Photocurrent, Gain, and Responsivity

If /p of (4.3) is a photosignal current then the current responsivity of the detector is 

In principle one could choose for a photon detector any material having an electronic excitation energy satisfying the requirement 

In practice, however, should satisfy the equality of (4.5) as closely as possible, both to maximize the operating temperature of the detector and to minimize the amount of background radiation limiting the performance of the detector. Thus near-satisfaction of the equality of (4.5) is the first condition on a photon detector material. 

In practice, however, should satisfy the equality of (4.5) as closely as possible, 

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