29-03-2012, 04:26 PM
Double and triple gate FIN FET
Double and triple gate FIN FET.pptx (Size: 173.78 KB / Downloads: 107)
INTRODUCTION
As MOSFET devices are shrinking further with advancement in technologies the problems with conventional (planar) MOSFETs are increasing. Industry is currently at the 90nm node ( which corresponds to gate lengths of about 70nm). As we go down to the 65nm, 45nm, etc nodes, there seem to be no viable options of continuing forth with the conventional MOSFET. Severe short channel effects (SCE) come into picture that affects the device performance.
Short Channel Effects
Drain induced barrier lowering (DIBL)
Hot electron effect
Vt roll off
Subthreshold S/D leakage
Drain induced barrier lowering (DIBL)
Drain induced barrier lowering or DIBL is a secondary effect in MOSFETs referring to a reduction of threshold voltage of the transistor at higher drain voltages.
Hot electron effect
Another problem, related to high electric fields, is caused by so-called hot electrons. This high energy electrons can enter the oxide, where they can be trapped, giving rise to oxide charging that can accumulate with time and degrade the device performance by increasing VT and affect adversely the gate’s control on the drain current.
Vt roll off
In a short-channel MOSFET, the depletion region has a complicated 2-D geometry and the channel region is influenced by the source/drain as much as by the gate. Before there is any positive bias on the gate, the channel is already depleted by the built-in potential between the channel and S/D regions. This influence becomes stronger when the p-n junctions between the S/D and the channel are reverse-biased, giving rise to drain-induced barrier lowering (DIBL). As a result of this undesirable coupling between the S/D region and the channel, which becomes stronger as the gate length is reduced, VT is lower for a transistor with shorter LG.
What is a DG-MOSFET?
Double gate MOSFETs (DG-FET) is a MOSFET that has two gates to control the channel.
The main idea of a Double Gate MOSFET is to control the Si channel very efficiently by choosing the Si channel width to be very small and by applying a gate contact to both sides of the channel. This concept helps to suppress short channel effects and leads to higher currents as compared with a MOSFET having only one gate.
Advantages of DG- MOSFET
improved gate-channel control.
Reduced short channel effects.
reduced leakage currents:
S/D leakage
Gate leakage
Symmteric DG-MOSFET
Symmetric DG-FETs have identical gate electrode materials for the front and back gates (ie. top and bottom gates). When symmetrically driven, the channel is formed at both the surfaces.
Asymmteric DG- MOSFET
In an asymmetric DG-FET, the top and bottom gate electrode materials can differ (eg. n+ poly and p+ poly). When symmetrically driven this would end up forming a channel on only one of the surfaces