To understand why solid-state devices function as they do, it is necessary to examine the composition and nature of semiconductors. The two most common materials used for semiconductors are germanium and silicon. The essential characteristic of these elements is that each atom has four valence electrons to share with adjacent atoms in forming bonds. While both elements are used in semiconductor construction, silicon is preferred in most modern applications due to its ability to operate over a wider range of temperatures. The nature of a bond between two silicon atoms is such that each atom provides one electron to share with the other. The two electrons shared are in fact shared equally between the two atoms. This form of sharing is known as a covalent bond. Such bonds are very stable and hold the two atoms together very tightly requiring much energy to break this bond. [Figure 12-209] In this case, all of the outer electrons are used to make covalent bonds with other silicon atoms. In this condition, because all of the outer shell atoms are used, silicon takes on the characteristic of a good insulator, due to the fact that there are no open positions available for electrons to migrate through the orbits.
For the silicon crystal to conduct electricity, there must be some means available to allow some electrons to move from place to place within the crystal, regardless of the covalent bonds present between the atoms. One way to accomplish this is to introduce an impurity, such as arsenic or phosphorus, into the crystal structure, which either provides an extra electron or create a vacant position in the outer shell for electrons to pass though. The method used to create this condition is called doping.
Doping is the process by which small amounts of additives called impurities are added to the semiconductor material to increase their current flow by adding a few electrons or a few holes. Once the material is doped, it then falls into one of two categories: the N-type semiconductor and the P-type semiconductor.
An N-type semiconductor material is one that is doped with an N-type or a donor impurity. Elements such as phosphorus, arsenic, and antimony are added as impurities and have five outer electrons to share with other atoms. This causes the semiconductor material to have an excess electron. Due to the surplus of electrons, the electrons are then considered the majority current carriers. This electron can easily be moved with only a small applied electrical voltage. Current flow in an N-type silicon material is similar to conduction in a copper wire. That is, with voltage applied across the material, electrons will move through the crystal towards the positive terminal just like current flows in a copper wire.
A P-type semiconductor is one that is doped with a P-type or an acceptor impurity. Elements such as boron, aluminum, and gallium have only three electrons in the valence shell to share with the silicon atom. Those three electrons form covalent bonds with adjacent silicon atoms. However, the expected fourth bond cannot be formed and a complete connection is impossible here, leaving a “hole” in the structure of the crystal. There is an empty place where an electron would naturally go, and often an electron moves into that space. However, the electron filling the hole left a covalent bond behind to fill this empty space, which leaves another hole behind as it moves. Another electron may then move into that particular hole, leaving another hole behind. As this progression continues, holes appear to move as positive charges throughout the crystal. This type of semiconductor material is designated P-type silicon material. Figure 12-210 shows the progression of a hole moving through a number of atoms. Notice that the hole illustrated at the far left of top depiction of Figure 12-210 attracts the next valance electron into the vacancy, which then produces another vacancy called a hole in the next position to the right. Once again, this vacancy attracts the next valance electron. This exchange of holes and electrons continues to progress and can be viewed in one of two ways. The first way that this flow can be seen as that of electron movement. The electron is shown in Figure 12-210 as moving from the right to the left through a series of holes. Likewise, the second depiction in Figure 12-210 of the motion of the vacated hole can be seen as migrating from the left to the right. This view is often called hole movement. The valence electron in the structure progresses along a path detailed by the arrows. Holes, however, move along a path opposite that of the electrons.
PN Junctions and the Basic Diode
A single type of semiconductor material by itself is not very useful. Useful applications are developed only when a single component contains both P-type and N-type materials. The semiconductor diode is also known as a PN junction diode. This is a two-element semiconductor device that makes use of the rectifying properties of a PN junction to convert alternating current into direct current by permitting current flow in one direction only.
Figure 12-211 illustrates the electrical characteristics of an unbiased diode, which means that no external voltage is applied.
