FinFET (Fin field-effect transistor) | Needs & short-channel effects

It is a multi-gate device, a metal-oxide-semiconductor field-effect transistor(MOSFET) built on a substrate where the gate is placed on two or three sides of the channel or wrapped around the region, forming a double gate structure. These devices called “finfets” because the source/drain region forms fins on the silicon surface. The FinFET devices have faster switching times and high current density than planar CMOS technology.

Fin field-effect transistor (FinFET)

FinFET is a type of non-planar transistor also called “3D” transistor. It is the basic fabrication of modern nanoelectronic semiconductor device.

FinFET is a type of multi-gate MOSFET. A multi-gate transistor incorporates multi gates into one single device. It is a non-planar single region, a double-gate transistor which based either on the Bulk Silicon-On-Insulator (SOI). There are two types of FinFets:

  1.  Bulk FinFet
  2.  SOI FinFet

The ‘types’ of finfets are base onto which it is fabricated. This means that FinFets can be made either on SOI or regular silicon wafers.

The gate of the FinFet is wrapped around which reduces leakage current thereby increasing effectiveness.

Fin field-effect transistor

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NEED OF FINFET

The channel length of the MOSFET has been shrinking constantly so as to fabricate FAST AND compact. Following are the parameters related to MOSFET that highlight the need for smaller, compact devices and explain why the MOSFET is not a suitable choice for the same. The shorter region of the gate electrode is known as length and the longer section is called width.

As the channel length is reduces, the short-channel effects increase. The short-region effects are attributed to two physical phenomena:

  • The limitation imposed on drift electron characteristics in the channel.
  • The modification of the threshold voltage is due to the short channel length.

There are three different distinguishable short-channel effects:

A.    Drain-induced barrier lowering

The two depletion layers merge as a result of depletion region surrounding the drain which extends to the source. This leads to the occurrence of punch through. It can be reduced with the help of thinner oxides, larger substrate doping and also with longer region.

B.     Surface scattering

As the channel length becomes smaller, the component of the longitudinal electric field increases, and the surface mobility becomes dependent on field. The charge carrier transport in a MOSFET is confined within the small narrow inversion layer. The surface scattering reduces the mobility of charge carriers. It is difficult for electrons to move parallel to the interface. This is necessary so that the average surface mobility is about half of the bulk mobility. Surface scattering are the collisions suffered by the electrons which are a move toward the interface.

FinFET

C.     Velocity saturation

In saturation mode velocity saturation reduces transconductance. When a strong electric field is applied, carrier velocity reaches to its maximum value known as saturation velocity. When this process occurs, the transistor state is known as velocity saturation. Velocity saturation occurs due to the increased scattering rate of highly energetic electrons, primarily due to optical phonon emission. This effect increases the transit time of carriers through the region.

 

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