We present the results of a systematic study of the dependence of intersubband infrared detector performance on the subband energies. By using samples cut from different positions on a single wafer grown with a known thickness variation it is possible to obtain a set of devices for which the well width varies in a controlled manner, but the barrier height remains fixed. Current versus voltage and responsivity measurements demonstrate the importance of the detector performance on the exact location of the upper state: optimum peak response is obtained when the subband is resonant with the top of barrier.