A piece of n-type semiconductor or p-type semiconductor is like a resistor, which is not so useful. But when a manufacturer dopes a single silicon crystal with p-type material on one side and n-type on the other side, something new comes into existence – the PN junction.

PN junctions are elementary building blocks of semiconductor devices such as diodes, transistors, solar cells, LEDs, and integrated circuits. Understanding this enables you to understand all these devices.

[!NOTE]

一块 N 型半导体或 P 型半导体就像一个电阻器，用处不大。但是，当制造商将单晶硅的一侧掺杂为 P 型材料，另一侧掺杂为 N 型材料时，就会诞生新的东西——PN 结。

PN 结是二极管、晶体管、太阳能电池、LED 和集成电路等半导体器件的基本构建块。了解这一点使你能够了解所有设备了。

The PN Junction（PN 结）

As we know that p-type semiconductor has trivalent atoms and each of them produces one hole, we can visualize it as shown in the figure. Each circled minus sign is the trivalent atom and each plus sign is the hole in its valence orbit.

[!NOTE]

我们知道 P 型半导体有三价原子，每个原子都会产生一个空穴，我们可以将其形象化，如图所示。每个圆圈的减号是三价原子，每个加号是其价轨道上的空穴。

We also know that n-type semiconductor has pentavalent atoms and each of them produces one free electron, we can visualize it as shown in the figure. Each circled plus sign is the pentavalent atom and each minus sign is the free electron it contributes.

[!NOTE]

我们还知道，N 型半导体有五价原子，每个原子都会产生一个自由电子，我们可以将其形象化，如图所示。每个圆圈的加号是五价原子，每个减号是它的自由电子。

A manufacturer can produce a single silicon crystal with p-type material on one side and n-type on the other side, as shown in the figure. The border between p-type and n-type is called the PN junction.

[!NOTE]

制造商可以生产一侧为 P 型材料、另一侧为 N 型材料的硅晶体，如图所示。P 型和 N 型之间的边界称为 PN 结。

A PN crystal is commonly known as junction diode. The word diode is a contraction of two electrodes, where di stands for two.

[!NOTE]

PN 晶体通常称为“结型二极管”。二极管一词是两个电极的缩写，其中 di 代表 “两个”。

There are three possible biasing conditions for the PN Junction:

1. Equilibrium or Zero Bias – No external voltage is applied to the PN junction.
2. Reverse Bias – The positive source terminal is connected to the n-type, and the negative source terminal is connected to the p-type.
3. Forward Bias – The negative source terminal is connected to the n-type, and the positive source terminal is connected to the p-type.

Let’s look at them one by one.

[!NOTE]

PN 结有三种可能的偏置条件：

1. 平衡或零偏置 – PN 结上未施加任何外部电压。

2. 反向偏置 – 正源极端子连接到 N 型，负源极端子连接到 P 型。

3. 正向偏置 – 负源极端子连接到 N 型，正源极端子连接到 P 型。

让我们一一看看。

Equilibrium (zero bias)（平衡（零偏差））

In a PN junction, without an external applied voltage, an equilibrium condition is reached. Let’s see how.

[!NOTE]

在 PN 结中，无需外部施加电压，即可达到平衡状态。让我们看看如何实现的。

The Depletion Region（耗尽区）

An N-type semiconductor has a larger number of free electrons than a P-type semiconductor. Due to this high concentration of electrons on the N-side, they repel each other.

Due to repulsion, free electrons spread (diffuse) in all directions. Some of them cross the junction. When a free electron enters the p region, it is attracted to the positive hole and recombines with it. When this happens, the hole disappears and the free electron becomes a valence electron.

When the free electron falls into a hole on the P-side, the P-side atom gains an extra electron. The atom which gains an extra electron has more number of electrons than protons, due to which it becomes a negative ion.

Similarly, each free electron that leaves the N-side atom creates a hole in the N-side atom. The atom that loses an electron has more number of protons than electrons, due to which it becomes a positive ion.

So each time an electron crosses the junction and recombines with a hole, it creates a pair of ions. Following figure shows these ions on each side of the junction.

