It depends on the state you are working in and the contract that you assign. In most states in the USA the default is that an employer is allowed to take claim of any and all intellectual property of employees if that is what is in their contract. A few states have made that illegal, California being a good example.
Any provision in an employment agreement which provides
that an employee shall assign, or offer to assign, any of his or her
rights in an invention to his or her employer shall not apply to an
invention that the employee developed entirely on his or her own time
without using the employer's equipment, supplies, facilities, or
trade secret information except for those inventions that either:
(1) Relate at the time of conception or reduction to practice of
the invention to the employer's business, or actual or demonstrably
anticipated research or development of the employer; or
(2) Result from any work performed by the employee for the
employer.
(b) To the extent a provision in an employment agreement purports
to require an employee to assign an invention otherwise excluded from
being required to be assigned under subdivision (a), the provision
is against the public policy of this state and is unenforceable.
They wouldn't have a law on the books if wasn't legal to own all the IP of your employee.