Cal. Corp. Code § 305

(a) Unless otherwise provided in the articles or bylaws and except for a vacancy created by the removal of a director, vacancies on the board may be filled by approval of the board (Section 151) or, if the number of directors then in office is less than a quorum, by (1) the unanimous written consent of the directors then in office, (2) the affirmative vote of a majority of the directors then in office at a meeting held pursuant to notice or waivers of notice complying with Section 307 or (3) a sole remaining director. Unless the articles or a bylaw adopted by the shareholders provide that the board may fill vacancies occurring in the board by reason of the removal of directors, such vacancies may be filled only by approval of the shareholders (Section 153).
(b) The shareholders may elect a director at any time to fill any vacancy not filled by the directors. Any such election by written consent other than to fill a vacancy created by removal, which requires the unanimous consent of all shares entitled to vote for the election of directors, requires the consent of a majority of the outstanding shares entitled to vote.
(c) If, after the filling of any vacancy by the directors, the directors then in office who have been elected by the shareholders shall constitute less than a majority of the directors then in office, then both of the following shall be applicable:
1. (1) Any holder or holders of an aggregate of 5 percent or more of the total number of shares at the time outstanding having the right to vote for those directors may call a special meeting of shareholders, or
2. (2) The superior court of the proper county shall, upon application of such shareholder or shareholders, summarily order a special meeting of shareholders, to be held to elect the entire board. The term of office of any director shall terminate upon that election of a successor.
  The hearing on any application filed pursuant to this subdivision shall be held on not less than 10 business days notice to the corporation. If the corporation intends to oppose the application, it shall file with the court a notice of opposition not later than five business days prior to the date set for the hearing. The application and any notice of opposition shall be supported by appropriate affidavits and the court’s determination shall be made on the basis of the papers in the record; but, for good cause shown, the court may receive and consider at the hearing additional evidence, oral or documentary, and additional points and authorities. The hearing shall take precedence over all other matters not of a similar nature pending on the date set for the hearing.
(d) Any director may resign effective upon giving written notice to the chairperson of the board, the president, the secretary or the board of directors of the corporation, unless the notice specifies a later time for the effectiveness of such resignation. If the resignation is effective at a future time, a successor may be elected to take office when the resignation becomes effective.