An avalanche photodiode for detecting x-rays and other radiation comprises a first substrate having a portion removed therefrom, a first insulating film formed on the first substrate, a second substrate comprising a floating zone silicon semiconductor substrate disposed on the first insulating film, an impurity region selectively formed in the second substrate at a surface corresponding to the removed portion, a PN junction formed on the second substrate, a glass substrate mounted to the second substrate, a first electrode formed on the first substrate for applying a voltage to the impurity region, a second electrode formed on the second substrate for applying a voltage to the second substrate, a third electrode formed on the glass substrate and electrically connected to the second electrode, and an integrated circuit package having a lead pin connected to the third electrode. Accordingly, a shallow depletion layer may be provided on a floating zone SOI substrate. The substrates may be joined to the glass substrate using a eutectic bonding process.