Why did they change from using vacuum tube to transistor?

Answer 1

There were several reasons:

• Transistors were smaller allowing either miniaturization or more circuits and thus more functions in the same space.
• Transistors generated less waste heat (mostly because they had no heater) and so were easier to cool.
• Transistors operated on lower voltages both making power supply design easier and allowing for potential speed increases.
• Transistors offered an increase in reliability and operating lifetime with resulting reductions in maintenance costs.
• Transistors were more rugged and thus did not break when exposed to shock and vibration, allowing use of onboard embedded computers in missile guidance systems, etc.
• etc.

Note: they did NOT switch to transistors because they were cheaper! In the late 1950s when the switch began almost any computer grade vacuum tube sold for less than $10 but each computer grade transistor usually still cost at least $20. This situation did not change significantly until the computer industry was already switching from transistors to integrated circuits.