An apparatus comprising a chemical field effect transistor array in a circuit-supporting substrate is disclosed. The transistor array has disposed on its surface an array of sample-retaining regions capable of retaining a chemical or biological sample from a sample fluid. The transistor array has a pitch of 10 μm or less and a sample-retaining region is positioned on at least one chemical field effect transistor which is configured to generate at least one output signal related to a characteristic of a chemical or biological sample in such sample-retaining region.