UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 6)*

GigOptix, Inc.

(Name of Issuer)

Common Stock, par value 0.001 per share

(Title of Class of Securities)

37517Y103

(CUSIP Number)

Avi S. Katz

GigOptix, Inc.

130 Baytech Drive

San Jose, CA 95134

(408) 522-3100

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

December 24, 2013

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.   ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 37517Y103

This Amendment No. 6 to Schedule 13D is being filed by Dr. Avi S. Katz, the Founder, Chief Executive Officer and Chairman of the Board of the Issuer, in his individual capacity. The Schedule 13D filed on August 19, 2011 (the “Schedule 13D”), as previously amended by Amendment No. 1 filed on January 5, 2012 (“Amendment No. 1”), Amendment No. 2 filed on March 29, 2012 (“Amendment No. 2”), Amendment No. 3 filed on August 14, 2012 (“Amendment No. 3”) Amendment No. 4 filed on January 29, 2013 (“Amendment No. 4”) and Amendment No. 5 filed on October 21, 2013 (“Amendment No. 5” and collectively with Amendment No. 1, Amendment No. 2, Amendment No. 3, Amendment No. 4 and Amendment No. 5 the “Prior Amendments”), is hereby further amended and supplemented as set forth in this Amendment No. 6.

Each Item below amends and supplements the information disclosed under the corresponding Item of the Schedule 13D. Unless otherwise indicated herein, capitalized terms used but not defined in this Amendment No. 6 shall have the same meaning herein as are ascribed to such terms in the Schedule 13D or the Prior Amendments, as applicable.

ITEM 3. Source and Amount of Funds or Other Consideration.

As the Founder, Chief Executive Officer and Chairman of the Board of the Issuer, the Issuer has issued Dr. Katz options, restricted stock units (“RSUs”), and warrants, as reported in Item 5 below, as part of his equity compensation, and as previously reported in the Schedule 13D. Dr. Katz has also acquired shares in the open market.

ITEM 4. Purpose of Transaction.

On December 24, 2013 the Issuer completed an underwritten public offering (the “Offering”) of 8,325,000 shares of the common stock of the Issuer (the “Common Stock”). In addition, the underwriters have exercised their overallotment option in full for an additional offering of 1,248,750 shares of Common Stock. Solely as a result of the increase in the number of shares of Common Stock issued and outstanding due to the Offering, the percentage of shares held by Dr. Katz has decreased by more than 1%. In addition, in connection with the Offering, on December 19, 2013 Dr. Katz, along with the other executive officers and directors of the Issuer, has entered into a Lock-Up Agreement (the “Lock-Up Agreement”) with Roth Capital Partners, as representative of the underwriters, the form of which is attached as an exhibit to the Underwriting Agreement filed as Exhibit 1.1 to the Issuer’s Current Report on Form 8-K filed with the SEC on December 19, 2013. Under the lock-up agreements, Dr. Katz may not, directly or indirectly, sell, offer to sell, contract to sell, or grant any option for the sale (including short sales), grant any security interest in, pledge, hypothecate, hedge, establish an open “put equivalent position” (within the meaning of Rule 16a-1(h) under the Securities Exchange Act of 1934, as amended), or otherwise dispose of, or enter into any transaction which is designed to or could be expected to result in the disposition of any shares of the Common Stock or securities convertible into or exchangeable for shares of Common Stock, or publicly announce any intention to do any of the foregoing, without the prior written consent of the representative, for a period of 90 days, subject to an 18 day extension under certain circumstances, from December 19, 2013. This consent may be given at any time without public notice. These restrictions on future dispositions by Dr. Katz is subject to exceptions for transfers (i) as a bona fide gift or gifts to immediate family members who agree to be bound by these restrictions, (ii) by will or the laws of descent and distribution or to one or more trusts for bona fide estate planning purposes, or (iii) to the Issuer or as may be required under any of the Issuer’s benefit plans. The Lock-Up Agreement does not restrict the ability of Dr. Katz from purchasing shares of Common Stock on the open market or under an employee stock purchase plan of the Issuer or exercising any options or other convertible securities granted under any benefit plan of the Issuer. The foregoing description of the Lock-Up Agreement does not purport to be complete and is qualified in its entirety by reference to the Lock-Up Agreement, a copy of which is attached hereto as Exhibit A and is incorporated herein by reference. The Offering and the entry into the Lock-Up Agreement have necessitated the filing of this Amendment No. 6.

