Professor John David
Head of Department
email : j.p.david@sheffield.ac.uk
tel: +44 (0) 114 222 5185
B.Eng.(Electronics) 1976, University of Sheffield, Ph.D. 1983, University of Sheffield, Research Assistant, University of Sheffield, 1982-1985, Research Fellow, University of Sheffield, 1985-present. Senior Member IEEE.
Research Activities
Impact Ionisation
A systematic study of impact ionisation and avalanche multiplication has been carried out in a wide range of materials and structures. The breakdown voltage is shown to be reasonably approximated by an average value for the band structure rather than just the conduction band minima.
Studies into the effect of GaAs/AlGaAs heterojunctions on the impact ionisation coefficients show that there is no enhancement of the ionisation coefficients. The extra scattering in the AlGaAs compensates for the energy gained by the band discontinuity. In structures with short avalanching distances, dead space effects are found to be significant. These reduce the multiplication below the value predicted by simple local models for the multiplication. More recently, P-I-N diodes with thin 'i' regions where these dead spaces are more significant were shown to reduce the excess avalanche noise below that predicted from the ionisation coefficient ratio.
Piezoelectric Structures
Strained III-V structures grown on a (111)B substrate have built-in piezoelectric fields.
These fields allow us to design novel opto-electronic devices such as blue shifting modulators, tuneable lasers and an integrated laser-modulators. Such devices have been demonstrated in the InGaAs/GaAs system and work is currently ongoing in the InGaAs/InP system for telecommunication applications.
Strain Relaxation Studies
The growth of strained structures allows flexibility in the design of novel electro-optic devices. Unfortunately the presence of strain can also cause the devices to degrade through the formation of misfit dislocations. We find that leakage currents are extremely sensitive to the presence of dislocations in the active regions of device structures and monitoring this is a simple way of estimating the magnitude of misfit dislocations.