It is possible to create a junction having properties similar to those of a PN junction by bonding aluminum (AI) to suitably doped Nstypc silicon! The junction that results is called a metal-semiconductor (MS) junction. Like a PN junction, the MS junction presents a low resistance to current flow when it is forward biased (metal p”~it’yp with respect to the N-type silicon) and a high resistance when reverse biased. A depletion region and a barrier potential arc junction by a mechanism similar to that described for PN junctions, except in this case carrier diffusion consists only or clecuons diffusing from the semiconductor’ to the metal. The electrons accumulate at the metal surface and the depletion region exists only in the semiconductor side of the junction.
In Chapter 3 we will discuss applications of diodes in switching circuits, where the bias on a diode is changed very rapidly from reverse to forward, or vice versa. In these applications it is necessary for a diode to respond very rapidly, that is, to change its nature very quickly from that of a high resistance to that of a low resistance, or vice-versa. Metal-semiconductor junctions are able to respond more rapidly in these situations than are their PN counterparts because only majority carriers (electrons in the N-type silicon) are involved in the process, Diodes formed and used this way are called Schottky barrier diodes. or simply Schottky diodes.Figure 2-18 shows the special symbol used to represent a Schottky diode.
A metal-semiconductor junction with Schottky-diede properties can also be formed by bonding gold (Au) to P-type germanium. This device is called a goldbonded tiiot/(‘and responds very rapidly in switching applications
As we shall study in Chapter 6, aluminum is widely used in the fabrication of electronic devices to provide electrical contacts at semiconductor surfaces, where terminals can be attached or where interconnections can be made to other devices. When aluminum is bonded to P-type silicon no diode junction is formed. The bond is simply called an ohmic contact because it exhibits a resistance that is independent of the voltage polarity across it
SUMMARY AND OVERVIEW
This section is a condensed version of the material covered in Sections 2-1 through 2-8. It does not contain the technical detail of those sections and is provided here for readers who do not need or wish to study semiconductor theory in great depth. It does, however provid sufficient background for all subsequent discussions of electronic devic theory
Modern electronic devices-diodes, transistors, and integrated circuit -are (;OBstructed from a special class of materials called semiconductors. As the name implies. a semiconductor is neither a good conductor of electrical current noran insulator. However, it is not the ability or lack of ability to conduct current that makes semiconductors useful. Rather, it is their ability to form crystals having special electrical properties that makes them so valuable. The element silicon (Si) is the most widely used semiconductor material, followed by germanium (Ge). .
Since neither silicon nor germanium occurs naturally in a state suitable for use as semiconductor material, each must be subjected to a complex manufacturing process in which crystals are “grown” from a batch of melted, highly purified material. Figure 2-2 is a diagram of the atomic structure of a semiconductor crystal. The only electrons shown in this example are those furthermost from the center (the nucleus) of each atom. These electrons are called valence electrons and are the ones most responsible for the electrical properties of the material. Notice that each atom shares valence electrons with four of its neighbors, as indicated by the
ovals enclosing electrons around each nucleus. Shared electrons tend to stay bound to their “parent” atoms in covalent bonds, which are symbolized in the figure by the ovals. Although the interlocking structure ‘of the crystal is highly stable, it is possible for aalence electron to acquire enough energy (usually heat energy) to overcome the covalent bond and thus escape its parent atom. We say that a covalent bond has been ruptured: The escaped electron becomes a free electron thin can wander- about 111 the material. There is enough heat energy at room temperature to free a large number of electrons, which are then available for the conduction of electrical current through the crystal, just as the flow of electrons a conductor constitutes current. (However, there are lastly more free electrons in a conductor than in a semiconductor.)
When an electron escapes a c valent bond, it leaves behind a hole in the crystal structure. Since electrically neutral atoms have as many positively charged protons in their nucleus as they do negatively charged electrons outside the nucleus, an atom that has a hole from having lost an electron has a net positive charge. The atom is called a positive ion. It is also possible for 11 wandering electron’to fall into a hole, thus returning the atoin to a neutral state. In those ca es, we say that a recombination, or annihilation, has occurred.
We have already noted that free electrons are available in a semiconductor to establish current flow through it. These electrons are called charge carriers, because they carry negative charge from one location to another when they move. Holes are also charge carriers-in this case, positive charge carriers. Recall that an atom having a hole is positively charged. If a valence electron leaves one atom to occupy a hole in an adjacent atom, the atom it left becomes positively charged and the atom it joined becomes neutral. In effect, positive charge has moved from one atom to another. See Figure 2-4. The movement of holes through a crystal in this way is called hole current. Thus, there are two types of current in a semiconductor: electron current and hole current. Note that the movement of positive charge in one direction is equivalent to movement ofnegative charge in the opposite direction so the two components add to equal the total current in the material. Also note – that a recombination (free electron falling into a hole) does not leave a hole behind, so there is no hole now in that case.
There arc no holes in a conductor, so electron current is the only type that can exist. This current results from the presence of an electric field through the material, created, for example, by an externally connected voltage source. The electric field . drives electrons from the negative terminal of the voltage source through the material to the positive terminal. Current that exists due to the force of an electric field is called drift current. In a semiconductor; both electron current and hole current are created by electric field forces, so both electron drift and hole drift occur when a voltage source is connected across the ends of a semiconductor. A given electric field will cause electrons to flow in one direction and holes to flow· in the oppo she direction. If the same number of electrons and holes are subjected to the same electric field. the electrons will move faster than the holes (electrons arc said to have greater mobility), so the component of drift current due to electrons will be greater than that due to holes.
