Quiet chaos

6 December 2012

Researchers in France have investigated the performance of high-permittivity (high-k) 28 nm CMOS devices in comparison with the standard 45 nm technology.  They have shown that power output is increased in the smaller device, even for lower bias voltages.

Reducing the gate length of these devices will have the effect of increasing the leakage current.  This is due to the fact that a smaller structure will be more affected by tunnelling from the device into the reservoir or substrate.  High-k CMOS was originally designed to overcome the consequent increase in DC power consumption.

However, the smaller structure does suffer from a reduced maximum operational frequency, caused by a degradation of the device’s gate resistance.  This means that the technology will be difficult to implement in standard radiofrequency systems, which will be an important challenge for the future.

Noise removal

As radiofrequency and microwave communications have advanced, more and more sophisticated signal processing techniques and devices have been necessary in order to deal with the increased channel noise.  For example, standard linear filters will not function if the noise and the signal overlap, or if the noise is not Gaussian in nature.  Modern signals, such as those occurring in image processing systems have detail at very high frequencies, which standard filters would remove as noise, leading to a loss of potentially important information.  The essence is that a linear filter treats all data equivalently.

However, non-linear filters are capable of extracting the appropriate information even from the noise section channel.  They can distinguish between noisy data and useful signal data, and treat them accordingly, filtering them of processing them respectively.  Furthermore, they are capable of processing more sophisticated signals and outputs as the information becomes more complex, for example, two-dimensional signals or functions of highly non-linear differential equations.

Modern processing systems need higher power than ever before, and the need for high-power but small sized integrated technology is increasing.  The standard nonlinear systems are based on silicon 45 nm CMOS processes, but eventually this will have to be reduced in size as a reflection of the familiar Moore’s law.  One candidate is the 28 nm titanium nitride gate, hafnium silicon dioxide high-k oxide investigated by the team in France.

Small and strong

To perform their comparison, the group investigated the current-voltage characteristics of the standard 48 nm technology compared to the smaller process.  The gate width was constant for the comparison and the team found that the smaller device did indeed produce higher power.  This is because the output current, to second order in voltage, is inversely proportional to gate length, and this higher current will lead to higher power.

In addition, the team also demonstrated the suitable nonlinear behaviour of the device. At one phase of the operation, the current is clearly linearly proportional to the voltage.  However, after one loop, the differential conductance changes sign and exhibits highly nonlinear behaviour, showing that these devices will prove ideal for nonlinear signal processing applications.

Broadening horizons

In the future the group hope to apply their non-linear measurement system for the characterisation of other families of transistors, in particular the AlGaN/GaN High Electron Mobility Transistor.  Alongside this, they will be attempting  to upgrade the system for higher power applications and pulse stimuli, and to study intermodulation and X-parameters characteristics which will greatly increase the already wide range of applications for their method.

Interestingly, the lead author of the paper, Rezki Ouhachi, explained that the team sees their on-going projects are as a whole. One research project includes “material study, physical simulation, technological fabrication, electrical and thermal characterisation, electrical and thermal modelling”, said Ouhachi. For example, he explained “I am working on the compact modelling of Silicon/Germanium based bipolar transistors in order to address future mixed analogue/digital high speed devices”.

There is an increasing demand for high data rates with an equally strong demand for low power-consumption at a reduced price in a broad range of applications. This challenge can be met only if the progress in the understanding of the material keeps pace with the demand.  As Ouhachi explained, “the technological fabrication but also new electrical characterisation techniques must be developed to ensure the optimisation of these devices of the future”.

Further reading

This article is based on the Letter: Large signal microwave performances of high k metal gate 28 nm CMOS technology (new window).

A PDF version (new window) of this feature article is also available.

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Cover of Electronics Letters, volume 50, issue 18

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