Kindly go through the following literature

Sensors (Basel). 2010; 10(3): 2088–2106.

Published online 2010 Mar 15. doi: 10.3390/s100302088

PMCID: PMC3264469

Metal Oxide Gas Sensors: Sensitivity and Influencing Factors

Chengxiang Wang, Longwei Yin,* Luyuan Zhang, Dong Xiang, and Rui Gao

There is a very good with explication of types of heterojunctions and related mechanisms: http://www.sciencedirect.com/science/article/pii/S0925400514009149

However, in the most cases the selectivity depend more on type of added material/dopant, i.e. on its catalytic effect or on type of heterojunction (p-p, n-n, n-p) which have essential influence of electronic properties at the interface (on electron depletion region in the case of n-type materials or hole accumulation layer in the case of p-type materials).

Another interesting work on hybrid material with modulation of selectivity is http://pubs.acs.org/doi/abs/10.1021/acsami.6b11337

Thank you very much

Gas sensing is a surface morphological phenomenon. So yes the selectivity of the gas depends upon the heterojunction.In fact, it depend on many factors  like preparation method, sintering temperature, crystallite size, temperature of the heater, dopant, doping concentration, effect of catalyst, thickness of the film, and type of the material whether its p- type or n-type, and even on the layering of the material on the substrate.