Area of work

Automatization of the Calibration Process

Search for values of parameters that reproduce a reference I-V characteristics. The black cross indicates the optimal values, circles represent different initial conditions, and lines represent the optimisation algorithm.

The first stage of investigating a novel phenomenon using physical simulations is to calibrate the simulator against the experimental data in well-known conditions, i.e. to reproduce the measured data in a simulation. In the case of transistors, it is usually the I-V characteristics of the device. This validates the simulator and the applicability of the used models to the investigated phenomenon, as well as yields the values of the used parameters. One way to go about the calibration is to use physical insights and understanding of how various parameters have impact on various parts of the I-V curve and manually change the values of those parameters to achieve an agreement between the measured and simulated data. This procedure usually yields good agreement and physical values of the parameters, but is also usually tedious and slow. Here, another approach opens up, which is complete or partial automatization of the calibration process. Physical insights are still used to inform the optimiser on selection of the parameters and their respective parts of the I-V characteristics at different stages of the process, but the search for values that give the best agreement in each stage as well as the succession of the stages is performed automatically. The simulator used in the development of the automated calibration is the Glasgow 'atomistic' device simulator GARAND.

TCAD-Breakdown

When investigating variability effects in devices, the simulator is first calibrated against data obtained from measurement using 'uniform' doping. Then, .using the same doping profile and the calibrated parameters, 'atomistic' simulations are performed. However, the mean of an ensemble of devices using the 'atomistic' doping with RDF (random dopant fluctuations) differes from the simulation using 'uniform' doping with the same dopant distribution. T This is a problem in investigating variability due to RDF (or other source), since the ensemble does not reproduce the experimental results. The aim of this project is to calibrate the parameters and the doping profile using 'atomistic' doping directly, omitting the intermediate step of calibrating the simulation using 'uniform' doping.

Contact

Email: Daniel dot Balaz at glasgow.ac.uk

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Recent grants worked on

2011 - present

EPSRC Platform Grant The grant allows for development of the optimization tool for the automated calibration of devices.

2006 - 2010

Novel Time-Resolved Thermal Imaging: AlGaN/GaN Heterostructure Field Effect Transistors (http://gow.epsrc.ac.uk/ViewGrant.aspx?GrantRef=EP/D04698X/1) Schematic illustration of an AlGaN/GaN HFET device. The red arrows represent spontaneous (SP) and piezoelectric (PE) polarization in the III-N semiconductor materials, the violet +/- circles represent the bound charge (also referred to as polarization doping) arising from the discontinuity in the polarization at the device interface and surface and the induced two-dimensional electron gas (2DEG) in the channel of the device. The illustration is not to scale, the GaN layer is typically two orders of magnitude thicker than the AlGaN layer. GaN-based HFETs (also referred to as HEMTs - High Electron Mobility Transistors) achieve excellent performance as high-power high-frequency devices. Yet, reliability problems as self-heating, current collapse or device degradation prevent these devices from wide penetration of the commercial market. The reliability of GaN HFETs was studied in this project. In the course of the project, the focus was shifted from thermal imaging to the current collapse and device degradation phenomena. First, a device in the absence of the current collapse and before degradation was calibrated to provide a starting point for the investigation of the mentioned phenomena. Then, the size of the impact of converse piezoelectric effect on the bound charge in the device was explored and found to be negligible. It is widely accepted that the current collapse is caused by 'virtual gate', i.e. electrons leaked to the surface of the device from the gate. We have found the charge distribution that reproduced the measured I-V characteristics in the presence of the current collapse for a wide range of gate and drain voltages accurately. Later, we have simulated the emission and transport of the electrons that leads to the found charge distribution. A proposed mechanism for the device degradation is that the high electric field via the converse piezoelectric effect induces high strain at the gate edges of the device leading to lattice dislocations and trap creation which cause the drain current to degrade permanently. We have investigated this hypothesis and found that while the measured and simulated effect on the I-V characteristics is in principle the same, we were unable to reproduce the I-V for a wide range of voltages within this model. The TCAD simulation platform used in this project was Sentaurus from Synopsys. This project was run in collaboration with the Applied Spectroscopy Group & Center for Device Thermography and Reliability at the University of Bristol lead by Prof. Martin Kuball and with the industrial partner QinetiQ represented by Prof. Michael J. Uren (now at the University of Bristol).

Publications

2013

• S.-Y. Liao, E. Towie, D. Balaz, C. Riddet, B. Cheng and A. Asenov, "Impact of the statistical variability on 15nm IIIV and Ge MOSFET based SRAM design," 14th Ultimate Integration on Silicon (ULIS): Coventry, UK, Mar. 19-21, 2013.
  

2009

• D. Balaz, K. Kalna, M. Kuball, M. J. Uren and A. Asenov, "Impact of the field induced polarization space-charge on the characteristics of AlGaN/GaN HEMT: Self-consistent simulation study," Physica Status Solidi (c), Vol. 6, No. S2, p. S1007–S1011, May 2009.
  
• D. Balaz, K. Kalna, M. Kuball, D. J. Hayes, M. J. Uren and A. Asenov, "Impact of surface charge on the I-V characteristics of an AlGaN/GaN HEMT," Workshop on Compound Semiconductor Devices and Integrated Circuits: Malaga, Spain, May 17-20, 2009.
  
• D. Balaz, K. Kalna, M. Kuball, M. J. Uren and A. Asenov, "Systematic simulation study of the impact of virtual gate geometry on the current collapse in AlGaN/GaN HEMTs," UK Semiconductors: Sheffield, June 1-2, 2009.
  

2008

• D. Balaz, K. Kalna, M. Kuball, M. J. Uren and A. Asenov, "Impact of the field induced polarization space-charge on the characteristics of AlGaN/GaN HEMT: Self-consistent simulation study," International Workshop on Nitride semiconductors: Oct. 6-10, 2008.