Introduction to Crystal 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. Principle of current amplification.

An NPN transistor 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 respectively called the emitter e, base b, and collector.

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 conscious to make the majority carrier concentration in the emitter region higher than that in the base region, while making the base region very thin and strictly controlling the impurity content. In this way, once the power is turned on, due to the correct emitter junction, the majority carriers (electrons) in the emitter region and the majority carriers (control holes) in the base region can easily cross the emitter structure and diffuse in opposite directions. 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:

β1=Ic/Ib

In the formula, β - is referred to as the DC amplification factor,

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.

2、 Characteristic curve of transistor

1. Input characteristics

The input characteristic curve of a transistor represents the relationship between Ib and Ube. Its characteristics are: 1) When Uce is in the range of 0-2 volts, the position and shape of the curve are related to Uce, but when Uce is higher than 2 volts, the curve Uce is basically independent. Usually, the input characteristic can be represented by two curves (I and II).

2) When Ube<UbeR, the segment of Ib ≈ O (0~UbeR) is called the "dead zone". When Ube>UbeR, Ib increases with the increase of Ube. When amplified, the transistor operates in a more straight segment.

3) The input resistance of a transistor is defined as:

The estimation formula for rbe=(△ Ube/△ Ib) Q point is:

Rbe=rb+(β+1) (26 millivolts/Ie millivolts)

Rb is the base resistance of a transistor, and for low-frequency low-power transistors, rb is approximately 300 ohms.

2. Output characteristics

The output characteristic represents the relationship between Ic and Uce (with Ib as the parameter), which is divided into three regions: cutoff region, amplification region, and saturation region.

When Ube<0 in the cutoff region, Ib ≈ 0, and there are no electrons injected into the base region in the emission region. However, due to the thermal motion of molecules, a small amount of current still passes through the collector, i.e. Ic=Iceo, which is called the penetration current. At room temperature, Iceo is about a few microamperes, and germanium tubes are about tens of microamperes to hundreds of microamperes. Its relationship with the reverse current Icbo of the collector is:

Icbo=(1+β)Icbo

At room temperature, the Icbo of a silicon transistor is less than 1 microampere, while the Icbo of a germanium transistor is about 10 microamperes. For a germanium transistor, the Icbo value doubles for every 12 ℃ increase in temperature, while for a silicon transistor, it doubles for every 8 ℃ increase in temperature. Although the Icbo value of a silicon transistor changes more dramatically with temperature, due to the fact that the Icbo value of a germanium transistor itself is larger than that of a silicon transistor, it is still heavily affected by temperature. In the amplification region, when the emitter junction of a transistor is in forward bias and the collector junction is in reverse bias operation, Ic approximately changes linearly with Ib, and the amplification region is the area where the transistor operates in the amplification state.

When both the emitter and collector junctions are in a positive bias state in the saturation region, Ic basically does not change with Ib and loses its amplification function. Based on the bias of the emitter and collector junctions of the transistor, its working state may be determined. The situation may determine its working status.

The input and output characteristics of a transistor

The cutoff region and saturation region are the areas where the transistor operates in a switching state. When the transistor is turned on, the operating point falls in the saturation region, and when the transistor is turned off, the operating point falls in the cutoff region.

3、 Main parameters of transistor

1. DC parameters

(1) The reverse saturation current Icbo between the collector and base, when the emitter is open (Ie=0) and a specified reverse voltage Vcb is applied between the base and collector, is only related to temperature and remains constant at a certain temperature. Therefore, it is called the reverse saturation current between the collector and base. A good transistor has a very small Icbo. The Icbo of a low-power germanium transistor is about 1-10 microamperes, while the Icbo of a high-power germanium transistor can reach several milliamps. The Icbo of a silicon transistor is very small, at the nanoampere level.

(2) When the collector emitter reverse current Iceo (penetration current) is open at the base (Ib=0), the collector current is determined by applying a specified reverse voltage Vce between the collector and emitter. Iceo is approximately β times that of Icbo, i.e. Iceo=(1+β). Icbo and Iceo are greatly affected by temperature and are important parameters for measuring the thermal stability of tubes. The smaller the value, the more stable the performance. The Iceo of low-power germanium tubes is larger than that of silicon tubes.

(3) When the collector is open and a specified reverse voltage is applied between the emitter and base, the current at the emitter is actually the reverse saturation current at the emitter junction.

(4) The DC current amplification factor β 1 (or hEF) refers to the ratio of the DC current output by the collector to the DC current input by the base when there is no AC signal input in the common emitter connection, that is:

β1=Ic/Ib

2. Communication parameters

(1) The AC current amplification factor β (or hfe) refers to the ratio of the change in collector output current △ Ic to the change in base input current △ Ib in the common emitter connection, that is:

β=△Ic/△Ib

The beta of a typical transistor is approximately between 10-200. html "target=" -blank "title=" 10-200 ">10-200. If the beta is too small, the current amplification effect is poor. If the beta is too large, although the current amplification effect is large, the performance is often unstable.

(2) The common base AC amplification factor α (or hfb) refers to the ratio of the change in collector output current △ Ic to the change in emitter current △ Ie when connected in a common base configuration, that is:

α=△Ic/△Ie

Because Δ Ic<Δ Ie, therefore α<1. If the alpha value of the high-frequency transistor is greater than 0.90, it can be used

The relationship between alpha and beta:

α=β/（1+β）

β=α/（1-α）≈1/（1-α）

(3) The cut-off frequencies f β and f α are 0.707 times the frequency when β drops to low frequencies, which is the cut-off frequency f β of the common emitter; When α decreases to 0.707 times the frequency of low frequency, it is the cut-off frequency f α of β for the common base. f α is an important parameter indicating the frequency characteristics of the tube, and their relationship is:

fβ≈（1-α）fα

(4) The characteristic frequency fT decreases as the frequency f increases, and when β decreases to 1, the corresponding fT is an important parameter that comprehensively reflects the high-frequency amplification performance of the transistor.

3. Limit parameter

(1) When the collector current Ic increases to a certain value, causing the β value to decrease to 2/3 or 1/2 of the rated value, the maximum allowable current ICM of the collector is called ICM. So when Ic exceeds ICM, although it does not cause damage to the tube, the β value significantly decreases, affecting the amplification quality.

(2) When the emitter is open, the reverse breakdown voltage of the collector junction is called BVEBO.

(3) When the collector is open, the reverse breakdown voltage of the emitter junction is called BVEBO.

(4) Collector emitter breakdown voltage BVCEO is the maximum allowable voltage applied between the collector and emitter when the base is open. If Vce>BVCEO during use, the transistor will be broken down.

(5) The maximum allowable dissipated power of the collector PCM when the collector current exceeds Ic and the temperature increases, and the parameter changes of the tube due to heating do not exceed the allowable value, is called PCM. The actual dissipated power of the tube is the product of the collector DC voltage and current, i.e. Pc=Uce × Ic. When used, Pc<PCM.

PCM is related to heat dissipation conditions, and adding heat sinks can improve PCM.