Transistors are among the most important inventions of humanity. They change the world as we have known it and set the basis for the information age. Modern computers and the internet allow us to communicate with our friends across the globe. Knowledge that previously was entitled to a very small number of scientists is now literally available at our fingertips. In a sense, the transistor expanded our reach across the globe while on the same time reducing the world to a size suitable for our trouser pockets. So what are these powerhouses of advancement? Is there just a single type of transistor or are there more? How do they work? We will try to answer at least some of these questions. Welcome to the first video on types of transistors. The bipolar chunk transistor or just bipolar transistor was the first design put into operation in 1947. Since then, another type has taken over but the bipolar transistor sure has earned its place in history. But what does a bipolar transistor look like? Well, they come in different sizes and shapes depending on their application. Their backhatch often serves certain needs like handling mechanical stress or dissipating heat. And as for life in general, it's the inside of the matters. Adoles of their backhatch, the cross section of a bipolar transistor shows three distinct layers. Depending on the sequence of these layers, we can either build an NPN or a PNP transistor. The most important one is the base in the middle. It's called base because it served as a mechanical base for the very first prototype. The other two layers in a bipolar transistor are called the emitter and collector on either side of the base. Their naming is rather straightforward. The emitter emits charge carriers into the base while the collector collects them back up again. The symbol of a bipolar transistor depicts the side view of the first experimental setup. It consists of a slab of semi-conducting material as the base. The contacts for collector and emitter are on either side of a plastic wedge which presses down on the semiconductor. To distinguish between an NPN and an MPNP transistor, the symbol shows a small error between base and emitter. The direction of this error gives the polarity of the base emitter diode. At the first glance, the bipolar transistor appears symmetrical and we might be tempted to swap emitter and collector. But a real bipolar transistor has a heavily-doped emitter and a lightly-doped collector relative to the doping of the base. This way physicists improve the transistor's performance. But this also means our bipolar transistor is no longer symmetrical. So be careful when you wire them into your circuit. How we can use a bipolar transistor in our circuits can be depicted by a couple of different characteristics. These characteristics show the behavior of the transistor from certain perspectives. There are the input characteristic, current gain, transfer characteristic and output characteristic. The input characteristic tells us how the transistor reacts to changes between base and emitter. It plots the base current IBE as a function of the base emitter voltage VBE. The space and emitter form a simple PN chunk tune or a diode. Unsurprisingly, the input characteristic of a bipolar transistor looks like the current voltage relation of a diode. The current gain is possibly the greatest perk of a bipolar transistor. The current gain is the ratio between collector current IC and base current IBE. The higher the current gain, the higher IC will be for a given IBE. If we plot IC as a function of IBE, we see a near constant slope at the beginning. This slope basically represents the current gain. A steep slope means a high current gain. Towards higher currents, the slope decreases as our transistor is no longer able to keep up and the current gain decreases. Next up, we have the transfer characteristic. It tells us how the collector current IC reacts to changes in base and emitter voltage VBE. As the collector current IC is linked to the base current IBE via the current gain B, the transfer characteristic is in first approximation a scaled version of the input characteristic. This means the transfer characteristic again looks like a current voltage relation of a diode. The output characteristic can be a bit daunting at first, but let's break things down a little bit and go one step at a time. With the output characteristic, we want to figure out how the collector current IC is related to the collector emitter voltage VCE. But as we have seen before, the collector current is also a strong function of either base current IBE or base emitter voltage VBE. So depending on what base current or base emitter voltage we use, the output characteristic will look differently. To represent these different characteristics, a couple of selected lines are drawn into the output characteristic at once. Each of these lines belongs to a certain base current IBE or base emitter voltage VBE. In fact, there is an infinite number of lines, but then things would become too crowded. If we apply a certain base current or base emitter voltage, we simply select one of these lines. This characteristic field can be divided into different regions of operation in which the transistor has different properties. Those are the cutoff region, the forward active region, the saturation region and the reverse active region. One last note on the input and output of the transistor. This is just a convention to describe the behavior and is totally unrelated to any circuits input and output. Of course we can connect the transistor in any way we want it. In this video, we started to talk about some different transistors available, namely the bipolar transistor. Please be aware that we barely scratched the surface of it, but covering the whole topic in detail in one video would be beyond the scope. In the following videos, we will give a brief introduction of two additional transistor types, namely the chafet and the MOSFET. I'm Kristoff with the Institute of Electronics. We hope you've learned something today, but anyways, thanks for watching.