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A random path length model for X-ray avalanche photodiodes (APDs) has been used to assess the effects of carriers' dead spaces and injection position on the avalanche gain distributions. Significant carrier's dead space, typically found in submicrometer avalanche regions, is found to reduce spread in the avalanche gain distribution, which is consistent with the reduced excess noise factors observed in conventional APDs. Mixed carrier-type injection increases the spread in the gain distribution along the avalanche region. The model was validated by comparing the results with experimental X-ray spectra from a GaAs/ Al0.8Ga0.2As separate absorption and multiplication APD. Good agreement was achieved between measured and simulated data. Avalanche multiplication shifted the detected photo peak away from the system noise, thereby improving its energy resolution and signal-to-noise ratio.