[!NOTE]

N 型半导体比 P 型半导体具有更多的自由电子。由于 N 侧电子高度集中，因此它们会相互排斥。

由于排斥力的作用，自由电子会向四面八方扩散，其中一些穿过结界。当自由电子进入 P 区时，它会被正空穴吸引并与之结合。当发生这种情况时，空穴消失，自由电子变成价电子。

当自由电子落入 P 侧的空穴时，P 侧原子会获得一个额外的电子。获得额外电子的原子的电子数多于质子数，因此它会变成负离子。

同样，离开 N 侧原子的每个自由电子，都会在 N 侧原子中产生一个空穴。失去电子的原子的质子数多于电子数，因此它会变成正离子。

因此，每次电子穿过结界并与空穴重新结合时，都会产生一对离子。下图显示了连接点两侧的这些离子。

Each pair of positive and negative ions at the junction is called a dipole. The creation of a dipole means that one free electron from the n-side and one hole from the p-side are taken out of circulation. As the number of dipoles builds up, the region near the junction is depleted of majority charge carriers. Therefore we call this charge-empty region The depletion region.

[!NOTE]

结界处的每对正离子和负离子称为偶极子。偶极子的产生意味着 N 侧的自由电子被 P 侧的空穴不断地带走。随着偶极子数量的增加，结界附近的区域会耗尽多数电荷载流子。因此，我们将这个电荷空区称为耗尽区。

Barrier Potential（势垒）

Each dipole has an electric field between positive and negative ions. Whenever a free electron tries to enter the depletion region, this electric field pushes it back into the n region.

The strength of the electric field increases with each electron-hole recombination inside the depletion region. Therefore the electric field eventually stops the diffusion of electrons across the junction and equilibrium is reached.

[!NOTE]

每个偶极子在正离子和负离子之间都有一个电场。每当自由电子试图进入耗尽区时，电场就会将其推回到 N 区。

电场强度随着耗尽区内的自由电子和空穴结合而增加。因此，电场最终会阻止自由电子穿过结界，耗尽区停止扩散并达到平衡。

The electric field between the ions is equivalent to a difference of potential called the barrier potential. At room temperature, the barrier potential equals approximately 0.3 V for germanium diodes and 0.7 V for silicon diodes.

[!NOTE]

离子之间的电场的电势差称为势垒。在室温下，锗二极管的势垒电位约为 0.3 V，硅二极管的势垒电位约为 0.7 V。

Forward Bias（正向偏置）

In forward bias, the p-type is connected with the positive source terminal and the n-type is connected with the negative source terminal. Following figure shows a forward bias diode.

[!NOTE]

在正向偏置中，P 型与电源正极相连，N 型与电源负极相连。下图显示了正向偏置二极管。

With a battery connected this way, the holes in the p region and the free electrons in the n region are pushed toward the junction. If the battery voltage is less than the barrier potential (0.7V), the free electrons do not have enough energy to get through the depletion layer. When they enter the depletion layer, the ions will push them back into the n region. Because of this, there is no current through the diode.

[!NOTE]

当电池以这种方式连接时，P 区域中的空穴和 N 区域中的自由电子被推向结。如果电池电压低于势垒（0.7V），自由电子就没有足够的能量穿过耗尽层。当它们进入耗尽层时，离子会将它们推回 N 区域。因此，没有电流通过二极管。

When the battery voltage is greater than the barrier potential (0.7V), the free electrons have enough energy to pass through the depletion layer and recombine with the holes. In this way they begin to neutralize the depletion region, reducing its width.

[!NOTE]

当电池电压大于势垒（0.7V）时，自由电子有足够的能量穿过耗尽层并与空穴结合。这样，它们开始中和耗尽区，减小其宽度。

When a free electron is recombined with a hole, it becomes a valence electron. As a valence electron, it continues to travel to the left, passing from one hole to the next until it reaches the left end of the diode. When it leaves the left end of the diode, a new hole appears and the process starts again. Since there are billions of electrons traveling at the same time, we get a continuous current through the diode.

[!NOTE]

当自由电子与空穴重新结合时，它变成价电子。作为价电子，它继续向左移动，从一个空穴传递到另一个空穴，直到到达二极管的左端。当自由电子离开二极管的左端时，会出现一个新的空穴，然后开始循环这个过程。由于有数十亿个电子同时行进，因此我们可以通过二极管获得连续的电流。

Reverse Bias（反向偏置）

Connecting the p-type to the negative terminal of the battery and the n-type to the positive terminal corresponds to reverse bias. Following figure shows a reverse bias diode.

[!NOTE]

将 P 型连接到电池的负极，将 N 型连接到正极，这就是反向偏置。下图显示了反向偏置二极管。

The negative battery terminal attracts holes, and the positive battery terminal attracts free electrons. Because of this, holes and free electrons flow away from the junction leaving the positive and negative ions behind. Therefore, the depletion region gets wider.

[!NOTE]

电池负极吸引空穴，电池正极吸引自由电子。因此，空穴和自由电子从结界流出，留下正离子和负离子。因此，耗尽区变得更宽。

The width of the depletion region is proportional to the reverse voltage. As the reverse voltage increases, the depletion region gets wider. The depletion region stops growing when its potential difference is equal to the applied reverse voltage. When this happens, electrons and holes stop moving away from the junction.

