A bipolar junction transistor (BJT) is a three-layer semiconductor device. The outputs are named Emitter, Base and Collector, of which the emitter is highly contaminated, the base less so, and the collector moderately contaminated. The aim is to control the conductivity of the semiconductor layers.
The sandwiched semiconductor types can also take the PNP order instead of NPN. These combinations create a component that is the foundation of the modern world. The processor and other elements of computers, so that information technology (smart devices, ADAS...) could not exist without transistors in their current form.
There are minimal differences between symbols of the two types. For the currents, Kirchhoff's current law can be applied, and for the voltages, the base-emitter and the collector-emitter voltage are worth inspecting to understand the basic operation of the transistor.
$ I_E = I_B + I_C $
$ V_{BE} \approx 0.7 [V] $
In this configuration the component is connected to two loops with a power supply in each of them, and they have their junction at the emitter. The resistors are there to limit current. There are two tests to be performed, one to reveal the input and the other to reveal the output characteristics of the NPN transistor.
First the base current is measured as a function of the base-emitter voltage. The characteristic is diode-like, which depends slightly on the CE voltage. The next test measures the emitter current as a function of the collector-emitter voltage. This measurement must be performed repeatedly at different base currents to obtain the full characteristics. Three regions can be identified, these are the cut-off region (purple), the saturation region (blue) and the active region. In the cut-off region, VBE is so low that no current flows.
The characteristics show that the collector current can be controlled by turning the base current on and off, and the base current can be orders of magnitude smaller, controlling a much larger current than the base current. For the transistor to behave as a switch, it must be used in the saturation region, where the CE voltage is less than the threshold defined by the manufacturer. Also, the ratio of the controlled to the control current should be much less than the maximum gain that can potentially be achieved, since this is when the "switch" is guaranteed to be fully open, acting as a low value resistor.
$ V_{CE} < V_{CE(sat)} $
$ \dfrac{I_C}{I_B} \ll \beta $
The operation of a transistor should not be thought of as an electric switch, but as a faucet instead, because it does not just have an off or on state. It is like a variable resistor whose current value depends on the base current. Amplifiers can be built from transistors that amplify low amplitude AC signals to higher amplitude, ideally without distortion. The active region is used for this exact purpose. Distortion is when the signal accidentally falls into one of the other two regions.
$ \beta = \dfrac{I_C}{I_B} \approx 50-400$