Ferroelectric capacitor
Encyclopedia
Ferroelectric capacitor is a capacitor
based on a ferroelectric
material. In contrast, traditional capacitors are based on dielectric materials. Ferroelectric devices are used in digital electronics as part of ferroelectric RAM
, or in analog electronics as tunable capacitors (varactors).
In memory applications, the stored value of a ferroelectric capacitor is read by applying an electric field
. The amount of charge
needed to flip the memory cell to the opposite state is measured and the previous state of the cell is revealed. This means that the read operation destroys the memory cell state, and has to be followed by a corresponding write operation, in order to write the bit back. This makes it similar to the ferrite core memory. The requirement for a write cycle for each read cycle, together with the high but not infinite write cycle limit, poses a potential problem for some special applications.
Here
is the film thickness, 
and 
are the electric fields in the film and electrode at the interface, 
is the spontaneous polarization, 
, and 
& 
are the dielectric constants of the film and the metal electrode.
With perfect electrodes,
or for thick films, with 
the equation reduces to:
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
based on a ferroelectric
Ferroelectricity
Ferroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was...
material. In contrast, traditional capacitors are based on dielectric materials. Ferroelectric devices are used in digital electronics as part of ferroelectric RAM
Ferroelectric RAM
Ferroelectric RAM is a random-access memory similar in construction to DRAM but uses a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-volatile memory technologies that offer the same functionality as Flash memory...
, or in analog electronics as tunable capacitors (varactors).
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. The amount of charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
needed to flip the memory cell to the opposite state is measured and the previous state of the cell is revealed. This means that the read operation destroys the memory cell state, and has to be followed by a corresponding write operation, in order to write the bit back. This makes it similar to the ferrite core memory. The requirement for a write cycle for each read cycle, together with the high but not infinite write cycle limit, poses a potential problem for some special applications.
Theory
In a short-circuited ferroelectric capacitor with a metal-ferroelectric-metal (MFM) structure, a charge distribution of screening charges forms at the metal-ferroelectric interface so as to screen the electric displacement of the ferroelectric. Due to these screening charges, there is a voltage drop across the ferroelectric capacitor with screening in the electrode layer that can be obtained using the Thomas-Fermi approach as follows :Here
is the film thickness, and are the electric fields in the film and electrode at the interface, is the spontaneous polarization, , and & are the dielectric constants of the film and the metal electrode.With perfect electrodes,
or for thick films, with the equation reduces to: