There is no law that requires you to keep a personnel file on each employee. Specific employee records are what you must retain under federal laws. The information that you keep depends on what the information is and which law covers the retention of that particular information. Laws may also dictate that certain information must be kept but that it cannot be kept with other personnel information for confidentiality and privacy reasons.
Technically, you don't even need a "file." If you have a better system for keeping all the information straight, use it! Many are opting for computer files. However, you must retain certain pieces of information that an employee or applicant must complete and sign, like the W-4 Form, the I-9, and the job application, and those obviously can't be used in a computer file (unless you scan them in). As a practical matter, personnel files are the easiest means of keeping track of employee information to comply with those requirements and to keep track of information for your own business purposes.
Bare bones information. File or no file, you have to keep certain information to comply with federal laws. Most of that information is basic and can be kept on a form (with the exception of payroll records) in a file or on a computer. For bare-bones compliance, include:
Information collected about employees and retained in personnel files should be strictly job-related. You might consider keeping some of the records required by law in files separate from an employee's personnel file and limit the personnel file to information that applies only to the individual employee (such as resumes, performance evaluations, and work history). If required files are kept in separate files, they may be easily produced to prove compliance to government agencies without providing all other personnel information not relevant to the government investigation.