Proton–deuteron identification at energies between 2.5MeV and 6MeV has been studied as a function of the detector working bias. Digital pulse shape analysis (DPSA) has been used to perform the separation from the two mono-energetic beams. The technique makes use of the current signal delivered by a 500 mm neutron transmutation doped (NTD) silicon detector, which was set up for low-field injection. It is shown that identification of the H isotopes is better when the detector working bias is close to the depletion voltage rather than over-depletion. The presence of high frequency noise diminished the possibility of identification, however, the use of a simple triangular smoothing algorithm counteracted this.