the change in the resistance by doping related to many factor:type and amount of dopant is the main factor.if your base material is a n-type semiconductor, your dopant will be an electron donor in the case of decreasing resistance.
the amount of dopant is also important, you may have a nanocomposite sensor that the work function and the fermi level of two components at hetrostructures determine electron accepting or electron donating of the base material.
Additives / Dopant, affect chemical and electronic properties of the material.
This modification reflects a change or alteration in order to enhance material or device features.
Modifying the microstructure of the material in the case of semi-conductor gas sensors implies the use of additives to create new active centers in relation to certain gases. May be this link will help you...
Effect of Modification/Doping on Gas Sensing Properties of SnO2