Using Hot Wire Chemical Vapor Deposition (HWCVD), also known as thermocatalytic decomposition, heterogeneous silicon thin films can be obtained with a widely varying degree of order and crystallinity. Specific parameter regimes have been identified which allow the deposition of films with a structure ranging from purely amorphous to fully polycrystalline. In polycrystalline Si, all hydrogen appears in isolated, compact monohydride bonds. A comparison of XTEM and Raman studies confirmed that the 2000 cm-1 infrared mode indeed originates from a completely crystalline region and that there is no amorphous tissue in these films. The 2000 cm-1 vibration is due to Si-H bonds at completely coalescent crystal faces (between adjacent crystals). In films with this type of crystallinity, oxygen incorporation is greatly reduced, both during growth and after completion. The heterogeneous growth has been utilized in two types of devices, thin film transistors (TFTs) and thin film solar cells. TFTs have been made exhibiting excellent stability. The field-effect channel of these transistors consists of amorphous silicon hosting nanocrystalline domains which yields TFTs with a high mobility of 1.5 cm2 V-1s-1, virtually without the usual threshold voltage instabilities. Solar cells with an intrinsic poly-Si absorber layer have also been further optimized by deliberately profiling the active layer. A stepwise profiling sequence has been developed, starting from immediate-nucleation growth of small random crystallites to continued singly oriented growth of columnar polycrystalline material at a deposition rate of 5 Å/s. These n-i-p solar cells on stainless steel substrates presently have 4.41 % conversion efficiency. The short circuit current density is as high as 19.95 mA/cm2 while the light absorbing i-layer is only 1.2 µm thick and no enhanced back reflector is used.