Exhibit 31.2

**CERTIFICATIONS**

| I, William M. Clancy,   certify that: |
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| 1.                                    | I have reviewed this   quarterly report on Form 10-Q of Siliconix incorporated; |
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| 2.                                    | Based on my knowledge, this   report does not contain any untrue statement of a material fact or omit to   state a material fact necessary to make the statements made, in light of the   circumstances under which such statements were made, not misleading with   respect to the period covered by this report; |
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| 3.                                    | Based on my knowledge, the   financial statements, and other financial information included in this   report, fairly present in all material respects the financial condition,   results of operations and cash flows of the registrant as of, and for, the   periods presented in this report; |
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| 4.                                    | The registrant’s other   certifying officer(s) and I are responsible for establishing and maintaining   disclosure controls and procedures (as defined in Exchange Act Rules   13a-15(e) and 15d-15(e)) for the registrant and have: |
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|                                  | a) | Designed such disclosure   controls and procedures, or caused such disclosure controls and procedures to   be designed under our supervision, to ensure that material information   relating to the registrant, including its consolidated subsidiaries, is made known   to us by others within those entities, particularly during the period in   which this report is being prepared; |
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|                                  | b) | [Intentionally omitted] |
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|                                  | c) | Evaluated the effectiveness   of the registrant’s disclosure controls and procedures and presented in this   report our conclusions about the effectiveness of the disclosure controls and   procedures, as of the end of the period covered by this report based on such   evaluation; and |
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|                                  | d) | Disclosed in this report   any change in the registrant’s internal control over financial reporting that   occurred during the registrant’s most recent fiscal quarter that has   materially affected, or is reasonably likely to materially affect, the   registrant’s internal control over financial reporting; and |
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| 5.                                    | The registrant’s other   certifying officer(s) and I have disclosed, based on our most recent   evaluation of internal control over financial reporting, to the registrant’s   auditors and the audit committee of the registrant’s board of directors (or   persons performing the equivalent functions): |
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|                                  | a) | All significant   deficiencies and material weaknesses in the design or operation of internal   control over financial reporting which are reasonably likely to adversely   affect the registrant’s ability to record, process, summarize and report   financial information; and |
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|                                  | b) | Any fraud, whether or not   material, that involves management or other employees who have a significant   role in the registrant’s internal controls over financial reporting. |

Date:  August 10, 2004

| /s/   WILLIAM M. CLANCY                                |   |
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| **William M.   Clancy   Principal Accounting Officer** |   |