The physical position of the base is between the emitter and the collector, and it is made of lightly doped high resistivity materials. The collector surrounds the base region, and it is difficult for electrons to be injected into the base region from here due to the reverse bias of the collector junction, resulting in that the base current gain of * * * is approximately equal to 1, while the emitter current gain of * * * is larger. It can be seen from the schematic cross-sectional view of the typical NPN bipolar transistor on the right that the area of collector junction is larger than that of emitter junction. In addition, the emitter has a relatively high doping concentration.
Generally speaking, several regions of bipolar transistors are asymmetric in physical characteristics and geometric dimensions. Assuming that the transistor connected in the circuit is located in the forward amplification region, if the connection between the collector and emitter of the transistor in the circuit is interchanged at this time, the transistor will leave the forward amplification region and enter the reverse working region. The internal structure of the transistor determines that it is suitable to work in the forward amplification region, so the * * * base current gain and * * * emitter current gain in the reverse operation region are much smaller than those in the forward amplification region. This functional asymmetry is basically caused by different doping levels of emitter and collector. Therefore, in the NPN transistor, although the collector and emitter are both N-doped, their electrical characteristics and functions are completely interchangeable. The emitter region has the highest doping degree, followed by the collector region and the base region has the lowest doping degree. In addition, the physical dimensions of the three regions are also different, in which the base region is very thin and the collector area is larger than the emitter area. Because the bipolar transistor has such a material structure, it can provide reverse bias for the collector junction, but the premise of doing so is that the reverse bias cannot be too large to damage the transistor. The purpose of heavily doping the emitter is to increase the efficiency of electron injection from the emitter to the base region in order to obtain the highest current gain as possible.
In the * * * emitter connection mode of bipolar transistor, a slight change in the voltage applied across the base and emitter will cause a significant change in the current between the emitter and collector. Using this characteristic, the input current or voltage can be amplified. With the base of bipolar transistor as the input terminal and the collector as the output terminal, this two-port network can be analyzed by using Thevenin theorem. Based on the equivalence principle, bipolar transistors can be regarded as voltage-controlled current sources or current-controlled voltage sources. In addition, from the left side of the two-port network, the input impedance at the base is reduced to the impedance of the base resistance, which reduces the requirement for the load capacity of the previous circuit. NPN transistor is one of two types of bipolar transistors, which consists of two N-doped regions and a P-doped semiconductor (base) between them. The small current input to the base will be amplified, resulting in a large collector-emitter current. When the base voltage of an NPN transistor is higher than the emitter voltage and the collector voltage is higher than the base voltage, the transistor is in a forward amplification state. In this state, there is a current between the collector and emitter of the transistor. The amplified current is the result of electrons injected from the emitter into the base region (minority carriers in the base region) drifting to the collector under the push of the electric field. Because electron mobility is higher than hole mobility, most bipolar transistors used now are NPN type.
The electrical symbol of NPN bipolar transistor is shown on the right, and the arrow between the base and emitter points to the emitter. Another type of bipolar transistor is PNP, which consists of two layers of P-doped regions and a layer of N-doped semiconductor between them. The tiny current flowing through the base can be amplified at the emitter. That is, when the base voltage of PNP transistor is lower than the emitter, the collector voltage is lower than the base, and the transistor is in the forward amplification region.
In the electrical symbol of bipolar transistor, the arrow between the base and emitter points to the direction of current, where the current is the opposite direction of electron flow. Contrary to NPN type, the arrow of PNP type transistor points from emitter to base. Heterojunction bipolar transistor is an improved bipolar transistor with the ability of high-speed operation. It is found that this transistor can handle ultra-high frequency signals with frequency as high as several hundred GHz, so it is suitable for RF power amplification, laser driving and other applications that require strict working speed.
Heterojunction is a kind of PN junction, and both ends of this junction are made of different semiconductor materials. In this bipolar transistor, the emitter junction usually adopts heterojunction structure, that is, the emitter region adopts wide band gap material and the base region adopts narrow band gap material. The common heterojunction uses gallium arsenide (GaAs) as the base region and Al-Ga-As solid solution (AlxGa 1-xAs) as the emitter region. With such heterojunction, the injection efficiency of bipolar transistor can be improved, and the current gain can also be improved by several orders of magnitude.
The doping concentration in the base region of a bipolar transistor with heterojunction can be greatly increased, which can reduce the resistance of the base and help to reduce the width of the base region. In the traditional bipolar transistor, that is, the homogeneous junction transistor, the carrier injection efficiency from emitter to base is mainly determined by the doping ratio of emitter and base. In this case, in order to obtain high implantation efficiency, the base region must be lightly doped, which inevitably increases the base resistance.
As shown in the schematic diagram on the left, it represents the potential difference of holes crossing from the base region to the emission region; And it represents the potential difference of electrons crossing from the emitter region to the base region. Because the emitter junction has a heterojunction structure, it can be used to improve the injection efficiency of the emitter. In the base region, the composition distribution of semiconductor materials is uneven, which leads to the slow change of band gap width in the base region, and its gradient is expressed as. This slowly changing band gap can provide an internal electric field for minority carriers to accelerate their passage through the base region. This drift motion will have a synergistic effect with diffusion motion, reducing the transit time of electrons through the base region, thus improving the high-frequency performance of bipolar transistors.
Although many different semiconductors can be used to construct heterojunction transistors, silicon germanium heterojunction transistors and aluminum gallium arsenide heterojunction transistors are more commonly used. The process of manufacturing heterojunction transistors is crystal epitaxy, such as metal organic vapor phase epitaxy (MOCVD) and molecular beam epitaxy.