How to Read SMD Diode Marking Codes — Finding Part Numbers from A7·T4·KJE·L4 on the Chip, Codes That Differ by Company and the Same-Code Trap

Image description: A customer peers through a magnifier at a tiny 3-lead chip and asks, “It only says A7 on the chip. What is this?” A female clerk in a red apron opens a table, points with her finger and answers, “If you look it up in the table, it's BAV99!”

Small SMD diodes such as SOT-23 · SOD-123 · SOD-323 have bodies too small to print the full part number, so only a 2–4 character marking code is printed. When repairing or sorting stock, you often have to find BAV99 from nothing but 'A7' on the chip, yet the same part number has different codes at different companies, and the same code is sometimes used for different parts. This article collects only the marking codes printed in the official datasheets of Nexperia · onsemi · Diodes Incorporated · Vishay · Littelfuse.

All codes are exactly as shown in the Marking tables · figure descriptions of each company's datasheet, and text in English quotation marks is the original wording. Transistor marking codes are covered in How to Read BJT Part Numbers (No. 60).

1. What the letters on the chip are made of

The letters on the chip usually consist of part code + date code + (manufacturing site · lead-free mark). The part you use to find the part number is the part code; the rest changes with production period and factory even for the same part number.

2. Same part number, different code at each company

For BAV99 · BAW56 · BAV70 · BAS16, Nexperia and onsemi use the same codes (A7 · A1 · A4 · A6), but Diodes uses different codes starting with K. For the BAT54 family, all three companies differ. Conversely, some codes are the same across companies, such as BZX84C5V1 (Z2) · BZX84C12 (Y2) · MMSZ5231B (E1) · MMSZ5242B (H2). When searching by code, you must also consider the manufacturer.

The Diodes BAS16 datasheet covers BAS16 · MMBD4148 · MMBD914 in one document and only states “A2, KA2”, without specifying which of the three part numbers uses which code.

3. Same code, different part — the biggest trap

Image description: A customer holds three chips of different sizes marked T4 in his palm and asks, “All three say T4, so they're the same, right?” A female clerk points at the chip sizes and answers, “The code is the same, but the sizes are different!”

4. Reading manufacturing site · date · bars

If a Nexperia chip reads like 'A4W', A4 is the part code and W is the manufacturing site (China) letter. Be careful not to mistake the single letter next to the code for part of the part number. Likewise, the line above the letters on a Diodes chip is not a polarity mark; it indicates which factory assembled it.

5. The band is the cathode — bidirectional TVS has no band

The band on the body is the cathode (−) side. All three companies agree: Nexperia “The marking bar indicates the cathode.”, Vishay “color band denotes the cathode end”, Diodes “Cathode Band or Cathode Notch”. However, bidirectional TVS (SMAJ…CA) has no polarity and therefore no band — Vishay “no marking on bidirectional types”, Littelfuse “Cathode Band (for uni-directional products only)”. The SMAJ marking codes also differ between unidirectional (AE) and bidirectional (WE) (per Littelfuse · Vishay).

In the Diodes SMAJ datasheet table the bidirectional column comes first (BI · UNI), while Littelfuse · Vishay list unidirectional first, so it is easy to swap them when copying.

6. Marking code → part number at a glance

Use Ctrl+F (find in page on a phone) to look up the code on the chip. If the same code appears in several rows, choose by company and package. % · * mark the position of the manufacturing site letter.

Only codes printed in each company's official datasheets are included. The same part numbers from companies not listed here (ROHM · Toshiba, etc.) may use yet different codes.

7. Steps to find a part number from a marking code

1. Package first — check whether it has 3 leads (SOT-23 · SOT-323) or 2 terminals (SOD-123 · SOD-323 · SMA), and measure the size with a ruler
2. Read only the part code — leave out the date (M · YM · YWW), manufacturing site (W · t · p in the % · * position) and the lead-free dot
3. Guess the company — 3-character codes starting with K (KJE · KL1) are often seen from Diodes; a letter in the % position is Nexperia style · a single M next to the code is onsemi style
4. Check the marking table in the candidate part's datasheet — the tables in this article are a starting point; the final word is that company's datasheet
5. Check in the circuit — measuring polarity · series (BAV99) · common (BAW56 · BAV70) structure and forward voltage with a diode tester narrows down the candidates

Sources: Nexperia BAV99 · BAW56 · BAV70 · BAS16 · BAS316 · BAT54 · BAS40 · BZX84 · BZT52H · BZX384 · PMEG, onsemi BAV99LT1 · BAW56LT1 · BAV70LT1 · BAS16LT1 · BAT54(A·C·S)LT1 · 1N4148WS · BZX84C2V4LT1 · MMSZ5221BT1 · MBR0520LT1, Diodes BAV99 · BAW56 · BAV70 · BAS16 · 1N4148W · WS · WT · BAT54 · BAS40 · BZX84C · MMSZ5221B · B0520LW · S1A · B120 · SMAJ, Vishay BZT52 · SS12 · S1A · SMAJ · Body Marking(88912), Littelfuse SMAJ datasheets. Marking codes may change depending on the company · production period.