Staircase solid-state photomultiplier and avalanche photodiodes with enhanced ionization rates ratio

The theory of the staircase avalanche photodiode (APD) is presented, and recent results obtained with a new class of APDs having an enhanced ratio of ionization coefficients are surveyed. A general expression is given for the excess noise factor F in terms of the number of stages and the multiplication per stage. For high ionization yields per dynode, the F factor is near unity regardless of the number of stages; this is seen as implying virtually noise-free multiplication at high gain, as in the case of a photomultiplier. Also presented is a comparison of the noise behavior of the staircase APD with that of a phototube. A microscopic theory of the ionization yield gamma is considered, and it is pointed out that to obtain a high gamma, the electrons must approach the dynode with energy of the order of 10 times the optical phonon energy. Attention is also given to the possible problem of residual hole-initiated ionization. Formulas are derived for the electron and hole-initiated multiplications, and it is shown that from a measurement of these quantities, the ionization yield and the residual hole ionization coefficient can be obtained directly.