Betavoltaic Cells are analagous to solar cells, with e-h pairs created by beta radiation instead of light. These e-h pairs are then collected and separated by a pn junction giving rise to usable power. The long half-lives of several soft beta sources allow the realization of long-lived easily packaged beta-powered batteries.
My PhD thesis was on SiC betavoltaics. You can get a copy of it here.
For more information, please see the following publications:
3. MVS Chandrashekhar et. al, Appl. Phys. Lett., 91, 062731 (2007)…in press