A set of (p) a‐Si:H/(i) a‐Si:H/(n) c‐Si heterostructures is investigated by coplanar conductance measurements. The thickness of the (i) a‐Si:H buffer layer is varied between 2 and 50 nm, well beyond the values used in heterojunction solar cells. The change in this thickness plays a role on band bending at the heterointerface and therefore impacts the level of inversion of carrier population at the c‐Si surface. Measurements are compared with 1D analytical calculations and 2D electrical modeling. It is demonstrated that the deep defect density, related to silicon dangling bonds, in the (i) a‐Si:H layer strongly increases from 1 × 1017 to 4 × 1018 cm−3 when the (i) a‐Si:H layer thickness is decreased from 50 to 2 nm. This result is interpreted in terms of defect formation and dependence of the defect density upon the position of the Fermi level with respect to the valence band edge. Quantitative analysis in the framework of the defect‐pool model demonstrates that the strong increase of defect density is also promoted by an increase in the width of the valence band tail in the thin (i) a‐Si:H layer, suggesting that a very thin layer also suffers from increased disorder.