Semiconductor memory is used mainly for primary storage even with its high cost. in another hand, the magnetic tape is the cheapest storage media and it is commonly preferred only as backup storage! use your own words to discuss how the storage cost, speed and capacity trade-offs can affect the choice of storage media

In general you want to use the cheapest storage medium who's speed is compatible with the function it needs to perform. For active storage that's handled while running programs, you need memory whose speed is closely matched to the processor speed. And that would be the rather expensive semiconductor memory which is close to ideal for the task. But semiconductor memory has the disadvantage of being expensive and it loses the values stored when power is lost. So slower, persistent storage is used such as SSD (Solid State Drives) and hard disks. That media is cheaper, but slower, but still fast enough to handle tasks such as loading programs and data into memory for execution, or storing data generated by programs to persistent storage. But as with all man made things, disasters happen. Computers break down, hard disks crash, floods and fires happen, etc., and as a result data is lost. So we make backups. Backups have to have a lot of storage and they have to be cheap. But they don't need rapid access, you can start at the beginning and read (or write) all the way to the end. And for that purpose, magnetic tape is ideal. Magnetic tape is actually quite fast when you're simply streaming a continuous stream of data without any need to randomly access any piece of that data. And it's cheap, so you're willing to make a back up copy of your system and store that backup off site so a single disaster won't destroy both the primary system and the backup.