How did the development of the transistor affect computers? What did the transistor replace?

Question

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The Answer is The answer to the first part of this question is subjective: The development of transistor has had a tremendous impact on bring the computer to our homes in the form of PCs and more recently into our hands (Personal Digital Assistants etc). The ability to package and integrate transistors on a chip at a rate that has been increasing exponentially according to Moore's Law has resulted in tremendous performance gains for programs without having to change any of them.
The transistor replaced the vacuum tube in 1951.

How Did the Development of the Transistor Affect Computers? What