The working principle and package form of NPN transistor

1. Composition and package of triode

I believe that everyone knows about the transistor, the NPN transistor is actually the name of a bipolar junction transistor with two N-type materials (or negative carriers), and only one P-type material (or positive carriers), the NPN transistor uses the following schematic notation. NPN transistors have two N-type materials sandwiching P-type materials, in the N-type material, the negative charge predominates, and in the P-type material, the positive charge predominates.

An NPN transistor can be seen as a diode with two anodes connected, as shown below. With diodes, where the anode is basically positive and the cathode is negative, the NPN transistor has two diodes that connect the anode because the P-type material is sandwiched between the two N-type materials.

Now NPN transistors have a variety of packaging forms, such as NXP Company is currently used more low-power NPN transistor BC817 has the following packages.

2.NPN transistor principle

A bipolar junction transistor is a current-controlled device, which means that it requires a current to flow to its base for it to function as expected. It is not the voltage level applied to the base that makes the transistor work, but the current flow to the base. Knowing this property, it is now clear that we need a resistor in series with the base to set the level of current flowing through the base. In fact, when dealing with NPN transistor circuits, we need to keep two things in mind:

a. Make the triode on

b. Set the working mode of the device

To turn the transistor on, the base-emitter junction voltage must be overcome. The voltage required to overcome the base junction voltage (VBE) is the same as the diode voltage drop, generally around 0.7V. In practical applications, the VBE must be based on the specifications of the transistors used. For example, BC817-25 of NXP Semiconductors, VBEsat varies with temperature and collector current, as shown in the figure below.

In common applications, the voltage applied to the base is always much higher than that of the VBEsat, so changes in the VBEsat due to temperature may not be a problem. However, if changes in the VBE are not properly considered, sometimes the operation of the transistor will be jeopardized, because the base current will change when operating in the active region, and then the collector current will also change. Once the VBE was overcome, the triode became a current control device. As long as the operation is in the amplifier area, the size of the collector current is largely determined by the size of the base current. The base and collector currents of NPN transistors are related to β (β). Once the NPN transistor is on, it can be set to operate in amplifier or switch mode. These depend on the base current.