MMST4126-7-F Equivalent & Substitute Parts

Part Overview

The MMST4126-7-F is a Surface Mount PNP Bipolar Junction Transistor manufactured by Diodes Incorporated, housed in the SOT-323 (SC-70) package. This component is rated for 25 V collector-emitter breakdown voltage and 200 mA maximum collector current, with a maximum power dissipation of 200 mW. The device operates across the temperature range of -55°C to 150°C and features a transition frequency of 250 MHz.

The MMST4126-7-F is classified as Obsolete. Identification of equivalent and substitute parts is necessary to support ongoing design requirements, system maintenance, and production continuity where this component is specified in existing schematics or bill of materials.

Key Parameters

Parameter	Value
Transistor Type	PNP
Voltage - Collector Emitter Breakdown (Max)	25 V
Current - Collector (Ic) (Max)	200 mA
Power - Max	200 mW
Frequency - Transition	250 MHz
Vce Saturation (Max) @ Ib, Ic	400 mV @ 5 mA, 50 mA
DC Current Gain (hFE) (Min) @ Ic, Vce	120 @ 2 mA, 1 V
Operating Temperature	-55°C to 150°C (TJ)
Package / Case	SC-70, SOT-323
Mounting Type	Surface Mount
RoHS Status	ROHS3 Compliant
Moisture Sensitivity Level (MSL)	1 (Unlimited)

Substitute Part Grouping Explanation

Substitution of the MMST4126-7-F is determined by the following critical parameters:

Mandatory Compatibility Criteria:

Transistor Type: PNP
Package / Case: SC-70 or SOT-323 (physical and electrical compatibility)
Mounting Type: Surface Mount
Operating Temperature Range: -55°C to 150°C minimum
RoHS Status: ROHS3 Compliant

Performance Envelope Parameters:

Voltage - Collector Emitter Breakdown (Max): Must equal or exceed 25 V
Current - Collector (Ic) (Max): Must equal or exceed 200 mA
Power - Max: Must equal or exceed 200 mW
Frequency - Transition: Must support application frequency requirements

Substitute parts are grouped into two categories:

MFR Recommended Substitute (MMST3906-7-F): Manufactured by Diodes Incorporated. Exceeds the voltage rating (40 V vs. 25 V), maintains identical current and power ratings, and offers improved transition frequency (300 MHz vs. 250 MHz). Product status is Active, ensuring long-term availability.
Alternative Substitute (BC858BWT1G): Manufactured by onsemi. Meets voltage requirements (30 V vs. 25 V) but operates at reduced maximum collector current (100 mA vs. 200 mA) and lower power dissipation (150 mW vs. 200 mW). This part is suitable only for applications where the full 200 mA current capability is not required. Product status is Active.

Parameter Comparison

Parameter	MMST4126-7-F (Main)	MMST3906-7-F (MFR Recommended)	BC858BWT1G (Alternative)
Manufacturer	Diodes Incorporated	Diodes Incorporated	onsemi
Transistor Type	PNP	PNP	PNP
Voltage - Collector Emitter Breakdown (Max)	25 V	40 V	30 V
Current - Collector (Ic) (Max)	200 mA	200 mA	100 mA
Power - Max	200 mW	200 mW	150 mW
Frequency - Transition	250 MHz	300 MHz	100 MHz
Vce Saturation (Max) @ Ib, Ic	400 mV @ 5 mA, 50 mA	300 mV @ 5 mA, 50 mA	650 mV @ 5 mA, 100 mA
DC Current Gain (hFE) (Min) @ Ic, Vce	120 @ 2 mA, 1 V	100 @ 10 mA, 1 V	220 @ 2 mA, 5 V
Operating Temperature	-55°C to 150°C (TJ)	-55°C to 150°C (TJ)	-55°C to 150°C (TJ)
Package / Case	SC-70, SOT-323	SC-70, SOT-323	SC-70, SOT-323
Mounting Type	Surface Mount	Surface Mount	Surface Mount
Product Status	Obsolete	Active	Active
RoHS Status	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant
Moisture Sensitivity Level (MSL)	1 (Unlimited)	1 (Unlimited)	1 (Unlimited)

