Doping of Silicon

In this presentation we review Silicon doping, or how to change the electrical conductivity of Silicon. 1

At the end of this module you will be able to: Define semiconductors Describe the doping process Explain the difference between n-type and p-type conductivity 2

In previous modules we acquired preliminary information that is necessary to understand Silicon doping. We learned about the electronic structure of the atom and atomic bonding. Let’s quickly review these topics before we tackle doping. 3

Atoms have the same number of positive protons in the nucleus as negative electrons outside the nucleus. The electrons move around the nucleus in well-defined orbits or shells. These orbits have a maximum electron capacity. Once the orbital is full, electrons populate the next available orbital. This process starts with the innermost orbital, closest to the nucleus, and proceeds outwards. The electrons in the outermost orbit, called valence electrons, control how atoms interact with each other and also participate in the bonding to form molecules or solids. The figure, on the right side of the slide, shows the electronic structure of an atom of Silicon, with a total of 14 electrons, and 4 valence electrons in the outermost shell. 4

The electronic bond between the atoms of Silicon is covalent. Actually, each atom forms 4 covalent bonds with its 4 neighbors. Each atom shares one its four valence electrons with each one of its four neighbors. The result is that each atom appears to have 8 electrons in the outermost orbital, which is an energetically favorable configuration. 5

We can now focus our attention on semiconductors and the doping process. Silicon is the most widely used semiconductor material but there are other semiconductors such as Germanium or Gallium Arsenide. Some general characteristics of all semiconductors include: The conductivity can be increased by doping or by increasing the temperature. The crystalline structure is hold by bonds that are mostly covalent. 6

kT = 0.026 eV at room temperature (~ 300K) For a material to be electrically conductive it needs to have available charge carriers, that is, loose electrons that can propagate easily through the material. In pure Silicon the valence electrons form the covalent bonds between the atoms and thus are bound to the atoms. These electrons cannot propagate throughout the lattice. So, pure Silicon is essentially an insulator at room temperature of below. This is where the unique properties of semiconductors come into play. The electrical conductivity of Silicon can be increased. Charge carriers can be induced in the material. [click forward – animation] There are two ways to add extra charge carriers to Silicon. One way is to elevate the temperature. However, this is not very practical. The temperature needs to be increased to 200°C or more to obtain some conductivity. The other, more efficient mechanism, is doping where appropriate impurities (atoms of foreign elements) are introduced into the Silicon lattice, providing the loose charge carriers needed for electrical conduction. 7

Semiconductors can be classified as intrinsic or extrinsic materials. An intrinsic semiconductor is pure material in which there are no impurities. All the atoms in the lattice belong to the same species, Silicon in our case. As mentioned earlier, all the valence electrons are tied making the covalent bonds, and there are no loose charge carriers to conduct electricity. Thus, the intrinsic semiconductor is an insulator. The figures in the slide are different representations of Silicon. On the top right the blue spheres represent the Silicon atoms and the red bars represent the covalent bonds between neighboring atoms. On the bottom right we see a portion of the three dimensional structure of the Silicon lattice. Again here, the spheres represent Silicon atoms and the bars the covalent bonds. Four of the spheres have been colored in blue to show a Silicon atom and its four nearest neighbors. On the left we see the two-dimensional representation of the Silicon lattice. The blue circles represent the Silicon atoms and the small open circles represent the valence electrons. Note that there are two electrons between each pair of atoms indicating the covalent bonds and there are eight electrons around each atom indicating the filled outermost shells. 8

An extrinsic semiconductor is one in which a small number of host atoms have been replaced by atoms of other species. The foreign species belong to a small group of elements that have the capacity to generate extra charge carriers and are consequently called dopants. The most commonly used dopants in Silicon are Boron, Phosphorus, and Arsenic. The figure in the slide shows a dopant, the red sphere, occupying a Silicon lattice site. But, what difference does it make, if any, whether the dopant is Boron or Phosphorus, or Arsenic? 9

The difference between using Boron or Phosphorus as a dopant is the type of conductivity that is induced in the semiconductor, Silicon in our case. When Phosphorus is used as the dopant, excess electrons are created. These negative electrons are the majority charge carriers and thus the material is defined as n-type. Arsenic produces exactly the same effect as Phosphorus, n-type doping. However, when Boron is used as a dopant, excess holes are created. Holes are actually missing electrons that behave as positive particles. These positive holes are the majority charge carriers and thus the material is defined as p- type. In both cases, n-type doping and p-type doping, the electrical conductivity of Silicon is increased with the addition of the dopants, but the charge carrier is different in each case, giving rise to slightly different properties. To summarize, intrinsic is the pure material, and extrinsic is the material with dopants added. Extrinsic can be n-type or p-type depending on the type of dopant used. 10

Do you remember how many electrons does a Phosphorus atom have, in its outermost shell? Phosphorus has 5 electrons in the outermost shell. When a Phosphorus atom replaces a Silicon atom, in the Silicon lattice, 4 of these electrons form the covalent bonds with the neighboring atoms. The fifth electron is extra and remains somewhat loose in the vicinity of the Phosphorus atom. If voltage is applied to the Silicon, the extra or excess electron can move easily throughout the lattice. If there are many of these excess electrons moving, an electric current is produced. We say that the donor atoms of Phosphorus provide excess electron charge carriers to form n-type Silicon. The larger the number of Phosphorus dopants added, the larger the number of excess electrons and the higher the electrical conductivity so induced. 11