Additionally, and as previously disclosed, as the Founder, Chief Executive Officer and Chairman of the Board, the Issuer has issued options, RSUs and warrants to Dr. Katz. The options and RSUs are vesting pursuant to the terms of such prior grants. The warrants expired unexercised as of July 1, 2013. The reported change in the number of shares of Common Stock held by Dr. Katz as set forth above is as a result of this continued vesting of those prior grants, and not a result of the Offering or any new grants or equity issuances to Dr. Katz. Dr. Katz will continue to vest in

his options and RSUs which will necessitate additional amendments to the Schedule 13D. In addition, it is currently expected that consistent with the terms of the Employment Agreement between Dr. Katz and the Issuer and past practice, that there may be future issuances of options and/or RSUs to Dr. Katz. It is also currently expected that other employees, consultants and directors of the Issuer may in the future be issued options and/or RSUs by the Issuer (as well as continue to vest into existing options and RSUs), and as Chief Executive Officer and Chairman of the Board, Dr. Katz will evaluate any such issuances.

As previously disclosed in the Schedule 13D, Dr. Katz has also previously purchased shares of Common Stock in the open market for investment purposes, and such purchases have been made in the ordinary course of business and in connection with his employment with the Issuer. Dr. Katz reserves the right to purchase additional shares of Common Stock or to dispose of shares of Common Stock in the open market, in privately negotiated transactions or in any other lawful manner in the future.

CUSIP No. 37517Y103

It is the current intent of the Issuer to continue to grow through strategic acquisitions, and as Chief Executive Officer and Chairman of the Board, Dr. Katz reviews and evaluates potential transactions and the consideration used, including shares of Common Stock.

Except as described above, Dr. Katz presently has no plans or proposals which relate to or would result in any action enumerated in subparagraphs (a) through (j) of the instructions for Item 4 of Schedule 13D.

ITEM 5. Interest in Securities of the Issuer.

(a), (b) Dr. Katz holds directly 123,310 shares of Common Stock of the Issuer, 50,691 restricted stock units that are scheduled to vest within the next 60 days, resulting in the issuance of shares of Common Stock at that time, and 2,486,839 options that are vested and exercisable or exercisable within the next 60 days. As a result, Dr. Katz may be deemed to be the beneficial owner with sole voting and dispositive power of 2,660,840 shares in the aggregate of Common Stock, or 7.9% of the class of securities. However, the Issuer will withhold shares of Common Stock subject to the restricted stock units at the time of vesting for the purpose of satisfying any tax withholding obligations which arise in connection with such vesting.

The calculation of percentage ownership is based on 31,248,605 shares of Common Stock outstanding as of December 24, 2013 following the Offering, plus 2,486,839 shares of Common Stock that would be issued upon the exercise of the options, plus the vesting of the 50,691 restricted stock units (not including any withholding of shares of Common Stock for the purpose of satisfying any tax withholding obligations).

(c), (d) and (e): Not applicable.

ITEM 6. Contracts, Arrangements, Understandings or Relationship with Respect to the Securities of the Issuer.

The information set forth above in Item 4 regarding the Lock-Up Agreement is incorporated herein by reference.

Except for the Lock-Up Agreement, no other contracts, arrangements, understandings, or relationships (legal or otherwise) exist between Dr. Katz and any person with respect to any securities of the Issuer, including, but not limited to, transfer or voting of any of the securities, finder’s fees, joint ventures, loan or option arrangements, puts or calls, guarantees of profits, divisions of profits or loss, or the giving or withholding of proxies.

Item 7. Material to be Filed as Exhibits

Exhibit A Lock-Up Agreement

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.