I believe they are intended for current mode use but with no reverse bias voltage. Often photodiodes are reverse biased and used as a current source for speed but large area photodiodes can have a high leakage at anything other than zero reverse bias so you don't reverse bias the but still use them as a current source. I have done that in the past for fluoroscopy with 25mm square photodiodes. Look at the reverse current versus voltage graphs and you should get the idea.
Keith