How Do Transistors Amplify Current?
How do transistors work? How can they ‘amplify' current? Well, they don't! More precisely, they replicate an electric signal using a stronger source.
Imagine sound waves hitting a microphone: what happens actually is that the sound signal does not pass into the microphone, but the microphone produces a signal corresponding to the sound signal; It is not the actual signal. Remember that the actual signals in real world cannot be amplified or attenuated. Can you catch a sound or any other real world signal? No. They are as they are, we can only make a system which can work on the effect of the real world signal; sound waves hit on a microphone, light hits on a camera lens etc. But when it comes to the case of a transistor, you apply an input signal to the base and you obtain a new signal corresponding to the input signal with greater amplitude in the collector. The output signal is totally a new signal of a grater amplitude, not the actual signal. (Source)
Devices that performed the function of transistors had been around for 30–40 years prior to the invention of the transistor. They were called vacuum tubes. A vacuum tube consisted of a vacuum chamber made from glass or metal, a heating element that heated the space inside the chamber, and electrodes that protruded into the chamber. One specific type of vacuum tube was called a triode; it had three electrodes. In a triode, a large current flowing through two of the electrodes (called the anode and the cathode ) could be regulated by placing a wire grid (called the control grid) between the cathode and the anode. Applying a small current to this grid slowed down the flow of electrons between the cathode and the anode. It didn’t take long to figure out that you could use a fluctuating signal such as a radio or audio wave on the control grid. When you did that, the current on the anode followed the fluctuations of the control grid current, but with much larger variations. Thus, small variations in current at the control grid were amplified to create large variations in current at the anode. A transistor performs the same function as a vacuum tube triode, but using semiconductor junctions instead of heated electrodes in a vacuum chamber. Although the transistor didn’t do anything that the vacuum tube triode didn’t already do, it did it in a radically different way that had huge advantages over the vacuum tube. The earliest transistors were small, required very little power to operate, generated much less heat, and lasted much longer than vacuum tubes. (Source: Electronics All-in-One For Dummies by Doug Lowe)