Engineering Selection Recommendations

MMST3906-7-F (Primary Recommendation):

The MMST3906-7-F is the manufacturer-recommended substitute for the obsolete MMST4126-7-F. Both devices are manufactured by Diodes Incorporated and share identical mechanical packaging (SOT-323). The MMST3906-7-F maintains full electrical compatibility with the original specification: 200 mA maximum collector current, 200 mW power dissipation, and -55°C to 150°C operating temperature range. The higher voltage rating (40 V vs. 25 V) provides additional design margin without performance degradation. The improved transition frequency (300 MHz vs. 250 MHz) offers enhanced high-frequency performance. The device is ROHS3 Compliant with MSL 1 rating, matching the original part's compliance profile. Active product status ensures long-term availability and supply chain continuity.

BC858BWT1G (Secondary Alternative):

The BC858BWT1G is an alternative substitute manufactured by onsemi, suitable for applications where the full 200 mA collector current specification is not required. This device meets the voltage requirement (30 V) and maintains the SOT-323 package form factor. However, the maximum collector current is limited to 100 mA, and maximum power dissipation is 150 mW. The transition frequency is significantly lower at 100 MHz. The device is ROHS3 Compliant with MSL 1 rating. Active product status ensures availability. Selection of this part requires confirmation that circuit design operates within the reduced current and power envelope.

Frequently Asked Questions (FAQ)

Q: Can the MMST3906-7-F directly replace the MMST4126-7-F without circuit modification?

A: Yes. The MMST3906-7-F is electrically and mechanically compatible with the MMST4126-7-F. Both devices are PNP transistors in SOT-323 packages with identical maximum collector current (200 mA) and power dissipation (200 mW). The higher voltage rating of the MMST3906-7-F (40 V vs. 25 V) provides additional design margin. No circuit modification is required.

Q: Under what conditions is the BC858BWT1G suitable as a substitute?

A: The BC858BWT1G is suitable only for applications where the circuit design operates at collector currents not exceeding 100 mA and power dissipation not exceeding 150 mW. If the original design requires the full 200 mA capability or 200 mW power handling, the BC858BWT1G is not appropriate. The lower transition frequency (100 MHz) may also limit suitability in high-frequency applications.

Q: Are all substitute parts available in the same packaging?

A: Yes. The MMST4126-7-F, MMST3906-7-F, and BC858BWT1G are all housed in the SC-70 or SOT-323 package. Physical dimensions and land pattern compatibility are identical, permitting direct PCB layout substitution without redesign.

Q: What is the significance of the Obsolete product status for the MMST4126-7-F?

A: Obsolete status indicates that the MMST4126-7-F is no longer manufactured and will not be restocked. Existing inventory may be available from distributors, but long-term supply cannot be guaranteed. Substitution with an Active product (MMST3906-7-F or BC858BWT1G) is necessary for new designs or production runs requiring assured supply continuity.

Q: Do all substitute parts meet the same compliance requirements?

A: Yes. The MMST4126-7-F, MMST3906-7-F, and BC858BWT1G are all ROHS3 Compliant with Moisture Sensitivity Level 1 (Unlimited). All parts are REACH Unaffected and classified under ECCN EAR99. Compliance profiles are identical across all three devices.

Q: How do the DC current gain characteristics differ among these parts?

A: The MMST4126-7-F specifies a minimum DC current gain (hFE) of 120 at 2 mA collector current and 1 V collector-emitter voltage. The MMST3906-7-F specifies a minimum hFE of 100 at 10 mA and 1 V. The BC858BWT1G specifies a minimum hFE of 220 at 2 mA and 5 V. These differences reflect different measurement conditions and device design. Circuit designs relying on specific hFE values must account for these variations.

Q: What is the impact of transition frequency differences on circuit performance?

A: The MMST4126-7-F operates at 250 MHz transition frequency, the MMST3906-7-F at 300 MHz, and the BC858BWT1G at 100 MHz. For applications operating below 100 MHz, all three devices are suitable. For applications between 100 MHz and 250 MHz, the BC858BWT1G is not appropriate. For applications above 250 MHz, the MMST3906-7-F is preferred due to its higher transition frequency.

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