Comparison of indium-doped and undoped GaAs minimum ionising particle detectors

Schottky diode particle detectors were fabricated on bulk semi-insulating GaAs to test the material for use as a minimum ionising particle (m.i.p.) detector. Using indium doped LEC GaAs and undoped Vertical Bridgman (VB) GaAs, detectors were fabricated which had large breakdown voltages. A high charge collection efficiency (c.c.e.) of up to 85% was obtained. Commercially available undoped LEC GaAs from different suppliers had lower breakdown voltages and a correspondingly lower c.c.e. was obtained. All the detectors fabricated could be operated at biases up to breakdown and still show spectra with good separation of signal and noise. The indium doped LEC GaAs detectors had a leakage current of 30 nA mm−2 at −1350 V bias and the undoped VB GaAs detectors had a leakage current of 75 nA mm−2 at −1250 V. With the VB material a threshold bias was found which optimised the resolution. At higher bias the c.c.e. kept increasing linearly with bias but the noise level increased more rapidly until it dominated the signal. This increase in noise observed before breakdown was less in the indium doped LEC detectors where the m.i.p. signal was clearly seperated from noise.