What is Ferroelectric RAM (FRAM)?
Ferroelectric RAM, or FRAM refers to a specific type of non-volatile storage that uses a special, thin magnetic film to store, read and write data at high speed. Like a Read-Only Memory (ROM) it can retain data when the power is turned off.
Technically, in this type of memory there is a ferroelectric layer used instead of a dielectric layer, and binary 1 and 0 values are stored according to the direction of the dipole in the capacitor, which can be changed by applying voltage.
Understanding Ferroelectric RAM (FRAM)
FRAM modules are built with a ferroelectric thin film capacitor that stores data and it forms electronic dipoles in its crystal structure.
It can attain a much larger dielectric constant, and being non-volatile in nature, it can retain data even when the power is cut off.
The characteristic features of Ferroelectric RAM modules are as follows:
- Low power consumption
- Faster writing speed
- Dynamic memory space
- Tamper resistance since data cannot be noticed by physical analysis
- Higher writing endurance
- Better write cycles
- Green memory since no battery back-up is needed
- Robust design
- Radiation and magnetic field resistant
These memory modules are typically designed on the principles of the ferroelectric effect.
This effect is common to Perovskites materials, which have an atom at the center of their crystalline structure.
The crystals, however, do not have any ferrous material and they are not influenced by magnetic fields.
The structure of the FRAM memory cell is the same as the DRAM cell, and consists of the following:
- A capacitor
- A transistor but with a ferroelectric dielectric
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Depending on the number of transistors and capacitors present in it, the FRAM structure can be divided into the three types as follows:
- 2T2C, where there are two transistors and two capacitors
- 1T2C, where there is one transistor and two capacitors
- 1T1C, where there is one transistor and one capacitor
There are two stable and equal states of low energy in the atom at the center of the structure. These two states are called:
- The up polarization, where the atom is at the top
- The down polarization, where the atom is in the bottom position
These states regulate the location of the atom. When a field is applied to the crystals in a desired place, the atom moves in that direction. This helps the module in:
- Determining the state of the material
- Using it to store data
The main aspect of the working process of an FRAM is the capacitance of the ferroelectric capacitor, which is variable. A relevant field is applied to polarize the capacitor up or down, and two things may happen in such situations, as follows:
- If it is not switched while the application of an electric field, the polarization will not change and it will behave in a regular linear fashion since no additional charge will be induced.
- If it is switched, then an extra charge will be induced due to the resultant increase in the capacitance, along with a small capacitance due to the transistor and other interconnection parasitics.
In a switched-capacitor, the induced charge is represented by Cbit and the capacitor by Cs.
This means that the Bit Line will have a voltage that is proportional to the capacitance ratio, Cs : Cbit. Therefore, the voltage on the line will be: Cs / Cbit x Vdd.
The Field Effect Transistors or FETs are used as an additional element to implement this effect in memory. This allows access to individual cells that have the following:
- A word line
- A bit line
The FRAMs also have a cycle time, just as the DRAMs, which is equal to the successive access time and not just one single access time.
It is due to the FRAM design and operation, where the charge on the switched capacitor is at least double the charge on the unswitched capacitor.
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How Does Ferroelectric RAM Work?
Ferroelectric RAM technology works using crystals in which there is a reversible electric polarization in the dielectric.
For its operations, it typically depends on the ferroelectric effect, which changes the polarization of the dielectric based on the applied voltage.
Memory read operation
There are a number of stages required for the read operation of the FRAM, which is quite similar to Dynamic RAM.
First, the voltage of the bit line is compared to a reference, which has the following levels set:
- Un-switched voltage
- Below the switch voltage
Then the sense amplifier acts as a comparator magnifying the difference and gives any of the two following specific logic states:
- Logic 1
- Logic 0
The memory access sequence will be initiated only when the cell is in an inactive state and it will have the following lines:
- A low Bit Line
- A low Plate Line
- A low Word Line
Then, voltages will be placed on the Word Line and the Plate Line.
Memory write operation
The same fundamental principles of read operation are followed for the writing operation in the FRAMs. In this particular process, the operation happens as follows:
- The memory control circuitry deploys a field across the capacitor in the desired direction.
- The data stays the same even when there is no power in the chip because it needs energy to switch from one state to another.
The entire operation of the FRAM memory is quite simple, and its long life and non-volatility make it a contender for several applications, even if it does not have the same density as other technologies, yet.
Typically operating on random access, the read and write processes in the FRAMs can be completed with no delays, often within a 2 to 3-digit ns range, comparable to the RAMs.
As a result of this aspect and its faster read and write cycles, these memory modules can complete a read or write process even if there is a sudden power outage. This ensures higher data integrity.
What is FRAM Used for?
FRAM is used in smaller, portable, and handheld consumer and industrial devices where both higher memory density and better memory organization are needed.
Typically, you will find such memory modules been used in the following applications:
- Portable computers
- Handheld devices
- Portable and direct data logging applications
- Radio Frequency Identification (RFID) tags
- Power meters
- Personal Digital Assistants (PDAs)
- Parameter storage
- Real-time data writing
- Back-up memory
- Spacecrafts
- Industrial microcontrollers
- Medical equipment
- Life-enhancing patient monitoring systems and other instrumentation
- Programmable Logic Controllers (PLC)
These memory chips are also embedded sometimes in the Complementary Metal Oxide Semiconductor (CMOS) based chips. This actually helps the Memory Control Units (MCUs) in the following ways:
- It helps them to have and use their own ferroelectric memories.
- It helps them to reduce the number of phases required to incorporate the memory into them.
- It helps them to save costs due to lower power consumption.
Is Ferroelectric RAM Volatile?
Ferroelectric RAM is typically a non-volatile and low-power memory module that allows fast random access.
Built with a combination of traditional non-volatile memories such as Electrically Erasable Programmable Read Only Memory (EEPROM) and Flash and rapid Static RAM and Dynamic RAM, they support a nearly unlimited number of read/write cycles.
The use of the ferroelectric material that acts as the dielectric layer within the chip offers it its non-volatile characteristic.
What Will Happen When Reading a Ferroelectric RAM Cell?
When you read a Ferroelectric RAM cell, it will erase all the data stored in that cell. This is because it is typically a destructive read process.
Conclusion
As you can see from this article, Ferroelectric RAM is quite useful and environmentally-friendly memory that consumes low power but supports high-speed programming.
It can be configured in a flexible way and its security features and functions foil unlawful reading, making it suitable for digital devices.