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)) and   internal control over financial reporting (as defined in Exchange Act Rules   13a-15(f) and 15d-15(f)) 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)  Designed   such internal control over financial reporting, or caused such internal   control over financial reporting to be designed under our supervision, to   provide reasonable assurance regarding the reliability of financial reporting   and the preparation of financial statements for external purposes in   accordance with generally accepted accounting principles; |
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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 control over financial   reporting. |

 

| Date: May 10, 2005           |   |
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| /s/ WILLIAM M. CLANCY        |   |
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| William M. Clancy            |   |
| Principal Accounting Officer |   |