Absorption cross section of RN and SdS extremal black hole

The nature of scalar wave functions near the horizon of Reissner Nordstrom (RN) extremal and Schwarzschild-de Sitter (SdS) extremal black holes are found using WKB approximation and the effect of reflection of waves from the horizon. The absorption cross section $\sigma_{abs}$ when RN extremal and SdS extremal black holes placed in a Klein-Gordon field is calculated.