FJX3904TF Equivalent & Substitute Parts

Part Overview

The FJX3904TF is an NPN bipolar junction transistor manufactured by onsemi, designed for general-purpose switching and amplification applications. This device features a 40 V collector-emitter breakdown voltage, 200 mA maximum collector current, and 300 MHz transition frequency in a surface mount SC-70 (SOT-323) package. The FJX3904TF is classified as obsolete, making identification of equivalent and substitute parts essential for ongoing design support and production continuity. Active alternatives with matching or superior electrical characteristics are available from onsemi's current product portfolio.

Key Parameters

Parameter	Value	Unit
Transistor Type	NPN	—
Voltage - Collector Emitter Breakdown (Max)	40	V
Current - Collector (Ic) (Max)	200	mA
Vce Saturation (Max) @ Ib, Ic	300 mV @ 5 mA, 50 mA	—
DC Current Gain (hFE) (Min) @ Ic, Vce	100 @ 10 mA, 1 V	—
Power - Max	350	mW
Frequency - Transition	300	MHz
Operating Temperature (TJ)	150	°C
Package / Case	SC-70, SOT-323	—
Mounting Type	Surface Mount	—
Product Status	Obsolete	—

Substitute Part Grouping Explanation

Substitution of the FJX3904TF is determined by strict alignment of electrical and mechanical parameters. The primary substitution criteria are:

Electrical Compatibility Requirements:

Transistor type must be NPN
Collector-emitter breakdown voltage must be ≥ 40 V
Maximum collector current must be ≥ 200 mA
Vce saturation characteristics must be compatible with the specified operating point (300 mV @ 5 mA, 50 mA)
DC current gain (hFE) minimum must be ≥ 100 @ 10 mA, 1 V
Transition frequency must be ≥ 300 MHz
Maximum power dissipation must support the application requirements

Mechanical Compatibility Requirements:

Package must be SC-70 or SOT-323 (surface mount)
Mounting type must be surface mount
Pin configuration must be compatible with SOT-323 footprint

Product Status Consideration:

Active product status is preferred for long-term availability and supply chain continuity
RoHS3 compliance and automotive qualification (AEC-Q101) are advantageous for modern applications

Substitute parts are grouped into two categories: Direct Electrical Equivalents (matching all primary electrical parameters) and Similar Alternatives (meeting minimum requirements with variations in secondary parameters such as current gain or transition frequency).

Parameter Comparison

Part Number	Manufacturer	Ic (Max) mA	Vce(br) V	Vce Sat (Max) mV	hFE (Min)	Freq MHz	Power mW	Package	Status
FJX3904TF	onsemi	200	40	300	100	300	350	SOT-323	Obsolete
SMMBT3904WT1G	onsemi	200	40	300	100	300	150	SOT-323	Active
MMBT3904WT1G	onsemi	200	40	300	100	300	150	SOT-323	Active
MMBT3904TT1G	onsemi	200	40	300	100	300	300	SOT-416	Active
BC847AWT1G	onsemi	100	45	600	110	100	150	SOT-323	Active
BC847BWT1G	onsemi	100	45	600	200	100	150	SOT-323	Active
BC847CWT1G	onsemi	100	45	600	420	100	150	SOT-323	Active
BC847CWT3G	onsemi	100	45	600	420	100	100	SOT-323	Active
MMBT2222AWT1G	onsemi	600	40	1000	100	300	150	SOT-323	Active
MMBT2222AWT3G	onsemi	600	40	1000	100	300	150	SOT-323	Active
MMBT4401WT1G	onsemi	600	40	750	100	250	150	SOT-323	Active

Engineering Selection Recommendations

Direct Electrical Equivalents (Recommended Primary Substitutes):

The SMMBT3904WT1G and MMBT3904WT1G are direct electrical equivalents to the FJX3904TF. Both devices match all critical electrical parameters: 200 mA collector current, 40 V breakdown voltage, 300 mV saturation voltage, 100 minimum hFE, and 300 MHz transition frequency. Both are active products with RoHS3 compliance and unlimited moisture sensitivity level (MSL 1). The SMMBT3904WT1G carries AEC-Q101 automotive qualification, providing additional reliability assurance for demanding applications. The primary difference between these equivalents is packaging format (Tape & Reel versus Cut Tape & Digi-Reel).

The MMBT3904TT1G provides an alternative in the SC-75 (SOT-416) package while maintaining identical electrical specifications and 300 mW power rating. This option is suitable for applications where the larger package footprint is acceptable or preferred.