Another type of current that can exist in a semiconductor is called diffusion current. Diffusion occurs when there is an imbalance in the number of carriers of a given type between two different regions of a semiconductor. Carriers tend to. migrate from a region where there are many of thei-r own type to a region where there are fewer, ‘thus correcting the imbalance. If, for example. there art: many more electrons at one end of a bar of semiconductor marterial than there are at the other end, electrons diffuse from the high-density end to the low-density end until their distribution is more or less uniform throughout. Unless the region containing the excess electrons is replenished with new electrons (as it often is in practical devices), diffusion current ceases when the imbalance has been corrected. Hole current can also be of the diffusion type.
P and N Materials
Pure semiconductor material is said to be intrinsic. In practice, certain impurities are introduced into intrinsic material during the manufacturing process to give it new properties. The process of introducing impurities is called doping, and material that has been doped is called extrinsic. The purpose of doping is to create a semiconductor that has more electrons than holes (N material) or more holes than electrons. (P material). T? create N material, the semiconductor.is doped with \mpurity atoms that have five instead of four valence electrons. When each such atom joins the crystal structure, four of its valence electrons create the usual covalent bonds with other atoms, and the fifth electron is free. This type of impurity is called a donor, because every such atom donates one free electron to -rhe material. Figure 2-6 illustrates a donor atom in a crystal structure. To create P material, the semiconductor is doped with atoms that have three instead of four valence electrons. Each such atom forms covalent bonds with three neighbors only and thus creates one hole in the structure. These atoms arc called acceptors, because they can each accept one electron, Figure 2-7 this case.
It is important to note that both Nand P materials are electrically neutral. Although every donor atom’ donates a free electron, the donor atom’s nucleus brought with it just the right number of positively charged, protons to neutralize the charge carried by all its electrons. Similarly, each acceptor atom has the-same Humber of protons as electrons and is also elect rically neutral. Although N material has more electrons than holes, il does have some holes, and, P material does have some electrons. In N material, electrons are called the majority carriers and holes, the minority cettiets. In P material, holes are majority carriers and electrons are minority carriers. N material that has been very heavily doped and that is, therefore, very conductive is said to be N+, and heavily doped P material is p
When a block of N material is constructed adjacent to a block of P material, the boundary between the two is called a PN junction. At the junction, holes’diffu~ from the P material into the N material and electrons diffuse from the N material into the P material. (Remember that diffusion occurs when there is an imbalance of carriers of a given t)pe.) Every electron crossing the junction leaves behind a donor atom with a net positive charge, and every hole leaves behind an acceptor atom with a net negative charge. See Figure 2-8. Consequently, after the diffusion there is a thin layer of positive ions on the N side of the junction and a thin layer of negative ions on the P side. There are no mobile charge carriers in this region, and it is called the depletion region because it is depleted of such charge. See Figure 2-9. The layers of opposite charge established an electric field (like a voltage source) directed from the N material toward the P material. The direction of the field opposes the flow of further electron current from ~ to P and hole current from P N. The voltage difference between the charged regions is therefore called ..l barrier voltage. The value of the barrier depends on temperature and levels, but it is typically about 0.7 V for silicon and 0.3 V for germanium.
Suppose now that an external voltage source is connected across the P and N material, as shown in Figure 2-10. Notice that the positive terminal of the source is connected to the P material and the negative terminal is connected to the N material. The polarity of the external source thus opposes the barrier voltage at the PN junction and enhances the flow of current through the material and across the junction. The PN junction is said to be forward biased. If the polarity of the external source is reversed (positive (0 N and negative to P), as shown in Figure 2-12, the barrier voltage is reinforced and very little current flows. In this case, the junction is said to be reverse biased. A diode is a PN junction and therefore has the property just derrribed: It permits a generous flow of current in one direction (when forward brs s. J) and permits virtually no current to flow in the opposite direction (when reverse biased). This property is responsible for many useful applications of diodes in electronic circuits. The P side of a diode is called the anode and the N side is called the cathode. Figure 2-13 illustrates forward and reverse biasing of a diode and shows the standard symbol for the device.
The current through a reverse-biased diode, called reverse current, is very small but not totally zero. The saturation current, I” in equation 2-18 is the current that flows through the reverse-biased diode (from cathode to anode) when voltage V is a few tenths of a volt negative. As we shall see in Example 2-8, equation 2-18 produces a negative (reverse) current when voltage V is negative (reverse bias). When voltage V is a few tenths of a volt positive (forward bias), equation 2-18 will show that the positive (forward) current becomes quite large. The forward current becomes large when the forward voltage approaches about 0.7 V in silicon and about 0.3 V in germanium. Figure 2-11 illustrates this fact for a forward-biased silicon diode. Figure 2-14 shows diode current versus voltage under both fo wardand reverse-bias conditions .
We can tell from the presence of temperature T in the diode equation that diode .current depends on temperature. Furthermore, the value of Is itself depends heavily on temperature. As a rule, Is approximately doubles in value for every 10 c rise in temperature. Much of the art of electronic circuit design using semiconductors is concerned with compensating for the effects of temperature changes on circuit performance.
If the reverse-biasing voltage across a diode is to a value called the breakdown voltage, the reverse current through the diode will no longer be limited to the small saturation When breakdown occurs, the diode conducts heavily in the reverse direction, limited only by whatever resistors or other components are in series with it. Breakdown does not necessarily result in permanent damage to a diode, If the reverse current is limited so that the power dissipation rating of the diode is not exceeded (P = VI), then no irreversible damage occurs. Figure 2-15 shows the increase in reverse current when the reverse voltage is near the bre value VI/H. The value of VOR depends on doping and other physical charade is ticsof a diode and may range in value from about 10 V to several hundred volts.