onsemi MMBT3904K

Overview

MMBT3904K from Fairchild Semiconductor is a general-purpose NPN epitaxial silicon transistor in SOT-23 package, rated for 40 V V_CEO, 200 mA I_C, 350 mW P_C, with h_FE = 100 at I_C = 10 mA and f_T = 300 MHz - widely used in low-power switching and amplification circuits in consumer power supplies and signal conditioning stages.

For engineers reviewing the MMBT3904K datasheet, pinout, applications, or equivalent options, key selection criteria include V_CEO/I_C derating at elevated temperature, saturation voltage behavior under defined I_B/I_C ratios, noise figure at audio frequencies, and SOT-23 thermal resistance limitations in high-duty-cycle operation.

Technical Context

This discrete NPN transistor employs epitaxial base construction to achieve consistent DC current gain across operating conditions and supports fast switching with t_ON = 70 ns and t_OFF = 250 ns under specified bias conditions. Its Cob = 4 pF at V_CB = 5 V enables stable performance in RF-coupled preamp stages up to ~100 MHz.

The device operates across –55 °C to +150 °C junction temperature range and exhibits BVEBO = 6 V, limiting base-emitter reverse bias tolerance. V_BE(sat) rises from 0.65 V to 0.95 V as I_C increases from 10 mA to 50 mA at fixed β = 10, indicating predictable base drive requirements in saturated switch designs.

Key Specifications

Pinout & Package

SOT-23 plastic surface-mount package with gull-wing leads; 1.90 mm × 1.30 mm footprint, 1.14 mm max height, and standard 0.95 mm lead pitch. Marking "1AK" identifies MMBT3904K on top surface.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN general-purpose transistor applications.

Compared with BC847BW and PN2222A, the MMBT3904K offers optimal balance of SOT-23 compactness, predictable gain at 10–100 mA, and proven thermal performance in high-density PCB layouts - making it preferred for volume production of consumer and industrial embedded controls.

Availability

MMBT3904K is available at Aetrix Electronics and suitable for LED driver circuits, microcontroller interface stages, audio pre-amplifiers, and power supply enable switches requiring stable component supply and long-term manufacturability.

Supply support for MMBT3904K includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Fairchild Semiconductor was a U.S.-based semiconductor company specializing in power management, analog, and discrete devices before its acquisition by ON Semiconductor in 2016.

The MMBT3904K belongs to Fairchild's legacy general-purpose bipolar transistor product line, designed for cost-sensitive, high-reliability linear and switching applications in consumer, computing, and industrial electronics.

FAQ

What is the maximum continuous collector current rating for MMBT3904K?

The MMBT3904K has a maximum continuous collector current (I_C) rating of 200 mA at T_A = 25 °C. This rating decreases linearly with increasing ambient temperature, as shown in Figure 6 of the datasheet. At 100 °C ambient, the derated I_C is approximately 120 mA. The MMBT3904K must be operated within this thermal envelope to avoid junction overheating and premature failure.

Does MMBT3904K support operation in linear (active) mode for analog amplification?

Yes, the MMBT3904K is characterized for linear operation with specified h_FE, V_BE(sat), and noise figure across multiple collector currents. Its f_T = 300 MHz and NF = 5 dB at audio frequencies confirm suitability for small-signal amplification. The MMBT3904K is commonly used in emitter-follower buffers and common-emitter preamps where predictable gain and low distortion are required.

What is the pin configuration of MMBT3904K in the SOT-23 package?

The MMBT3904K uses standard SOT-23 pinout: Pin 1 = Base, Pin 2 = Emitter, Pin 3 = Collector. This matches JEDEC TO-236AB outline and is compatible with automated pick-and-place equipment. The top-marking "1AK" is laser-etched adjacent to Pin 1, enabling visual orientation verification during manual inspection or rework of the MMBT3904K.

How does the saturation voltage of MMBT3904K vary with operating conditions?

The MMBT3904K specifies V_CE(sat) = 0.2 V at I_C = 10 mA / I_B = 1 mA and 0.3 V at I_C = 50 mA / I_B = 5 mA. This increase reflects rising base resistance and carrier injection effects at higher currents. Designers should verify V_CE(sat) at actual operating I_C/I_B using the curves in Figure 2 of the MMBT3904K datasheet to ensure sufficient voltage headroom remains across the load.

Is MMBT3904K suitable for automotive applications?

The MMBT3904K is not AEC-Q101 qualified and lacks automotive-grade screening, traceability, or extended temperature validation beyond its specified –55 °C to +150 °C junction range. While it may function in non-safety-critical automotive subsystems (e.g., interior lighting), the MMBT3904K is intended for industrial and consumer applications. For automotive use, designers should select AEC-Q101-compliant alternatives such as the NSS40201MR6T1G.