Can my employer become a co-owner or owner of my intellectual property or patented invention?
Absolutely, in two ways. One is by way of the contract you signed with the company, so you always want to read those carefully. In many cases, the company will say that everything you invent while you’re employed by them, or before you’re employed, or after you leave, or after you die, belongs to them. So they can be quite aggressive, and the company would gain rights to your IP that way. Or you could have signed it to the company, certainly if you wished. If they have the money to proceed with your idea and you don’t, and then you could assign some rights to it.
There is also a thing called a shop right. If you work in a gas station and you invent a new way to change tires that’s faster and easier, and you use your employer’s equipment and tool, and work on this during the time you’re employed during the workday, then even though you would own the patent on that, they would have a right to use that idea without paying you any money in their shop. But that’s a slightly scarce thing these days.