The noise measurements show three distinct regions in the spectra, the first being the low frequency, 1/f, excess noise which has a corner frequency of 500Hz for bias voltages below 75V. Above this the excess noise has a second component which is steeper, being almost 1/f3 in nature, and has a corner frequency of 50Hz which increase with bias. As the diode approaches breakdown the corner frequency and the magnitude of the excess noise increases dramatically due to injection at the forward biased contact. The second feature of the spectra is the lower than expected value of the shot noise, being about half that of simple theory. This is due to trapping of the charge carriers in the bulk which reduces the amplitude of the individual pulses that the carriers produce while increasing their frequency. The third region is the corner frequency of the white noise between 20-30kHz. This is noted to be close to the reciprocal of the dielectric relaxation time for SIU-GaAs.
Further noise measurements need to be made on more samples using different types of contacts to see if there is any correlation between the noise of the sample and the properties of the material and fabrication. Diodes that have been irradiated also need to be tested to improve the present understanding of the effects of irradiation. The technique can also be extended to examine strip detectors made from GaAs. The biasing structure used at present utilizes a punch-through mechanism via a MSM diode. There is a possibility that this may introduce more noise and it is important to understand if this is true.