Floating body effect
Encyclopedia
The floating body effect is the effect of dependence of the body potential of a transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 realized by the silicon on insulator
Silicon on insulator
Silicon on insulator technology refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance and thereby improving performance...

 (SOI) technology on the history of its biasing and the carrier recombination processes. The transistor's body forms a capacitor against the insulated substrate. The charge accumulates on this capacitor and may cause adverse effects, for example, opening of parasitic transistors in the structure and causing off-state leakages, resulting in higher current consumption and in case of DRAM
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...

 in loss of information from the memory cells. It also causes the history effect, the dependence of the threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...

 of the transistor on its previous states. In analog devices, the floating body effect is known as the kink effect.

One countermeasure to floating body effect involves use of fully depleted devices. The insulator layer in FD devices is significantly thinner than the channel depletion width. The charge and thus also the body potential of the transistors is therefore fixed. However, the short-channel effect is worsened in the FD devices, the body may still charge up if both source and drain are high, and the architecture is unsuitable for some analog devices that require contact with the body. Hybrid trench isolation is another approach.

While floating body effect presents a problem in SOI DRAM chips, it is exploited as the underlying principle for ZRAM
ZRam
zRam is an available feature for certain distributions of Linux, that was previously called "compcache". It increases performance by avoiding paging on disk and instead uses a compressed block device in RAM in which paging takes place until it is necessary to use the swap space on the hard disk drive...

 and TTRAM
TTRAM
Twin Transistor RAM is a new type of computer memory in development by Renesas.TTRAM is similar to conventional one-transistor, one-capacitor DRAM in concept, but eliminates the capacitor by relying on the floating body effect inherent in a silicon on insulator manufacturing process...

technologies. For this reason, the effect is sometimes called the Cinderella effect in the context of these technologies, because it transforms a disadvantage into an advantage.http://www.onversity.net/load/zram.pdf
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