United States, 520 U.S. 651, 663 (1997). Assuming this line of cases serves as a guide in deciding who is a civil officer subject to impeachment, it appears that employees, as non-officers, are not subject to impeachment, while principal officers, such as the head of a cabinet-level executive department, are.
Article II, Section 4Article II, Section 4: The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
The Constitution requires a two-thirds vote of the Senate to convict, and the penalty for an impeached official upon conviction is removal from office. In some cases, the Senate has also disqualified such officials from holding public offices in the future. There is no appeal.
The Constitution gives Congress the authority to impeach and remove "The President, Vice President, and all civil Officers of the United States" upon a determination that such officers have engaged in treason, bribery, or other high crimes and misdemeanors.
(1) The House of Representatives shall have the exclusive power to initiate all cases of impeachment.
Article III of the Constitution establishes and empowers the judicial branch of the national government.
1 : to charge with a crime or misconduct specifically : to charge (a public official) before a competent tribunal (as the U.S. Senate) with misconduct in office. Note: Impeachment is the first step in removing an officer from office.