Neutron Transmutation Doped (NTD) monocrystalline silicon is grown by irradiating undoped Float Zone silicon with neutrons. During irradiation and subsequent annealing of the ingots silicon atoms Si₃₀ is converted into P₃₁ which is a n-type doping. By controlling the dose and the width of the irradiating neutron beam the resistivity variation can be kept at record low values over a large range of bulk resistivities ranging from 10 Ohm-cm to 4000 Ohm-cm. No other method of producing monocrystalline silicon can produce these low resistivity variations over the whole of the crystal. Topsil have been selling NTD products for over 30 years and have the largest selection of NTD products on the market.