A simple Monte Carlo model has been developed to simulate the avalanche multiplication process in In0.52Al0.48As. The model reproduces avalanche multiplication and excess noise factor measured on a wide range of In0.52Al0.48As p+-n-n+, n+-n-p+, and p+-n+ diodes and confirms that very low excess noise factor can be obtained using pure electron injection in very thick diodes with avalanche region greater than 2.21 μm or in very thin diodes with avalanche region lesser than 0.11 μm. In addition we investigated the effect of an electric field gradient in the avalanche region of avalanche photodiodes and found that the excess noise factor can be reduced with electric field gradients. However in thin diodes with avalanche region lesser than 0.20 μm, the onset of tunneling current negates the excess noise reduction achieved using the electric field gradient. Therefore ideal p+-i-n+ diodes still provide the overall preferred structure.

New types of detectors based on the wide band gap material AlGaAs have been developed for soft X-ray spectroscopy applications. We report on the spectroscopic performance of simple p-i-n diodes and avalanche photodiodes (APDs). A number of diode types with different layer thicknesses have also been characterised. X-ray spectra from 55 Fe and 109 Cd radioactive sources show these diodes can be used for spectroscopy with promising energy resolution (1.0-1.25 keV) over a -30 to +90 • C temperature range. The temperature dependence of the avalanche multiplication process at soft X-ray energies in Al 0.8 Ga 0.2 As APDs was also investigated at temperatures from -20 to +80 • C. The temperature dependence of the pure electron initiated multiplication factor (M e ) and the mixed carrier initiated avalanche multiplication factor (M mix ) were extracted from the X-ray spectra. The experimental results are compared with a spectroscopic Monte Carlo model for Al 0.8 Ga 0.2 As diodes from which the temperature dependence of the pure hole initiated multiplication factor (M h ) is determined.Monte Carlo simulations for the avalanche gain of absorbed X-ray photons have also been developed to study the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes. The model showed that the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and hence the photon energy), the relationship between the two ionization coefficients, and the overall mean

Impact ionization involving transport across a single heterojunction has been studied by measuring the electron and hole initiated multiplication, Me and Mh, in a series of p+in+ AlxGa1−xAs(500 Å)/GaAs(500 Å) heterostructures with x ranging from 0.3 to 0.6. At low electric fields, because of dead space effects, Me and Mh in these devices are very different and are primarily determined by the ionization properties of the material in the latter half of the structure. As the electric field increases, feedback from the opposite carrier type causes Me and Mh to converge to the values measured in the equivalent alloy. The effects of the band-edge discontinuities at the heterojunction interface on Me and Mh in these heterostructures are compensated by the different energy-loss rates in AlxGa1−xAs and GaAs. A simple Monte Carlo model using effective conduction and valence bands is used to interpret the experimental results.

The electron and hole multiplication characteristics, M e and M h , respectively, have been measured in two series of Al x Ga 1Ϫx As (xϭ0.15 and 0.30͒ p-i-n diodes where the i-region thicknesses, w, vary from 1.0 down to 0.025 m. From these, the effective electron and hole ionization coefficients, ␣ and ␤, respectively, have been determined and in the thicker structures agreement is found with data published previously in the literature. However, in the devices where w р 0.1 m, the dead space effect reduces multiplication below their bulk values at lower values of bias. As the bias is increased, ␣ and ␤ increase very rapidly suggesting that overshoot effects are compensating for the dead space.

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