双极晶体管（BJT） - 建筑和操作细节

A bipolar transistor or a BJT is a 3 terminal semiconductor device which is able to amplify or switch small signal input voltages and currents to significantly larger output signal voltages and currents.

How Bipolar Junction Transistor BJTs Evolved

During 1904–1947, the vacuum tube was unquestionably the electronic device of great curiosity and growth. In 1904, the vacuum-tube diode was launched by J. A. Fleming. Soon afterwards, in 1906, Lee De Forest enhanced the device with a third feature, known as the control grid, producing the first amplifier, and named as the triode.

在随后的几十年中，广播和电视引起了巨大的启发。制造业从1922年的大约100万管上升到1937年的大约1亿个。1930年代初，4个元素四极和5个元素Pentode在电子管业务中获得了受欢迎程度。

在随后的几年中，制造业发展成为最重要的部门之一，并为这些模型，生产方法，高功率和高频应用以及微型化方向创造了快速的改进。

On December 23, 1947, however, the electronics industry was going witness the arrival of an absolutely brand new "direction of interest" and improvement. It turned out on the mid-day that Walter H. Brattain and John Bardeen exhibited and proved the amplifying function of the very first transistor at the Bell Telephone Laboratories.

The very first transistor (which was in the form of a point-contact transistor) is demonstrated in Fig. 3.1.

Image Courtesy:https://commons.wikimedia.org/wiki/file:replica-of-first-transistor.jpg

The positive aspects of this 3 pin solid-state unit in contrast to the tube were instantly noticeable: It turned out to be much smaller, could work without an "heater" or heating losses, was unbreakable and strong, was more efficient in terms of power usage, could be stored and accessed with ease, didn't require any initial warming start up, and it worked at much lower operating voltages.

TRANSISTOR CONSTRUCTION

A transistor is basically a device built with 3 layer of semiconductor material in which either 2 n- type and a single p- type layer of material is used or 2 p- type and a single n- type layer of material is used. The first type is called an NPN transistor, while the second variant is named as the PNP type of transistor.

Both of these types could be visualized in the figure 3.2 with appropriate DC biasing.

We have already learned how inBJTs DC biasing对于建立所需的操作区域和交流扩增至关重要。为此，与底侧相比，发射极侧层的掺杂量更高，而浓度的掺杂量较小。

The outer layers are created with layers much greater in thickness compared to the p- or n- type sandwiched materials. In Fig 3.2 above, we can find that for this type the proportion of the total width in comparison to the central layer is around 0.150/0.001 : 150:1. The doping implemented over the sandwiched layer is also relatively lower than the outside layers which ranges typically across 10 : 1 or even lesser.

This kind of reduced doping level lowers the conduction capacity of the material and increases the resistive nature by restricting the quantity of thefree moving electrons or the "free" carriers.

在偏见图中，我们还可以看到，使用发射器的大写字母E，Coltocor和B用于基础的大写字母E显示了该设备的终端，在我们将来的讨论中，我会解释为什么对这些终端的重要性呈现出来。

同样，BJT一词用于缩写双极晶体管，并指定为这3个末端设备。短语“双极”表示掺杂过程中涉及的孔和电子相对于相反极化物质的相关性。

TRANSISTOR OPERATION

现在，让我们在图3.2的PNP版本的帮助下了解BJT的基本工作。如果电子和孔的参与简单地互换，则NPN对应物的操作原理将完全相似。

As can be seen in Figure 3.3, the PNP transistor has been redrawn, eliminating the base to collector biasing. We can visualize how the depletion region looks narrowed in width due to the induced biasing, which causes a massive flow of themajority carriersacross p- to the n- type materials.

In case the base-to-emitter bias of the pnp transistor is removed as demonstrated in Fig 3.4, the flow of the majority carriers becomes zero, allowing the flow of only minority carriers.

Briefly we can understand that, in a biased situationone p-n junction of a BJT becomes reverse biased while the other junction is forward biased.

在图3.5中，我们可以看到两个偏置电压都应用于PNP双极晶体管，这会导致指定的多数和少数族裔流动流。在这里，从耗竭区域的宽度中，我们可以清楚地看到哪个连接处具有前向偏向状态，哪些是反向偏置的。

如图所示，大量多数载体最终在偏离的P-N连接中扩散到N型材料中。这就提出了一个问题，这些载体能否发挥任何重要作用来促进基本电流IB或使其能够直接流入P型材料？

Considering that the sandwiched n-type content is incredibly thin and possesses minimal conductivity, an exceptionally few of these carriers is going to take this particular route of high resistance across the base terminal.

The level of the base current is normally around microamperes rather than milliamperes for the emitter and collector currents.

The bigger range of these majority carriers are going to diffuse along the reverse-biased junction into the p type material attached to the collector terminal as pointed out in Fig. 3.5.

The actual cause behind this relative ease with which the majority carriers are allowed to get across the reverse-biased junction is quickly realized by the example of a reverse biased diode where the induced majority carriers turn up as minority carriers in the n-type material.

换句话说，我们发现将少数载体引入了N型基本区域材料中。借助这些知识，以及对于二极管而言，耗尽区域的所有少数载体都跨越了反向偏置的连接，从而导致电子流，如图3.5所示。

Assuming the transistor in Fig.3.5 to be a single node, we can apply Kirchhoff's current law to get the following equation:

I_E= i_C+ I_B

Which shows that the emitter current is equal to the sum of base and collector current.

However, the collector current is made up of a couple of elements, which are namely the majority and the minority carriers as proven in Fig.3.5.

The minority-current carrier element here constitutes the leakage current, and is symbolized as ICO (current IC having an open emitter terminal).

因此，按照以下等式3.2中给出的净收集器电流是：

I_C= i_Cmajority+ I_统治

Collector current IC is measured in mA for all general purpose transistors, while ICO is calculated in uA or nA.

ICO的行为将像反向偏置的二极管一样，因此可能容易受到温度变化的影响，因此必须在测试时适当注意，尤其是在旨在在温度范围广泛变化的电路中，否则结果可能是巨大的。由于温度因子而受到影响。

也就是说，由于现代双极晶体管的构造布局的许多高级增强，ICO大大减少了，当今所有BJT都可以完全忽略。

In the next chapter we will learn how to configure BJTs in the common base mode.

参考：https://en.wikipedia.org/wiki/john_bardeen