How bipolar transistors work

The basic working principle of BJT is reflected in the interaction between the emitter junction (J1) and the collector junction (J2). When the base of the BJT is suspended or shorted to the emitter junction, the voltage between the collectors is positively biased, that is, when UCE>0, the collector junction (J2) is in a reverse bias state, and the external bias voltage is applied. Without the supply of electrons, the entire BJT is not conducting and is in a positive blocking state.

At this time, a forward voltage is applied between the base and the emitter of the BJT, that is, UBE>0. The minority emitter injection effect caused by the positive junction of the emitter junction causes the electrons in the emitter region to pass through the emitter and into the human base region. As the electrons penetrate deep into the base region, many electrons recombine with the holes in the base region, and the holes lost due to the recombination are replenished by the base contacts. If the width of the base region is much smaller than the diffusion length of the electrons, a significant portion of the electrons will reach the collector junction (J2) where they are captured by the electric field and transported to the collector region. In this way, the current begins to flow in the circuit, and the energy band diagram of the BJT in the on state is as shown in FIG.

Figure 1 Band diagram of the conduction state BJT

These electrons passing through the collector junction reduce the voltage drop of the collector junction, and also produce a conductance modulation effect in the collector region, reducing the voltage drop in the collector region. When the voltage between the base emitters is sufficiently large, the BJT operates in a saturated conduction state, and the BJT collector voltage after entering the saturation conduction state is very low. The collector current depends only on the impedance of the external circuit and is no longer affected by the base. control. The BJT operates in saturation, which is the biggest difference between a bipolar Power Transistor and a transistor that acts as a signal processor.

From the PN junction analysis, it is known that the base emitter voltage UBE determines the injection level of the emitter junction, that is, the current in the collector region is adjusted. When the base drive is removed, that is, the voltage between the base emitters is removed, the BJT current drops rapidly because no more electrons are injected into the base, and the remaining excess electrons cannot be combined with holes or flow. The collector is connected, and the collector junction is returned to the external reverse bias voltage state, and the BJT is turned off.

Figure 2 Carrier distribution of the PN junction attachment in the on state

The above is the basic working principle of controlling the BJT to turn on and off by the base emitter voltage UBE (which can also be said to be the base current). The effects of the two PN junctions can be analyzed by some calculations. The carrier distribution near the two PN junctions in the on state is shown in Fig. 2. A schematic diagram of the distribution of electrons and holes in the corresponding BJT is shown in FIG.

Figure 3 Carrier distribution in BJT during conduction

In the figure, the direction of the electron current is opposite to the direction of the current, and the direction of the hole flow is the same as the direction of the current. IB, IC, and IE are the base, collector, and emitter currents of the BJT, respectively. IpE and InE are the hole and electron currents through the emitter junction. InC is the part of the electron current passing through the collector junction in InE. I0C is the leakage current of the collector junction under bias conditions, which is much smaller than other currents and can be ignored. .

For the current flowing through the emitter junction to be composed of two parts, and the injection efficiency of the emitter junction is γ, then

In the base region, there is a recombination of the minority carriers of electron holes, and the current in the current InE that can finally enter the collector through the collector junction is InC, and the electrons are "transported" through the base to the collector, so the transport coefficient is defined. B is

Obviously, the coefficient is also less than 1, according to B and γ, the current amplification factor α of the BJT can be defined as

At this point, consider

Then there is

The coefficient α essentially indicates the effect of interaction and interaction between two PN junctions in a BJT. In general, α is not a constant constant and is affected by many factors, such as current flowing through the BJT and junction temperature of the device. It can be seen from the above equation that the BJT without the base current is not conductive unless the reverse biased collector junction breaks down. This is a fundamental feature of a three-layer, two-junction transistor. A typical BJT current amplification system with collector current and junction temperature is shown in Figure 4.

Figure 4 Current amplification factor as a function of collector current and junction temperature

The above is the basic working principle of bipolar transistors.