and Electrical E
The University Of Sheffield
ray photodiodes and Avalanche Photodiodes using GaAs
2.Quenching circuit for Single Photon Avalanche Photodiodes
ray detector arrays are found in a wide range of applications, such as medical imaging, medical dosimetry
and astronomy observations. Some of the materials currently used in X
ray detector arrays include Si, Ge and
CdTe. Although GaAs X
ray photodiode ar
rays were investigated in the last decade, research into them has
pped until the last 2 years.
GaAs does offer significant advantages due to its material properties. The high Z numbers of Ga and As (31
and 33) compared to that of Si (14) means that GaAs
ray photons more strongly than Si and Ge.
Epitaxial growth of GaAs and its substrates are part of an established industry, which is overwhelming
favourable compared to the production of CdTe wafers. In addition to GaAs X
ray photodiodes, using G
technology leads to possibility of developing GaAs
ray Avalanche Photodiodes (APDs), whose
internal gain can help to improve signal
In this talk, development of GaAs X
ray photodiodes carried out at Sheffield will be presented.
of different diode configurations will be shown. Design guidance for X
ray APDs, based on both experimental
work and avalanche gain simulation will also be discussed.
Single photon avalanche photodiodes (SPADs) are “binary” devices that can det
ect very weak optical signals
at single photon level. With single photon detection being central to applications such as Quantum
resolved Fluorescence, and Non
invasive assessment of VLSI circuits, and other
technologies for single photo
n detection being rather impractical, SPADs are much in demand.
The performance of a SPAD is however heavily influenced by its quenching circuit, which serves the
purpose of quenching SPAD’s avalanche current once it has been detected and delivering a
reverse bias pulses to the SPAD. Different types of quenching circuits, including the Capacitive Quenching
Circuit developed at Sheffield, can be found in literature. The talk will summarize the pros and cons of the most
popular type of quenchin
g circuits, i.e. gated mode passive quenching circuit, before describing the operation of
Capacitive Quenching Circuit. Results obtained using the two types of circuit will also be compared.
Shien Ng obtained her BEng and PhD degrees in Electronic
Engineering in 1999 and 2003,
respectively, from the Department of Electronic and Electrical Engineering (EEE), The University Of
Sheffield (TUOS). From 2003 to2006, she was with the National Centre for III
V Technologies at
TUOS. She was responsible for
the Material Assessment area within the centre. She became a Royal
Society University Research Fellow in 2006 and has recently been appointed to the position of Senior
Lecturer Elect at the TUOS. Her research interests include X
ray and near infrared photo
avalanche photodiodes, and Single Photon Avalanche Photodiodes. To date her work has been published
in 41 peer
reviewed international journal papers and have received funding from a variety of sources,
including the Royal Society, UK research counc
ils, UK Ministry of Defence, European Union
Frameworks, and European Space Agency.