ARS Technica takes a look at the history behind the solid state disk, you can read it over here.
Solid-state memory has been around since very nearly the beginning of computing, but a great many of the details have changed since that earlier time when ferrous ring memory was programmed with copper coils. The first nonvolatile semiconductor memory technology even theoretically suitable for use as a disk was Electrically Erasable Programmable Read Only Memory (EEPROM), invented by Intel in 1978. Using floating-gate transistors to store bits of information, and with specific read, write, and erase circuitry for each cell, EEPROM achieved roughly the read performance of RAM with no volatility and the ability to be rewritten many times. It could, in theory, with the right device engineering, have been used as main storage for a PC..