Alternative Substitutes (Application-Dependent Selection):

The MMBT2222AWT1G and MMBT2222AWT3G series offer higher collector current capability (600 mA) while maintaining 40 V breakdown voltage and 300 MHz transition frequency. These devices are suitable for applications requiring higher current handling than the original 200 mA specification.

The BC847 series (BC847AWT1G, BC847BWT1G, BC847CWT1G, BC847CWT3G) provides higher breakdown voltage (45 V) and higher current gain variants. However, these devices feature reduced maximum collector current (100 mA) and lower transition frequency (100 MHz), making them suitable only for applications with lower current and frequency requirements.

The MMBT4401WT1G supports 600 mA collector current and 40 V breakdown voltage with 250 MHz transition frequency, offering a middle-ground option for higher-current applications.

Product Status and Compliance:

All recommended substitute parts are active products with RoHS3 compliance and REACH unaffected status. The SMMBT3904WT1G includes AEC-Q101 automotive qualification, meeting stringent reliability requirements for automotive and industrial applications.

Frequently Asked Questions (FAQ)

Q: Can the SMMBT3904WT1G directly replace the FJX3904TF without circuit modification?

A: Yes. The SMMBT3904WT1G is a direct electrical equivalent with matching collector current (200 mA), breakdown voltage (40 V), saturation voltage (300 mV @ 5 mA, 50 mA), current gain (100 minimum @ 10 mA, 1 V), and transition frequency (300 MHz). Both use the SOT-323 package with identical pinout. No circuit modification is required.

Q: What is the difference between SMMBT3904WT1G and MMBT3904WT1G?

A: Both devices are electrically identical with matching specifications. The primary difference is packaging format: SMMBT3904WT1G is supplied in Tape & Reel format, while MMBT3904WT1G is supplied in Cut Tape & Digi-Reel format. Selection depends on procurement and assembly requirements. The SMMBT3904WT1G includes AEC-Q101 automotive qualification.

Q: Why does the MMBT3904TT1G use a different package (SOT-416) than the original FJX3904TF (SOT-323)?

A: The MMBT3904TT1G is housed in the SC-75 (SOT-416) package, which is physically larger than the SC-70 (SOT-323) package. While electrically equivalent, this package change requires PCB footprint modification. The SC-75 package accommodates the full 300 mW power rating without thermal derating, whereas the SC-70 package is rated for 150 mW. Use MMBT3904TT1G only if the larger footprint is acceptable or if higher power dissipation is required.

Q: Can the BC847 series replace the FJX3904TF?

A: The BC847 series (BC847AWT1G, BC847BWT1G, BC847CWT1G, BC847CWT3G) is not a direct replacement. These devices feature reduced maximum collector current (100 mA versus 200 mA) and lower transition frequency (100 MHz versus 300 MHz). The BC847 series is suitable only for applications with lower current and frequency requirements. Higher current gain variants (BC847C with hFE of 420) may be beneficial in low-signal applications but do not compensate for reduced current and frequency ratings.

Q: What are the advantages of the MMBT2222AWT1G over the FJX3904TF?

A: The MMBT2222AWT1G supports higher collector current (600 mA versus 200 mA) while maintaining 40 V breakdown voltage and 300 MHz transition frequency. This device is suitable for applications requiring higher current switching capability. However, saturation voltage is higher (1000 mV @ 50 mA, 500 mA versus 300 mV @ 5 mA, 50 mA), which may increase power dissipation in saturated switching applications.

Q: Are all substitute parts RoHS3 compliant?

A: Yes. All recommended substitute parts (SMMBT3904WT1G, MMBT3904WT1G, MMBT3904TT1G, BC847 series, MMBT2222 series, and MMBT4401WT1G) are RoHS3 compliant and REACH unaffected.

Q: Which substitute part offers the best long-term availability?

A: The BC847CWT1G has the highest inventory level (2,798,200 pieces) among all listed substitutes, indicating strong long-term availability. However, this part does not meet the current and frequency requirements of the original FJX3904TF. For direct electrical equivalents, the SMMBT3904WT1G and MMBT3904WT1G both have substantial inventory (17,961 and 85,200 pieces respectively) and active product status, ensuring reliable long-term supply.

Q: Does the FJX3904TF have any moisture sensitivity concerns?

A: No. The FJX3904TF has Moisture Sensitivity Level (MSL) 1, indicating unlimited shelf life without moisture control. All substitute parts listed also have MSL 1 rating, eliminating moisture sensitivity as a substitution constraint.

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