Percolation model for a selective response of the resistance of composite semiconducting np-systems towards reducing gases

It is shown that a two-component percolation model can explain an experimentally observed behavior, namely that a network built up by a mixture of sintered nanocrystalline semiconducting n- and p-grains can exhibit selective behavior, i.e. respond with a resistance increase when exposed to a reducing gas A and with a resistance decrease in response to another reducing gas B. To this end, a simple model is developed based on realistic assumptions about the reactions on the grain surfaces. The resistance is calculated by random walk simulations with nn-, pp- and np-bonds between the grains and the results are found in very good agreement with the experiments. Contrary to former assumptions, the np-bonds are crucial to obtain this accordance.