Low noise avalanche photodiodes

The maximum useful gain in avalanche photodiodes (APDs) is limited by the noise associated with the random fluctuations of the avalanche process which increase with multiplication. The conventional McIntyre theory relates this excess noise F, to the value of the mean multiplication, M and the ratio of the hole to electron ionization coefficients (β and α respectively), k. For electron initiated multiplication the excess noise factor is: F(M)=kM+(2-1/M)(1-k). The lowest noise is obtained when k is very small or very large and the carrier type with the larger ionization coefficient initiates the multiplication. As k tends to unity at high electric fields, conventional III-V APDs use thick avalanching regions to reduce the field and maximise the α/β ratio, but at the expense of low speed and high operating voltage. Recent work has shown that low excess avalanche noise can be obtained very simply by operating with thin avalanche multiplication regions. In such structures the electric field is considerably higher than in thick, bulk structures and this is found to be mainly responsible for the reduction in noise observed