The vacuum tubes tended to leak, and the metal that emitted electrons in the vacuum tubes burned out. The tubes also required so much power that big and complicated circuits were too large and took too much energy to run. In the late 1940's, big computers were built with over 10,000 vacuum tubes and occupied over 93 square meters of space.
The problems with vacuum tubes lead scientists and engineers to think of other ways to make three terminal devices. Instead of using electrons in vacuum, scientists began to consider how one might control electrons in solid materials, like metals and semiconductors.
Already in the 1920's, scientists understood how to make a two terminal device by making a point contact between a sharp metal tip and a piece of semiconductor crystal. These point-contact diodes were used to rectify signals (change oscillating signals to steady signals), and make simple AM radio receivers (crystal radios). However, it took many years before the three terminal solid state device - the transistor - was discovered.
The first time I ever saw a transistor was on a lab project Dad had from his days working at Bell Labs during summers while he was an instructor at Southern Tech. He gave the project to me, a speech synthesizer that could say "aeeeieeohhhuhuhuhhhhuuuuu" when attached to a reel-to-reel tape recorder.
Then, at 16, I because a DJ at WRWH-AM Radio in Cleveland, GA after school. I used to love to watch the glow of the vacuum tubes in the transmitter when I had a min. And, oh, the thrill if the power was off a bit and I had to use the toggle switch to adjust the power! I loved the equipment and meter readings more than I liked to talk on the air, when I had to read a brief summary of news from UPI every hour. Those were the days. Hyper days, but fun.