MMBT3904LT1G

The MMBT3904LT1G is a 200 mA, 40 V NPN general-purpose bipolar junction transistor manufactured by onsemi, housed in the industry-standard SOT-23-3 (TO-236) surface-mount package. It is the SMD equivalent of the classic 2N3904 through-hole transistor and is one of the most widely used small-signal NPN transistors in electronics.

Key specifications include a collector-emitter breakdown voltage (VCEO) of 40 V, collector-base voltage (VCBO) of 60 V, emitter-base voltage (VEBO) of 6 V, and continuous collector current of 200 mA. The device achieves a DC current gain (hFE) ranging from 40 to 300 depending on operating point (100 minimum at IC = 10 mA, VCE = 1 V), collector-emitter saturation voltage (VCE(sat)) of 0.2 V maximum at IC = 10 mA, and transition frequency (fT) of 300 MHz minimum. Output capacitance (Cobo) is 4.0 pF and input capacitance (Cibo) is 8.0 pF.

Switching characteristics include delay time of 35 ns, rise time of 35 ns, storage time of 200 ns, and fall time of 50 ns. Total power dissipation is 225 mW on FR-5 board with thermal resistance of 556°C/W (junction-to-ambient). The device operates over a junction temperature range of -55°C to +150°C.

The MMBT3904LT1G is Pb-free, halogen-free/BFR-free, RoHS compliant, and AEC-Q101 qualified with PPAP capability. The LT1G suffix denotes tape and reel packaging with 3,000 units per reel. MSL-1 rated (unlimited floor life). The top mark is typically “1AM”.

The MMBT3904LT1G operates as an NPN bipolar junction transistor (BJT), using current-controlled current amplification to switch or amplify signals.

Active Mode: When the base-emitter junction is forward-biased (VBE > 0.65 V typical) and the base-collector junction is reverse-biased, the transistor operates in the active region. A small base current (IB) controls a much larger collector current (IC) through current gain (hFE = IC/IB). With hFE ranging from 40 to 300, the MMBT3904LT1G can amplify small signals or drive moderate loads with minimal base drive.

Saturation Mode: When both junctions are forward-biased (VCE drops to VCE(sat) ≈ 0.2 V), the transistor is fully on, acting as a closed switch between collector and emitter. The low saturation voltage minimizes power dissipation in switching applications. To ensure hard saturation, the base current should exceed IC/hFE by a factor of 2-10 (overdrive factor).

Cutoff Mode: With the base-emitter junction reverse-biased or at zero bias, both junctions are reverse-biased and only tiny leakage currents flow (ICEX < 50 nA). The transistor acts as an open switch.

Switching Behavior: Transition between cutoff and saturation is governed by charge storage in the base region. Turn-on delay (35 ns) and rise time (35 ns) are fast due to rapid charge injection. Turn-off involves removing stored base charge, with storage time (200 ns) dominating the total turn-off period. Fall time is 50 ns. The 300 MHz transition frequency indicates the gain-bandwidth product, beyond which current gain drops below unity.

Small-Signal Operation: In the active region, the transistor presents an input impedance (hie) of 1.0-10 kΩ, voltage feedback ratio (hre) of 0.5-8.0 × 10⁻⁴, small-signal current gain (hfe) of 100-400, and output admittance (hoe) of 1.0-40 μmhos at IC = 1 mA. These parameters make it suitable for low-frequency amplification stages.

Thermal Behavior: The VBE temperature coefficient is approximately -2 mV/°C, meaning the base-emitter voltage decreases with rising temperature. This characteristic must be accounted for in bias circuit design to maintain stable operating points across temperature.