BAT54

The BAT54 is the small-signal Schottky barrier diode of the surface-mount era: a 30 V, 200 mA device in the SOT-23 package, marked L4P and shipped by onsemi (formerly Fairchild) in reels of 3,000 pcs. Its metal-semiconductor junction gives it a forward drop of 320 mV maximum at 1 mA and 400 mV at 10 mA — roughly half of a silicon switching diode — combined with a 5 ns maximum reverse recovery time and only 10 pF of junction capacitance at 1 V. That combination of low voltage drop, low charge and low capacitance is exactly what clamping, steering and detection circuits need at signal levels.

The full onsemi specification reads: 30 V maximum repetitive reverse voltage with a 30 V minimum breakdown at IR = 10 µA, 200 mA average rectified forward current, 600 mA non-repetitive peak forward surge current, maximum forward voltage of 240 mV at 0.1 mA, 320 mV at 1 mA, 400 mV at 10 mA, 500 mV at 30 mA and 0.8 V at 100 mA, reverse leakage of 2 µA maximum at VR = 25 V, 10 pF maximum total capacitance at VR = 1 V and 1 MHz, 5 ns maximum reverse recovery measured at IF = IR = 10 mA with IRR = 1 mA and RL = 100 Ω, 290 mW package dissipation with 430 °C/W junction-to-ambient thermal resistance, and a -55 °C to +150 °C junction range. Note that the single BAT54 occupies a three-lead SOT-23 outline but uses only two of those leads — the third is internally unconnected, so the footprint stays compatible with the BAT54A, BAT54C and BAT54S variants.

Designers should respect two limits. The 30 V rating keeps the BAT54 on 3.3 V and 5 V rails and low-voltage RF; it is not a mains or 24 V part. And 2 µA of leakage at 25 V, which grows steeply with temperature and reverse bias, is far too much for high-impedance precision nodes — for sample-and-hold amplifiers and picoamp front ends use a low-leakage pair such as the BAV199 or BAS416 instead. Where a low-drop clamp on an ADC input, an RF detector or a logic steering network is needed, the BAT54 remains the cheapest and most available answer.

Supply is effectively unlimited: onsemi BAT54LT1G, Nexperia BAT54,215, Diodes Incorporated BAT54-7-F, STMicroelectronics, Vishay, ROHM, PANJIT and MCC all produce the part, and pricing is fractions of a cent in reel quantities. Counterfeit risk is modest but the failure mode is subtle — remarked BAT54A, BAT54C or BAT54S devices (markings L42, L43 and L44) that share the footprint but have completely different internal connections, plus silicon substitutes that read 0.6 V instead of 0.3 V. Verify the L4P marking under magnification, measure forward drop at 1 mA, and sample-check capacitance and recovery on incoming reels.