- Bipolar Transistors are NOT current amplifiers. Most books get this wrong. In fact they're voltage-controlled insulators.

- Overall, bipolar transistors act like a thin layer of insulator which has a variable thickness. The thickness of the layer can be electrically altered in order to control electric current. It's like closing a switch: make the insulating layer very thin, and the transistor turns "on."

- A bipolar transistor is like three hunks of silicon in a row (usually it's NPN; two hunks of n-type silicon separated by a hunk of p-type.) At the places where the n-type silicon is touching the p-type, a very thin layer of insulator spontaneously appears. Because the three hunks of silicon are normally separated by insulator, a transistor is normally turned off.

- The center region is called the "Base." One of the end regions is called the "Emitter." By applying a small voltage between Base and Emitter, we can make the thin layer of insulator become even thinner. If it's thin enough it stops insulating and charges flow across it. (Imagine bringing two wires closer and closer until the electrons start jumping across the microscopic gap.)

- The transistor's Base/Emitter voltage controls the insulator, which then controls the whole transistor. A large voltage will shrink the insulating region and turn the transistor on. A smaller Base/Emitter voltage can turn the transistor half-way on, and this gives us linear amplifying action rather than just on-off switching.

- But a transistor is not a diode! Exactly. There's a second insulating region between the Base region and the third hunk of silicon (called the "Collector.") This insulator acts very weird, since it doesn't block the charges coming from the Base. It's not even an insulator? However, it does block the effects of any voltage placed between Collector and Base. Whether the Collector voltage is low or high, we get the same amperage through that layer. Therefore the Collector acts as if it is insulated from the Base. Yet charges flow right through the insulating region, from Emitter through Base and into Collector. Therefore the Collector acts as if it's NOT insulated from the Base. Which is it? Both!

A transistor is like a valve, a valve where a tiny change in voltage can open or close the huge valve by different amounts. A small amount of "signal" energy can control the large current provided by a power supply. That's what amplifiers are all about.

- Most transistors are Silicon, and Silicon junctions "turn on" at around 0.4V to 0.8V. Place half a volt on the Base of a transistor and the transistor barely starts turning on.

- But why do so many books say "Transistors Amplify Current?" It's because the authors are confused by the leakage current coming out of the Base wire. Yes, the Base/Emitter voltage controls the thickness of the insulating region and thus controls the main transistor current. But also a tiny bit of charge leaks out through the Base connection. It SEEMS like this leakage is simportant.

- It just so happens that the tiny leakage current in the Base connectino is proportional to the transistor's main Emitter/Collector current. (This makes perfect sense, since both the Base leakage current and the main Emitter/Collector current are controlled by the insulator thickness, which is set by the Base/Emitter voltage.) ...so it SEEMS as though the Collector current is controlled by the Base current. You can even simplify things by pretending that this is true. But in reality, it's the Base/Emitter voltage which runs things. You'll never understand the simple physics if you think that Base current can control Collector current. It can't.

- A bipolar transistor has a voltage-controlled input, while it's output is a variable controller which creates a constant current. It's vaguely like a resistor, but where the voltage in one place creates a current in a second place, yet the voltage in that second place does not affect the current. The effects of voltage are TRANSFERRED to another spot in the circuit. "Transfer Resistor." Transistor.