We often get inquiries from customers looking for replacements on a variety of transistors - some of which aren’t available in our inventory due to obsolescence or a variety of other reasons. This guide will help break down the steps we commonly take when looking for alternatives and which important parameters to use to help find the best options for a suitable replacement.
Here are the general steps taken to help to determine potential alternatives:
Identify Transistor Type Based on Markings
Transistors typically come with full or partial type numbers printed on the body of the part. Fig. 1 below has an example of markings.
Fig. 1 - Example of markings on a transistor
Parts similar to the above transistor can be found by typing “bc547” into the Digi-Key search bar, or by typing in “bc547 transistor” in a search engine.
Typically the production status and ID on your transistor can be found by searching for the markings online, however please note not all markings on the part will be relevant to a part number. Some markings can indicate other things such as date codes, or some smaller parts may have encoded markings that aren’t easily searchable.
Tips to effectively search for an ID on your original part online:
- You may have to search with different segments of your transistor part markings before you get a result
- Common strings to help identify some information on your part marking for what a potential part identifier could be are below (but not comprehensive):
BJTs: 2Nxxxx, BCxxx, TIPxxx, MMBTxxx, 2SAxxx, 2SBxxx, 2SCxxx, 2SDxxx
FETs: 2N700x, 2SJxxx, 2SKxxx, IRFxxx, MPSxxx, DMNxxx
What if the part you’re searching for is obsolete and you can’t get the exact part? We’ll outline some additional steps and helpful search parameters below.
Please note: this guide is written with the assumption that a datasheet/spec sheet is available for the reference part that provides relevant information for searching for an alternate part. If no technical datasheet/identifying part information is available for the reference part, searching for alternatives increases substantially in difficulty.
Looking for an Alternative: Transistor Type
If you’re able to ID your transistor based on the markings but the specific part is no longer available, look for an alternative by taking some of the most important identifying characteristics and narrowing it down from there. The first thing to consider when looking for an alternative is finding a part of the same type, such as BJT, FET/MOSFET, IGBT, JFET, etc. You can begin your product search on our site by typing in “transistor” into the search bar, then selecting an appropriate subcategory of transistors from the list of results:
Transistor Parameters to Consider
The first step when searching for a transistor alternative on our site is making sure to have as many technical specs on your original part as possible and all the possible filter options available. Once you’re in the appropriate sub-category for transistor type, our parametric searches have a “more filters” drop-down that enables all possible filters for the parts (seen in Fig. 2.) This applies only to the stacked filter display option on our site - if you have the scrolling display option selected, you can navigate through filter columns by scrolling horizontally in the filters section.
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Fig. 2 - “More Filters” location on DK site
The most important characteristics to pick first from the filters section (as applicable) to find a suitable alternative are the following:
- Transistor Type/FET Type/IGBT Type:
The wording of which type will vary based on what subcategory of transistors being browsed on our site, but the “type” filters typically designate the part’s polarity and construction. It’ll be important to designate if the part is NPN, PNP, or has any other defining features at the beginning of the search (Fig. 3)
Fig. 3 - Example of what transistor “type” filter options look like on Bipolar (BJT) - Single product page.
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Package Type:
.If replacing a through-hole or surface mount transistor, make sure the part’s footprint is the same for it to fit properly. Common packages are shown in Fig. 4. Package types can be applied under the “package/case” filter on the transistor’s parametric search pages.
Fig. 4 - Common transistor package types and pinouts -
Breakdown Voltage:
Ensure the breakdown/test voltage is equal or slightly higher than the original transistor. The exact names for each filter in the applicable subcategories are:
Transistors - Bipolar (BJT) - “Voltage - Collector Emitter Breakdown (Max)”
Transistors - IGBTs - “Voltage - Collector Emitter Breakdown (Max)”
Transistors - JFETs - “Voltage - Breakdown (V(BR)GSS)”
Transistors - FETs, MOSFETs - “Drain to Source Voltage (Vdss)”
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Max Current:
Select an option in the filters that can pass the required current. The filter is typically called either "Current Drain (Id) - Max or “Current - Collector (Ic) (Max)”. Make sure the current ratings are greater than or equal to the original transistor. -
Current Gain (for BJT Transistors):
Current gain is significant when looking for a substitute BJT transistor, as total gain depends on how other components interact with your circuit. If your original transistor has high gain, try to match it. The filter on our BJT page for gain shows up as “DC Current Gain (hFE) (Min) @ Ic, Vce”. -
“On” Resistance (For FET Types)
RDS(ON) is the DC resistance between the drain and source terminals when a specified gate to source voltage is applied to bias the FET to an on-state. Select as close of a resistance value to the original reference part as possible. -
Verify Pinout
Ensure the pinout of your alternative is consistent with your reference part. Pinout configurations for base/collector/emitter or gate/source/drain can be found in the individual part datasheets under “Documents and Media” on each product page on our site. -
Filter for Any Additional Applicable Parameters:
With the above filters applied, most results pages will be vastly narrowed. At this point, it’s best to check the specification sheet or datasheet for your original transistor to see if there are any further characteristics you can narrow down via the remaining filters on our site to find the closest option we carry for an alternative.
Please keep in-mind while some transistors are easy to find a suitable replacement for, some transistors no longer have direct substitutes. It’s best practice to narrow down the most similar options electrically and physically and then to examine the manufacturer datasheet on the product to ensure it’ll meet your application needs before ordering.
If you require any assistance, feel free to reach out to our Technical staff via web chat, e-mail us at applications.engineering@digikey.com, or make an inquiry here on the forum.