SCALING in MOSFET
MOSFET scaling is the reduction in the parameters (like current, voltage, electric field etc.) due to
reduction in length of the transistor with the advancement in technology. For example:180 nm
technology, supply voltage can be roughly of value 1.8 volts, but with newer technologies like 45
nm technology, supply voltage is around 1 volt.
Reducing the channel length of a MOSFET can be described in terms of scaling theory. A
scaling parameter S (S < 1) is used to scale the dimensions of a MOSFET. The value of S is
typically in the neighbourhood of 0.7 from one CMOS technology generation to the next. For
example, if a process uses a VDD of 2 V, a next generation process would use a VDD of 1.4 V.
In other words, VDD'= VDD.S The channel length of the scaled process is reduced to L'=L. S
while the width is reduced to W=W.S
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The design of high-density chips in MOS VLSI (Very Large-Scale Integration)
technology requires that the packing density of MOSFETs used in the circuits is as high
as possible and, consequently, that the sizes of the transistors are as small as possible.
The reduction of the size, i.e., the dimensions of MOSFETs, is commonly referred to
as scaling.
To meet the demand of high-density chips in MOS technology, it is required that
MOSFET are scaled down i.e., reduction in the size of transistor, so that high packaging
density can be achieved.
Thus, scaling of MOSFET transistor is concerned with systematic reduction of overall
dimension of the device as allowed by available technology, while preserving the
geometric ratios found in the larger devices.
A constant scaling factor ‘s’ is introduced. The scaled device is obtained by dividing
all horizontal and vertical dimensions of the large size transistor by this scaling factor.
It is expected that the operational characteristics of the MOS transistor will change with
the reduction of its dimension.
Need of scaling:
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To make the transistors smaller so you can fit more of them on the same size of die.
To reduce the current through them, and therefore reduce the power needed and the heat
produced.
• To reduce the voltage across them, and therefore reduce the power needed and the heat
produced.
Advantages of scaling:
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Increases the switching speed.
Reduces chip size.
Reduces power dissipation.
Disadvantages of scaling:
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Effect on SiO2 thickness – on scaling oxide thickness eventually a sage will reach when
oxide will lose its dielectric property.
Subthreshold current – For smaller device, in addition to VGS voltage, VDS voltage also
has greater influence on the carrier flow in the channel. The carrier in the channel can
be increased by increasing VDS voltage & also potential barrier can be reduced even
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VGS < VT. Thus, resulting current through the channel for higher values of VDS is called
sub-threshold current.
Noise problem – Scaling process inevitably results in noise problem, which degrades
the reliability of high-density chip.
Types of scaling:
Scaling is shrinking the device dimension. There are distinct types of scaling models used in
MOSFETs.
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Constant electric field scaling model
Constant voltage scaling model
Combination of the above two
Scaling helps in reducing the device dimensions in terms of length, width, thickness, etc by
making use of any of the above-mentioned modelling style. If the device dimensions are reduced,
in turn it reduces the area consumption, design becomes simplex, thereby reducing the cost of the
overall system.
1. Full scaling/ Constant field scaling:
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The Full scaling technique attempts to preserve the magnitude of internal electric fields
in the MOSFET, while the dimensions are scaled down by factor of ‘s’.
To achieve this goal the potentials must be scaled down proportionally, by the same
scaling factor.
In Full scaling, power supply voltage & all terminal voltages are scaled down
proportionally with the device dimensions, but the scaling of voltages may not be very
practical in many cases.
In particular, the peripheral and interface circuitry may require certain voltage levels
for all inputs and output voltages, which in turn would necessitate multiple power
supply voltages & complicated level – shifter arrangements.
Hence for these reasons, constant voltage scaling is usually preferred over full scaling.
2. Constant voltage scaling:
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In Constant voltage scaling, all dimensions of the MOSFET are reduced by a factor of
‘s’, but power supply & terminal voltage remain unchanged.
In Constant voltage scaling, drain current density and power density are increased
which may eventually cause serious reliability problem for the scaled transistor such as
electro-migration, hot carrier degradation, oxide breakdown & electrical overstress.