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EPSRC Reference: GR/J49549/01
Title: IMPACT IONISATION PROCESSES IN LOW DIMENSIONAL STRUCTURES AND DEVICES
Principal Investigator: David, Professor J
Other Investigators:
Skolnick, Professor M Rees, Professor GJ Cockburn, Professor J
Wood, Dr C Robson, Professor P
Researcher Co-Investigators:
Project Partners:
Department: Electronic and Electrical Engineering
Organisation: University of Sheffield
Scheme: Standard Research (Pre-FEC)
Starts: 17 January 1994 Ends: 16 July 1997 Value (£): 179,645
EPSRC Research Topic Classifications:
Electronic Devices & Subsys. Materials Characterisation
EPSRC Industrial Sector Classifications:
Related Grants:
Panel History:  
Summary on Grant Application Form
(i) To investigate experimentally the ionisation process in short and low dimensional structures. (ii) To develop modelling techniques to interpret the experimental results.(iii) To use the knowledge gained to understand avalanche breakdown in microwave devices and to improve device design. Progress:A two dimensional self-consistent bipolar Monte Carlo semiconductor device simulation code including impact ionisation has been implemented on a Silicon Graphics workstation. A pragmatic approach to the problem of ionisation has been adopted with the use of analytical band structure and Keldysh scatter probability functions. The threshold energies, software parameters and band nonparabolicity are chosen to give the dependence of electron and hole drift velocities and impact ionisation rates on electric field, in good agreement with the literature. Bulk GaAs p-i-n diodes with short avalanching lengths are currently being investigated using this model. The electron and hole initiated multiplication curves obtained from the simulation are in close agreement with experiment, providing an insight into the nature of the dead space, the distance carriers have to travel to gain the threshold energy. In addition to this, fully self consistent simulations of GaAs photodiodes and MESFETS are under investigation. In the case of photodiodes, our results reveal impact ionisation to set in much earlier than might be expected from the nominally applied field because of screening within the device and a correlation has been found between the (optically) created charge density and the charge multiplication. MESFETS also were found to break down at much lower nominal electric fields than might be expected as a result of wandering Gunn domains which cause growing oscillations in the electric field and increasing periodic bursts of impact ionisation. The experimental programme has been investigating the ionisation process in multilayer structures by measuring the breakdown voltages in a series of GaAs p-i(MQW)-n structures. By normalising the breakdown to that of bulk GaAs we have found that the critical dimensions are the momentum and energy relaxation lengths. No enhancement in the ionisation coefficient ratio is apparent for the wide range of dimensions considered.
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Organisation Website: http://www.shef.ac.uk