A layer of single crystal CVD diamond having a thickness of greater than 2 mm, wherein the layer has one or more of the following characteristics: (i) a high charge collection distance at 300 K of at least 100 μm measured at an applied field of 1 V/ m; (ii) a high value for the product of the average carrier mobility and lifetime μτ such that it exceeds 1.0 x 10 -6 cm 2 /V at 300 K; (iii) an electron mobility (μ e) measured at 300 K greater than 2400 cm 2 V -1 s -1; (iv) a hole mobility (μ h) measured at 300 K greater than 2100 cm 2 V -1 s -1; and (v) in the off state, a resistivity at 300 K greater than 10 12 Ωcm at an applied field of 50 V/μm. and in addition one or more of the following characteristics: (a) a level of any single impurity of not greater than 1 ppm and a total impurity content of not greater than 5 ppm where impurity excludes hydrogen and its isotopic forms; (b) a photoluminescence line related to the cathodoluminescence line at 575 nm, measured at 77 K under 514 nm Ar ion laser excitation (nominally 300 mW incident beam), which has a peak height <1/25 of the diamond Raman peak at 1332 cm -1; (c) a strong free exciton emission, the strength of the free exciton emission excited by 193 nm ArF excimer laser at room temperature is such that the quantum yield for free exciton emission is at least 10 -5; (d) in electron paramagnetic resonance, a single substitutional nitrogen centre [N-C]° at a concentration < 100 ppb; (e) in EPR, a spin density < 1 x 10 17 cm -3 at g=2.0028. (i) a high charge collection distance at 300 K of at least 100 μm measured at an applied field of 1 V/ m; (ii) a high value for the product of the average carrier mobility and lifetime μτ such that it exceeds 1.0 x 10 -6 cm 2 /V at 300 K; (iii) an electron mobility (μ e) measured at 300 K greater than 2400 cm 2 V -1 s -1; (iv) a hole mobility (μ h) measured at 300 K greater than 2100 cm 2 V -1 s -1; and (v) in the off state, a resistivity at 300 K greater than 10 12 Ωcm at an applied field of 50 V/μm. and in addition one or more of the following characteristics: (a) a level of any single impurity of not greater than 1 ppm and a total impurity content of not greater than 5 ppm where impurity excludes hydrogen and its isotopic forms; (b) a photoluminescence line related to the cathodoluminescence line at 575 nm, measured at 77 K under 514 nm Ar ion laser excitation (nominally 300 mW incident beam), which has a peak height <1/25 of the diamond Raman peak at 1332 cm -1; (c) a strong free exciton emission, the strength of the free exciton emission excited by 193 nm ArF excimer laser at room temperature is such that the quantum yield for free exciton emission is at least 10 -5; (d) in electron paramagnetic resonance, a single substitutional nitrogen centre [N-C]° at a concentration < 100 ppb; (e) in EPR, a spin density < 1 x 10 17 cm -3 at g=2.0028. (a) a level of any single impurity of not greater than 1 ppm and a total impurity content of not greater than 5 ppm where impurity excludes hydrogen and its isotopic forms; (b) a photoluminescence line related to the cathodoluminescence line at 575 nm, measured at 77 K under 514 nm Ar ion laser excitation (nominally 300 mW incident beam), which has a peak height <1/25 of the diamond Raman peak at 1332 cm -1; (c) a strong free exciton emission, the strength of the free exciton emission excited by 193 nm ArF excimer laser at room temperature is such that the quantum yield for free exciton emission is at least 10 -5; (d) in electron paramagnetic resonance, a single substitutional nitrogen centre [N-C]° at a concentration < 100 ppb; (e) in EPR, a spin density < 1 x 10 17 cm -3 at g=2.0028.