It is part of the BC817 family, distinguished by its -40 hFE grade — a minimum DC current gain of 250 at 100 mA collector current and 1 V VCE. This gain band makes it a common choice for general-purpose switching and linear amplification in low-power circuits, such as relay drivers, LED drivers, and signal preamplifiers on compact PCBs.
The -40 suffix: why the hFE grade matters
The BC817 family offers two hFE grades: the -25 (minimum gain 160 at 100 mA) and the -40 (minimum gain 250 at 100 mA). The -40 grade provides higher guaranteed gain, which is useful when the base drive is limited — for example, when driving the transistor directly from a microcontroller GPIO without a separate driver stage. If your design relies on a tight gain window, verify that the -40's 250 minimum meets your saturation margin; the -25 variant may not hold up under the same base current.
Obsolete status and sourcing reality
The BC817-40 is supplied in a SOT-23-3 package (also referred to as TO-236-3 or SC-59). It is a surface-mount device — not through-hole — so it suits automated assembly and compact layouts. The SOT-23-3 footprint is widely used across small-signal transistors, which simplifies board-level replacement if a pin-compatible substitute is sourced.