Most biometric devices would be input devices although they may
also utilize storage devices and output devices in their
functioning.
A biometric device usually takes in biometric information about
a person and compares it to information stored in a database - much
like username/password methods of authentication compare the
username and password to the stored information for a user.
Fingerprint scanners, read the fingerprint, convert it to a data
stream and then compare it to the stored data previously recorded
for the individual, retinal scanners read information about the
retinal patterns of a user, convert it to data and compare it to
stored data, etc. Note that biometric devices must rely on a
storage device, but usually the storage device is not part of the
biometric device. Instances of OUTPUT associated with a biometric
device might be a signal to open a door or trigger an alarm, but
again, the output usually comes from another device that the
biometric device is connected to.
