A numerical study of the blocking characteristics of the Permeable Base Transistor (PBT) is presented. The PBT is regarded as a promising transistor structure for high voltage and high frequency applications. Numerical studies of the PBT were focused on the high frequency figure of merits or the breakdown characteristics of the Schottky gate. A device designed for high frequency and high voltage switching should be optimized for large blocking and fast switching. The trade off between blocking and speed is a complicated matter which depends strongly on the geometry and doping level. In this work we studied the blocking characteristics for a Silicon PBT with regard to the doping level and doping profile, gate thickness and gate to drain distance. A scaling formalism was developed in order to estimate the transistor performance for a wide range of doping levels and geometrical combinations. A design example is presented of a normally off transistor that can block a drain to source voltage of 10 V while the unity current gain frequency fT value for Vgs = 0.2 V is higher than 7 GHz.