate and leaving the drain floating. In this condition electrons are attracted towards the source and they tunnel off the floating gate, passing through the thin oxide layer. This leaves the floating gate devoid of charge.

Generally the erase process is only made to last a few milliseconds. When complete each Flash memory cell in the block is checked to ensure it has been completely erased. If not a second erase cycle is initiated.

Programming Flash memory

In the early days of flash memories one of the limiting factors in their uptake was the topic of programming Flash memory because they had a limited number of erase programme cycles. This was caused by the destructive breakdown of the thin gate oxide layer. Some of the early examples of flash memories only had a few hundred cycles. Now Flash memory technology is vastly improved and manufacturers quote figures that mean the Flash memory life is no longer a concern.

Most of this improvement in Flash memory has been brought about by improving the quality of the oxide layer. When samples of flash memory chips are found to have a lower lifetime it is usually caused by the manufacturing process not being optimised for the oxide growth. Now programming Flash memory is not a problem and when using Flash memory the chips are, within reason, not treated as items with a limited life.

Flash memory access

Flash memory is different to most other types of electronic memory in that while reading data can be performed on individual addresses on certain types of flash memory, erase and write activities may only be performed on a block of a Flash memory. A typical block size will be 64, 128, or 256 kB. In order to accommodate this, the low level control software used to drive Flash memories, needs to take account of this if the read and write operations are to be performed correctly.

By Ian Poole

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