onsemi has marked the 4N25FR2M as obsolete. Quantities are lot-specific and confirmed at RFQ; no stock-holding claim is made here.
Minimum current transfer ratio of 20% at 10 mA forward current means the output transistor conducts at least 2 mA collector current when the LED is driven at 10 mA — enough to saturate a TTL or CMOS logic input without an additional gain stage. Typical turn-on and turn-off times of 2 µs each set the switching ceiling at roughly 250 kHz — adequate for 24 VDC digital input modules sampling at 1 kHz or for isolated serial data links running at 115.2 kbaud.
Supplied in a 6-SMD Gull Wing package on Tape & Reel — the 0.050-inch (1.27 mm) lead pitch and gull-wing footprint are standard for automated pick-and-place assembly. The 6-SMD supplier device package is the same body as the DIP-6 but with formed leads for surface mounting. Operating temperature range of -55°C to 100°C covers industrial and outdoor enclosure environments — the lower end suits cold-start conditions in telecom shelters, the upper end stays within the derating curve for most control cabinet designs.