[!NOTE]

耗尽区的宽度与反向电压成正比。随着反向电压的增加，耗尽区会变宽。当耗尽区的电位差等于施加的反向电压时，耗尽区会停止增长。当这种情况发生时，电子和空穴停止向结界移动。

Reverse Current（反向电流）

The reverse current in a diode consists of a minority-carrier current and a surface-leakage current. This reverse current is so small that you cannot even notice it and it is considered almost zero.

[!NOTE]

二极管中的反向电流由少数载流子电流和表面漏电流组成。该反向电流非常小，甚至无法察觉，几乎可以视为零。

Reverse Saturation Current（反向饱和电流）

As we know that thermal energy continuously creates pairs of free electrons and holes. Suppose thermal energy has created a free electron and hole inside the depletion region.

The depetion region pushes the newly created free electron into the n region, forcing it to leave the right end of the diode. When it reaches the right end of the diode, it enters the external wire and flows towards the positive battery terminal.

On the other hand, the newly created hole is pushed into the p region. This extra hole on the p side allows one electron from the negative battery terminal to enter the left end of the diode and fall into a hole.

[!NOTE]

我们知道，热能可以不断地产生成对的自由电子和空穴。假设这些热能在耗尽区内产生了自由电子和空穴。

耗尽区将新产生的自由电子推入 N 区，迫使其离开二极管的右端。当它到达二极管的右端时，它进入外部导线并流向电池正极端子。

另外，新产生的空穴被推入 P 区。P 侧的额外空穴允许来自电池负极端子的电子进入二极管的左端并落入空穴中。

Since thermal energy continuously produces electron-hole pairs inside the depletion region, a small continuous current flows in the external circuit. Such reverse current caused by the thermally produced minority carriers is called the Saturation current. The name saturation means that increasing the reverse voltage will not increase the number of thermally produced minority carriers.

[!NOTE]

由于热量在耗尽区内不断产生自由电子和空穴，因此外部电路中会流过微小的连续电流。这种由热能产生的少数载流子引起的反向电流称为饱和电流。饱和这个名字的意思是增加反向电压不会增加热产生的少数载流子的数量。

Surface-Leakage Current（表面漏电流）

There exist another current in a reverse-biased diode. A small current flows on the surface of the crystal known as the Surface-leakage current.

The atoms on the top and bottom surface of the crystal have no neighbors. They have only six electrons in the valence orbit. This means that each surface atom has two holes. The following image shows these holes along the surface of the crystal.

[!NOTE]

反向偏置二极管中存在另一种电流。晶体表面流过的小电流称为表面漏电流。

晶体顶部和底部表面的原子没有相邻的原子。它们的价轨道上只有六个电子。这意味着每个表面原子都有两个空穴。下图显示了晶体表面的这些空穴。

Due to this, electrons travel through the surface holes from the negative battery terminal to the positive battery terminal. In this way, a small reverse current flows along the surface.

[!NOTE]

因此，电子会通过表面空穴从电池负极流向电池正极，从而形成微小的反向电流沿表面流动。

Breakdown（击穿）

There is a limit to how much reverse voltage a diode can withstand before getting destroyed. If you continue to increase the reverse voltage, the diode will eventually reach the breakdown voltage.

Once the breakdown voltage is reached, a large number of minority carriers are produced in the depletion region by Avalanche effect and the diode begins to conduct heavily in the reverse direction.

[!NOTE]

二极管在损坏之前能承受的反向电压是有限的。如果继续增加反向电压，二极管最终会达到击穿电压。

一旦达到击穿电压，通过雪崩效应在耗尽区中产生大量少数载流子，并且二极管开始在相反方向上释放大量的电子。

Avalanche Effect（雪崩效应）

As we know there is a small minority-carrier current in a reverse-biased diode. When the reverse voltage increases, it forces minority carriers to move faster. These minority carriers moving at high speed collide with the atoms of the crystal and knock valence electrons loose, producing more free electrons. These new minority carriers join the existing minority carriers and collide with other atoms that knock off more electrons.

One free electron dislodges one valence electron, resulting in two free electrons. These two free electrons then dislodge two more electrons, resulting in four free electrons. In this way the number of electrons increases in Geometric progression: 1, 2, 4, 8, . . .

