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                                                                     EXHIBIT 5.1



                          ON SEMICONDUCTOR CORPORATION

                                                               October 10, 2001

ON Semiconductor Corporation
5005 McDowell Rd.
Phoenix, AZ  85008

Dear Sirs:

            I refer to the Registration Statement on Form S-8 ("Registration
Statement") to be filed by ON Semiconductor Corporation today with the
Securities and Exchange Commission ("Commission") under the Securities Act of
1933, as amended ("Act"), relating to the shares of Common Stock, $.01 par
value, of ON Semiconductor Corporation to be issued under the ON Semiconductor
Corporation (fka SCG Holding Corporation) 2000 Stock Incentive Plan (last
amended and restated as of May 23, 2001), and the ON Semiconductor Corporation
(fka SCG Holding Corporation) 2000 Employee Stock Purchase Plan (last amended
and restated as of May 23, 2001) (collectively referred to as "Plans").

            In connection with the foregoing registration, I have acted as the
general counsel of ON Semiconductor Corporation and have examined and am relying
on originals, or copies certified or otherwise identified to my satisfaction, of
such corporate records and such other instruments, and I have made such
investigations of law, as I have deemed appropriate as a basis for the opinion
expressed below.

            Based on the foregoing, it is my opinion that the shares of ON
Semiconductor Corporation issuable under the Plans are duly authorized and, when
issued in accordance with the terms of the Plans, at prices in excess of the par
value thereof, will be validly issued, fully paid and nonassessable.

            I hereby consent to the filing of this opinion as an exhibit to the
Registration Statement. By giving such consent, I do not thereby admit that I am
an expert with respect to any part of the Registration Statement, including this
exhibit, within the meaning of the term "expert" as used in the Act or the rules
and regulations of the Commission issued thereunder.

                                    Very truly yours,

                                    /S/ GEORGE H. CAVE
                                    George H. Cave, Esq.
                                    Vice President, General Counsel and
                                    Secretary