I think there are two types of Denso A/F sensors. The older type (Denso refers to it as a "cup type") is used on older new age WRXes. I think the newer type (Denso refers to it as a "planar") is used on STIs. The cup type sensor is only accurate to about 12:1 (assuming 14.5:1 is stoichiometric) whereas the planar sensor is accurate to about 10:1.
I'm not aware of a spec sheet for these sensors, but there is an SAE paper which describes the planar sensor and compares it with the cup sensor at
http://www.denso.co.jp/DTR/vol7_no1/dissertation5-i.pdf. As you can see, the sensor's signal is current flow. I believe a sensor controller subsystem in the ECU translates that current to a voltage which is what the main ECU processor sees.
Here's a figure from that paper which shows the response curves of the two types of sensors: