BSN20

The BSN20 from Nexperia (formerly NXP/Philips Semiconductors) is an N-channel enhancement mode small-signal MOSFET in a subminiature SOT-23 surface-mount package, using TrenchMOS technology. It is designed for low-power switching applications where a small footprint, logic-level gate drive, and fast switching speed are required.

The BSN20 occupies a niche in the MOSFET landscape as a small-signal, low-current device with relatively high on-resistance (RDS(on) of 15 ohm maximum at VGS=10V). While this RDS(on) is far too high for power switching applications, it is perfectly adequate for driving small loads such as relay coils, LED indicators, and digital logic lines. The high RDS(on) is a consequence of the small die size required to fit in the tiny SOT-23 package while maintaining a 50V drain-source rating.

The TrenchMOS technology used in the BSN20 provides a vertical current flow path through trench-gate structures, achieving better RDS(on) per unit area than planar MOSFET technology. This allows the BSN20 to offer 173mA continuous drain current and 15 ohm RDS(on) in a die that fits within the SOT-23 package. Without TrenchMOS, a comparable planar device would have either higher RDS(on) or lower current rating.

The logic-level compatibility is a key feature. The threshold voltage (VGS(th)) is typically 1.0V, meaning the device can be turned on with gate voltages as low as 2-3V. This makes it directly compatible with 3.3V and 5V logic outputs from microcontrollers, FPGAs, and other digital ICs, without requiring a gate driver. At VGS=5V, the RDS(on) is 20 ohm maximum, which is sufficient for driving relay coils (typically 50-500 ohm) and small indicators.

The very fast switching speed makes the BSN20 suitable for high-speed digital applications such as level translation and line driving. The small gate charge (Qg typically less than 1nC) allows the device to switch in tens of nanoseconds when driven from a low-impedance source. This is much faster than bipolar transistors, which have storage time delays that limit switching speed.

The BSN20 has a long history, originally designed by Philips Semiconductors and later transferred to NXP and then Nexperia as part of the standard products divestiture. The original BSN20 (VDS=50V) is still listed in some catalogs, but Nexperia currently promotes the BSN20BK (VDS=60V, improved ESD protection) as the production version. The BK variant is electrically similar but offers a higher voltage rating and integrated gate-source ESD protection diodes (2kV HBM), making it more robust in handling and assembly.

The SOT-23 marking for the original BSN20 is M8p (with the p being a Philips/Nexperia manufacturing code). The BSN20BK has a different marking (%4S). When replacing a BSN20 with a BSN20BK, the designer should verify that the higher VDS rating (60V vs 50V) and slightly different RDS(on) characteristics are acceptable for the application.

The BSN20 operates as an N-channel enhancement mode MOSFET using TrenchMOS vertical structure technology.

TrenchMOS Structure: The BSN20 uses a trench-gate MOSFET structure where vertical trenches are etched into the silicon and lined with gate oxide and gate electrode (polysilicon). The channel is formed on the vertical sidewalls of the trench when a positive gate voltage is applied. This vertical structure provides several advantages over planar MOSFETs: (1) higher channel density per unit area, reducing RDS(on); (2) shorter current path, reducing parasitic resistance; (3) better scaling to smaller die sizes.

N-Channel Enhancement Mode: As an N-channel enhancement mode MOSFET, the BSN20 is normally off when VGS = 0V. No conductive channel exists between the drain and source. When a positive voltage is applied to the gate (VGS > VGS(th), typically 1.0V), an inversion layer of electrons forms at the silicon surface beneath the gate oxide, creating an N-type conductive channel from source to drain. Current flows from drain to source (conventional current direction) when VDS is positive.

RDS(on) vs Gate Voltage: The RDS(on) varies significantly with gate voltage because the channel conductivity depends on the strength of the inversion layer. At VGS=10V (full enhancement), RDS(on) is 15 ohm maximum. At VGS=5V (logic level), RDS(on) increases to 20 ohm maximum. At lower gate voltages near the threshold, RDS(on) increases dramatically. For reliable switching, VGS should be at least 2-3 times VGS(th) to ensure low RDS(on) and consistent switching behavior.

Small-Signal Operation: The BSN20 is a small-signal MOSFET, meaning it is designed for low-current applications (173mA maximum). The small die size limits the current handling and results in relatively high RDS(on) compared to power MOSFETs. However, the small die also means low gate charge (typically less than 1nC), enabling very fast switching. The low gate charge allows the MOSFET to be driven directly from digital logic outputs without a gate driver, as the logic output can supply the small gate charge quickly.

ESD Protection (BSN20BK): The BSN20BK variant includes integrated gate-source ESD protection diodes rated for 2kV HBM. These diodes clamp the gate-source voltage during ESD events, preventing gate oxide rupture. The original BSN20 may have limited or no ESD protection. The ESD diodes add a small amount of gate-source capacitance but do not significantly affect normal switching performance.

Body Diode: Like all MOSFETs, the BSN20 has an inherent body (parasitic) diode from source to drain (for N-channel). This diode is forward-biased when VDS is negative (drain below source). In switching applications, the body diode can carry the freewheeling current from inductive loads. The body diode forward voltage is approximately 0.7-1.0V, and the reverse recovery time is not specified for this small-signal device.