The P-side in the illustration is shown to have many holes, while the N-side shows many electrons. The electrons on the N-side tend to diffuse out in all directions. When an electron enters the P region, it becomes a minority carrier. By definition, a minority carrier is an electron or hole, whichever is the less dominant carrier in a semiconductor device. In P-type materials, electrons are the minority carrier and in N-type material, the hole is considered the minority carrier. With so many holes around the electron, the electron soon drops into a hole. When this occurs, the hole then disappears, and the conduction band electron becomes a valence electron.
Each time an electron crosses the PN junction, it creates a pair of ions. Figure 12-211 shows this area outlined by dashed lines. The circled plus signs and the circled negative signs are the positive and negative ions, respectively. These ions are fixed in the crystal and do not move around like electrons or holes in the conduction band. Thus, the depletion zone constitutes a layer of a fixed charge. An electrostatic field, represented by a small battery in Figure 12-211, is established across the junction between the oppositely charged ions. The junction barrier is an electrostatic field, which has been created by the joining of a section of N-type and P-type material. Because holes and electrons must overcome this field to cross the junction, the electrostatic field is usually called a barrier. Because there is a lack or depletion of free electrons and holes in the area around the barrier, this area is called the depletion region. [Figure 12-211] As the diffusion of electrons and holes across the junction continue, the strength of the electrostatic field increases until it is strong enough to prevent electrons or holes from crossing over. At this point, a state of equilibrium exists, and there is no further movement across the junction. The electrostatic field created at the junction by the ions in the depletion zone is called a barrier.
Forward Biased Diode
Figure 12-212 illustrates a forward biased PN junction. When an external voltage is applied to a PN junction, it is called bias. In a forward biased PN junction or diode, the negative voltage source is connected to the N-type material and the positive voltage source is connected to the P-type material. In this configuration, the current can easily flow. If a battery is used to bias the PN junction and it is connected in such a way that the applied voltage opposes the junction field, it has the effect of reducing the junction barrier and consequently aids in the current flow through the junction.
The electrons move toward the junction and the right end of the diode becomes slightly positive. This occurs because electrons at the right end of the diode move toward the junction and leave positively charged atoms behind. The positively charged atoms then pull electrons into the diode from the negative terminal of the battery.
When electrons on the N-type side approach the junction, they recombine with holes. Basically, electrons are flowing into the right end of the diode, while the bulk of the electrons in the N-type material move toward the junctions. The left edge of this moving front of electrons disappears by dropping into holes at the junction. In this way, there is a continuous current of electrons from the battery moving toward the junction.
When the electrons hit the junction, they then become valence electrons. Once a valence electron, they can then move through the holes in the P-type material. When the valence electrons move through the P-type material from the right to the left, a similar movement is occurring with the holes by moving from the left side of the P-type material to the right. Once the valence electron reaches the end of the diode, it then flows back into the positive terminal of the battery.
- Electron leaves negative terminal of the battery and enters the right end (N-type material) of the diode.
- Electron then travels through the N-type material.
- The electron nears the junction and recombines and becomes a valence electron.
- The electron now travels through the P-type material as a valence electron.
- The electron then leaves the diode and flows back to the positive terminal of the battery.
Reverse Biased Diode
When the battery is turned around as shown in Figure 12-213, then the diode is reverse biased and current does not flow. The most noticeable effect seen is the widened depletion zone.
The applied battery voltage is in the same direction as the depletion zone field. Because of this, holes and electrons tend to move away from the junction. Simply stated, the negative terminal attracts the holes away from the junction, and the positive terminal attracts the electrons away from the barrier. Therefore, the result is a wider depletion zone. This action increases the barrier width because there are more negative ions on the P-side of the junction and more positive ions on the N-side of the junction. This increase in the number of ions at the junction prevents current flow across the barrier by the majority carriers.
To summarize, the important thing to remember is that these PN junction diodes offer very little resistance to current when the diode is forward biased. Maximum resistance happens when the diode is reversed biased. Figure 12-214 shows a graph of the current characteristics of a diode that is biased in both directions.