[!NOTE]

我们知道，反向偏置二极管中存在少量载流子电流。当反向电压增加时，它会迫使少数载流子移动得更快。这些高速移动的少数载流子与晶体的原子碰撞，使价电子被撞散，产生更多的自由电子。这些新的少数载流子加入现有的少数载流子，与其他原子发生碰撞，撞出更多电子。

一个自由电子撞击一个价电子，产生两个自由电子。这两个自由电子又取代两个电子，产生四个自由电子。这样，电子数量以几何级数增加：1、2、4、8……

This constant collision with atoms generates a large number of minority carriers that produce a significant amount of reverse current in the diode. And this process continues until the reverse current becomes large enough to destroy the diode.

[!NOTE]

这种与原子的不断碰撞会产生大量少数载流子，从而在二极管中产生大量反向电流。这个过程一直持续到反向电流大到足以摧毁二极管。

Diode Schematic Symbol（二极管原理图符号）

Following figure shows the schematic symbol of a diode. The symbol looks like an arrow that points from the p side to the n side. The p side is called the anode and the n side the cathode.

[!NOTE]

下图显示了二极管的示意图符号。该符号看起来像一个从 P 侧指向 N 侧的箭头。P 侧称为阳极，N 侧称为阴极。

Diode I-V Characteristics（二极管的伏安特性曲线）

Following figure shows a basic diode circuit in which the diode is forward biased. A series resistor $R_S$ is usually used to limit the amount of forward current $I_F$.

[!NOTE]

下图显示了一个基本的二极管电路，其中二极管是正向偏置的。通常使用串联电阻 $R_S$ 来限制正向电流 $I_F$ 的大小。

After connecting this circuit, if you measure the voltage and current of the diode for forward and reverse bias, and plot it, you will get a graph that looks like this:

[!NOTE]

连接此电路后，如果测量二极管正向和反向偏置的电压和电流并绘制图表，将得到下图：

This plot is called Current-Voltage (I-V) characteristics. This is the most important diode characteristic because it defines how much current flows through the diode for a given voltage.

A resistor is a linear device because its IV curve is a straight line. A diode, however, is different. It is a nonlinear device as its IV curve is not a straight line. This is due to the barrier potential.

Depending on the voltage applied across it, a diode will operate in one of three regions: Forward bias, Reverse bias, and Breakdown.

[!NOTE]

该图称为伏安特性曲线。这是最重要的二极管特性，因为它定义了给定电压下有多少电流流过二极管。

电阻器是线性器件，因为其伏安特性曲线是一条直线。然而，二极管则不同。它是一种非线性器件，因为其伏安特性曲线不是直线。这是由于势垒势能造成的。

根据施加在其上的电压，二极管将在三个区域之一中工作：正向偏置、反向偏置和击穿。

Forward Bias Region（正向偏置区域）

When the diode voltage is less than the barrier potential, a small current flows through the diode. When the diode voltage exceeds the barrier potential, the current flowing through the diode increases rapidly.

[!NOTE]

当二极管电压小于势垒电势时，二极管中流过很小的电流；当二极管电压超过势垒电势时，流过二极管的电流迅速增大。

The voltage at which current starts to increase rapidly is called the forward voltage ($V_F$) of the diode. It is also called cut-in voltage or knee voltage. Typically, a silicon diode has a VF around 0.7V and a germanium-based diode has around 0.3V.

[!NOTE]

电流开始快速增加的电压称为二极管的正向电压 ($V_F$)。它也被称为切入电压或拐点电压。通常，硅二极管的 $V_F$ 约为 0.7V，而锗基二极管的 $V_F$ 约为 0.3V。

Reverse Bias Region（反向偏置区域）

The reverse bias region exists between zero current and breakdown.

In this region, a small reverse current flows through the diode. This reverse current is caused by the thermally produced minority carriers. This reverse current is so small that you cannot even notice it and it is considered almost zero.

[!NOTE]

反向偏置区位于零电流和击穿区之间。

在此区域，二极管中流过小的反向电流。该反向电流是由热能产生的少数载流子引起的。该反向电流非常小，甚至无法察觉到它，并且几乎可以认为是零。

Breakdown Region（击穿区域）

If you continue increasing the reverse voltage, you will eventually reach the so-called breakdown voltage of the diode.

At this point, a process called Avalanche Breakdown occurs in the semiconductor depletion layer and the diode starts conducting heavily in the reverse direction, destroying the diode.

[!NOTE]

如果继续增加反向电压，最终将达到二极管的击穿电压。

此时，半导体耗尽层中会发生雪崩效应的过程，并且二极管开始在相反方向上大量导电，从而损坏二极管。

You can see from the graph that the breakdown has a very sharp knee, followed by an almost vertical increase in current.

[!NOTE]

从图中可以看出，击穿有一个非常陡峭的拐点，随后电流几乎垂直增加。