[This slide needs to be shown in presentation mode since it contains an animation. Click forward twice and wait for the electrons to move all the way to the + side] This animation shows electrical conductivity in n-type Silicon. Several Phosphorus dopants have been added to the lattice generating several electron carriers. [click forward – animation] Watch what happens when a voltage difference is created across the material. [click forward and start talking as the electrons move] As voltage is applied to the material, the electrons flow towards the more positive side. We can conclude that the electrical conductivity in n-type Silicon is due to the flow of the excess electrons through the lattice. [This slide needs to be shown in presentation mode since it contains an animation. Click forward twice and wait for the electrons to move all the way to the + side] This animation shows electrical conductivity in n-type Silicon. Several Phosphorus dopants have been added to the lattice generating several electron carriers. [click forward – animation] Watch what happens when a voltage difference is created across the material. [click forward and start talking as the electrons move] 12 As voltage is applied to the material, the electrons flow towards the more

Do you remember how many electrons does a Boron atom have, in its outermost shell? Boron has 3 electrons in the outermost shell. When a Boron atom replaces a Silicon atom, in the Silicon lattice, 3 of these electrons form the covalent bonds with the neighboring atoms. But one of the covalent bonds is left undone because one electron is missing. It happens that the missing electron behaves like a positive particle that is called a hole. If voltage is applied to the Silicon, the extra or excess hole can move throughout the lattice but in opposite direction to the electron motion. Still, if there are many of these excess holes moving, an electric current is produced. We say that the donor atoms of Boron provide excess positive hole charge carriers, to form p-type Silicon. The larger the number of Boron dopants added, the larger the number of excess holes and the higher the electrical conductivity so induced. 13

[This slide needs to be shown in presentation mode since it contains an animation]. Click forward one time; then click forward four more times waiting each time for the electrons to move in one direction and the holes to move in the opposite direction. It ends when all the holes are near the V- side] This animation shows electrical conductivity in p-type Silicon. Several Boron dopants have been added to the lattice generating several hole excess carriers. The holes, the bonds with a missing electron, are initially indicated by the red circles. [First click] Watch what happens when a voltage difference is created across the material. [Second to fifth click waiting for electrons and holes to move in-between clicks. The narration goes on while the particles move. The animation It ends when all the holes are near the V- side] As voltage is applied to the material, the electrons flow towards the more positive side. But this movement is equivalent to the holes, indicated by the plus signs, flowing towards the more negative side V-. [Wait until all the holes, the + signs, accumulate near the V- end] The current flow in this case can be described as the flow of positive particles, the holes, towards the negative side of the material. This explains why we say that Boron doping creates p-type material. 14 The flow of current in the Silicon can be described as the motion of positive

To conclude this module let’s summarize some of the most important concepts we have introduced: electrons and holes, n-type and p-type conductivity, doping, and semiconductors. 15

In a crystalline solid a hole is a missing electron and carries positive charge, equal to the electron charge but with positive sign. When we apply a voltage difference at the ends of a material that contains holes, the holes move towards the negative side V-, just opposite to the electrons that move towards the positive side V+. [click forward] Both electrons and holes can be responsible for the conduction of electricity in Silicon and other semiconductors. In p-type materials, the majority of the carriers are the holes, while in n-type materials the majority of the carriers are the electrons. 16

Here is a question for you: what determines if a semiconductor will become n- type or p-type? 17

The answer is: the difference in valency, the number of electrons in the outermost orbital, between the dopant and the host, is the factor that determines if a semiconductor will become n-type or p-type when doped. 18

Can you replace the term X-type that appears in bold face with the correct term, in the two sentences shown in the slide? 19

If the valency of the dopant is higher than that of the host material, usually by just one electron, then the semiconductor will become n-type. 20

If the valency of the dopant is lower than that of the host material, usually by just one electron, then the semiconductor will become p-type. [note that a table appears when you click] Let’s now fill the empty cell with the type of semiconductor that is formed when the dopant is added to the host material. If Silicon is doped with Boron, which type it becomes? 21

If we add Boron with valency 3 to Silicon that has valency 4, then the Silicon becomes p-type. If Silicon is doped with Phosphorus, which type it becomes? 22

If we add Phosphorus with valency 5 to Silicon that has valency 4, then the Silicon becomes n-type. And if Silicon is doped with Arsenic, which type it becomes? 23

Arsenic and Phosphorus produce the same effect. If we add Arsenic with valency 5, to Silicon that has valency 4, then the Silicon becomes n-type. 24

Here is another question for you: What is doping? [wait a few seconds and then click forward] Doping is the process by which the electrical conductivity of a semiconductor is modified, usually increased. It consists in introducing, into the semiconductor lattice, appropriate impurities called dopants that generate extra charge carriers. The most widely used dopants in Silicon are…? Can you tell? [wait a few seconds and then click forward] Boron, Phosphorus, and Arsenic are the commonly used dopants in Silicon. Boron induces p-type doping, while Phosphorus and Arsenic produce n-type doping. 25

The figure in the center of the slide shows a Silicon substrate in blue where two regions have been doped. The red region has become p-type due to the incorporation of Boron dopants. The yellow region has become n-type due to the incorporation of Phosphorous dopants. In both cases, n-type and p-type doping, the new regions are more conductive than the Silicon substrate, but in the n-type region, electrons are the majority carriers, while in the p-type region, holes are the majority carriers. 26

How would you explain why is it that semiconductors are so special materials? [click forward] Semiconductors are very special materials, because we can control the electrical conductivity and, moreover, we can create regions with two different types of conductivity, n-type and p-type, by appropriate selection of dopants. In another module we will see that we can create a diode by putting an n-type region next to a p-type region and also we can create a transistor by selective doping of Silicon. But returning to the n-type and p-type regions, how is it that they are created? N-type and p-type regions are created by doping, that is by the introduction of controlled amounts of selected impurities into the Silicon lattice. 27

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