MOSFET Scaling

MOSFET Scaling

Scaling definition and types

•        High integration of MOSFETs on IC, requires reduction of size of MOSFET. Reducing the size of MOSFET is called as scaling.
•        Scaling of MOS transistors is concerned with systematic reduction of overall dimensions of the devices as allowed by the available technology, while preserving the geometric ratios found in the larger devices
•        There are two types of scaling

o   Constant field scaling/full scaling

o   Constant voltage scaling

Scaling factor

•        constant scaling factor S > 1.

•        All horizontal and vertical dimensions of the large-size transistor are then divided by this scaling factor to obtain the scaled device.

•        W, L, toxare decreased by S, while ND , NA doping concentrations are increased by S.



Constant Field Scaling/voltage scaling

•        This scaling option attempts to preserve the magnitude of internal electric fields in the MOSFET, while the dimensions are scaled down by a factor of S.

•        the Poisson equation describing the relationship between charge densities and electric fields dictates that the charge densities must be increased by a factor of S in order to maintain the field conditions.

Constant Field Scaling/voltage scaling


Constant Field Scaling/Full scaling

•        The gate oxide capacitance per unit area, on the other hand, is changed as follows.


Constant Field Scaling/Full scaling

•        The aspect ratio WIL of the MOSFET will remain unchanged under scaling. Consequently, the transconductance parameter kn will also be scaled by a factor of S. Since all terminal voltages are scaled down by the factor S as well, the linear-mode drain current of the scaled MOSFET can now be found as:


Constant Field Scaling/Full scaling

•        Now consider the power dissipation of the MOSFET. Since the drain current flows between the source and the drain terminals, the instantaneous power dissipated by the device (before scaling) can be found as:


Notice that full scaling reduces both the drain current and the drain-to-source voltage by a factor of S; hence, the power dissipation of the transistor will be reduced by the factor S*S.


 

Advantages Constant Field Scaling

• Consider the gate oxide capacitance defined as Cg=WL COX, Since the gate oxide capacitance C is scaled down by a factor of S, we can predict that the transient characteristics, i.e., the charge-up and charge-down times, of the scaled device will improve accordingly

•        the proportional reduction of all dimensions on-chip will lead to a reduction of various parasitic capacitances and resistances as well, contributing to the overall performance improvement.



Disadvantages Constant Field Scaling

•        While the full scaling strategy dictates that the power supply voltage and all terminal voltages be scaled down proportionally with the device dimensions, 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 input and output voltages, which in turn would necessitate multiple power supply voltages and complicated level shifter arrangements. For these reasons, constant-voltage scaling is usually preferred over full scaling.

Constant Voltage scaling

•        In constant-voltage scaling, all dimensions of the MOSFET are reduced by a factor of S, as in full scaling.

•        The power supply voltage and the terminal voltages, on the other hand, remain unchanged.

•        The doping densities must be increased by a factor of s*s in order to preserve the charge-field relations.

Constant Voltage scaling



Constant Voltage scaling

Under constant-voltage scaling, the changes in device characteristics are significantly different compared to those in full scaling, as we will demonstrate. The gate oxide capacitance per unit area Cox is increased by a factor of S, which means that the transconductance parameter is also increased by S. Since the terminal voltages remain unchanged, the linear mode drain current of the scaled MOSFET can be written as:



Constant Voltage scaling

•        Also, the saturation-mode drain current will be increased by a factor of S after constant voltage scaling. This means that the drain current density (current per unit area) is increased by a factor of S3, which may cause serious reliability problems for the MOS transistor.


 

Constant Voltage scaling

Consider the power dissipation. Since the drain current is increased by a factor of S while the drain-to-source voltage remains unchanged, the power dissipation of the MOSFET increases by a factor of S.



Constant Voltage scaling

Finally, the power density (power dissipation per unit area) is found to increase by a factor of S3 after constant-voltage scaling, with possible adverse effects on device reliability.



Advantages and Disadvantages of Constant Voltage scaling

•        Adv: constant-voltage scaling maybe preferred over full (constant- field) scaling in many practical cases because of the external voltage-level constraints.

•        Disadv: It must be recognized, however, that constant-voltage scaling increases the drain current density and the power density by a factor of S*S*S. This large increase in current and power densities may eventually cause serious reliability problems for the scaled transistor, such as electromigration, hot-carrier degradation, oxide breakdown, and electrical over-stress

Reference

•        Sung Mo Kang & YosufLeblebici, “CMOS Digital Integrated Circuits: Analysis and Design”, Tata McGraw-Hill, Third Edition.

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