Working principle of transistor

There are two types of transistor based on their materials: germanium transistor and silicon transistor. And each of them has two structural forms, NPN and PNP, but the most commonly used are silicon NPN and PNP transistors. Except for the power polarity, their working principles are the same. The following only introduces the current amplification principle of NPN silicon transistors.

For an NPN transistor, it is composed of two N-type semiconductors sandwiching a P-type semiconductor. The PN junction formed between the emitter and base regions is called the emitter junction, while the PN junction formed between the collector and base regions is called the collector junction. The three leads are called the emitter e, base b, and collector c.

When the potential at point b is a few volts higher than the potential at point e, the emitter junction is in a forward biased state, while when the potential at point C is a few volts higher than the potential at point b, the collector junction is in a reverse biased state, and the collector power supply Ec is higher than the base power supply Ebo.

When manufacturing a transistor, it is intentional to make the majority carrier concentration in the emitter region higher than that in the base region. At the same time, the base region should be made very thin, and the impurity content should be strictly controlled. In this way, once the power is turned on, due to the positive bias of the emitter junction, the majority carriers (electrons) in the emitter region and the majority carriers (holes) in the base region can easily diffuse across the emitter junction towards each other. However, because the concentration of the former is higher than that of the latter, the current passing through the emitter junction is basically an electron current, which is called the emitter current Ie.

Due to the thinness of the base region and the reverse bias of the collector junction, most of the electrons injected into the base region cross the collector junction and enter the collector region, forming a collector current Ic. Only a small number (1-10%) of electrons are left to recombine in the holes of the base region. The recombined holes in the base region are replenished by the base power source Eb, thus forming a base current Ibo According to the principle of current continuity: Ie=Ib+Ic

This means that by adding a small Ib to the base, a larger Ic can be obtained at the collector, which is called current amplification. Ic and Ib maintain a certain proportional relationship, that is:

In the equation β 1=Ic/Ib, β 1- is called the DC amplification factor, and the ratio of the change in collector current △ Ic to the change in base current △ Ib is β=△ Ic/△ Ib

In the formula, β - is referred to as the amplification factor of AC current. Due to the small difference in the values of β 1 and β at low frequencies, sometimes for convenience, the two are not strictly distinguished. The value of β is about tens to hundreds.

A transistor is a current amplification device, but in practical use, it often utilizes the current amplification effect of a transistor, which is converted into voltage amplification effect through resistance.