Diodes and Transistors

Department of EECS EE100/42-43 Spring 2007 Rev. 1 Diodes and Transistors 1. Introduction So far in EE100 you have seen analog circuits. You started with simple resistive circuits, then dynamical systems (circuits with capacitors and inductors) and then op-amps. Then you learned how circuit elements do not operate the same at all frequencies. Now you will learn about two very important circuit elements – diodes1 and transistors. These elements make the world of digital electronics “tick”. Digital electronics has revolutionized industry over the last 40 years. In the second half of this course, you will learn some of the fundamental concepts (Boolean algebra, microcontrollers, processor I/O and digital systems interfacing) behind this revolution. However, before you do so, you should understand the two devices – diodes and transistors – that were responsible for this revolution. 2. Organization of this document In this document, we will talk about diodes and transistors. First we will discuss very basic semiconductor physics. We won’t discuss the details because the point of this course is electronic circuits, not semiconductor physics. A detailed understanding of semiconductor physics is important only when you deal with microelectronic circuits. We are just building breadboard circuits in this class, thus it is enough if you have an intuitive understanding of the semiconductor physics. If you want to learn the detailed physics behind these devices, you should consider taking EECS 130 at the University of California, Berkeley. Next we will talk about diodes, followed by the bipolar junction transistor. In the case of both devices, we will talk about the nonlinear models, load line analysis and applications. We will conclude this chapter by looking at how transistors can be used as logic devices. This will lead naturally to our discussion of digital systems. Please note that I have chosen to discuss the bipolar junction transistor instead of the field effect transistor. The reason: bipolar transistors are the mainstay of interface elements to microcontrollers. Thus you will be seeing a lot of BJTs when you work with sensor interfaces. 3. Basic Semiconductor Physics [4] [2] [6] A semiconductor is a solid whose electrical conductivity is in between that of a metal and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. Semiconductors are tremendously important technologically and 1 Diodes are really not used in digital circuits anymore. However they are fundamental in circuits like power electronics. Moreover, they are simpler to understand than transistors. So we start our discussion of nonlinear electronics with diodes. Department of EECS EE100/42-43 Spring 2007 Rev. 1 economically. Silicon is used to create the most semiconductors commercially but dozens of other materials are used as well. Semiconductor devices, electronic components made of semiconductor materials, are essential in modern electrical devices, from computers to cellular phones to digital audio players. Elemental or intrinsic semiconductors are materials consisting of elements from group IV of the periodic table. Figure 1 below depicts the lattice arrangement for silicon (Si), one of the more common semiconductors. At sufficiently high temperatures, thermal energy causes the atoms in the lattice to vibrate; when sufficient kinetic energy is present, some of the valence electrons break their bonds with the lattice structure and become available as conduction electrons. These free electrons enable current flow in a semiconductor. Figure 1. Lattice structure of silicon However, if you have to heat up a semiconductor for electron flow, then how can it be used at room temperature? The answer to this is: doping. Doping consists of adding impurities to the crystalline structure of the semiconductor. For example, if we add phosphorous atoms to silicon, then we will have an extra electron floating around in the lattice because phosphorous has five valence electrons. Thus phosphorous atoms are also called donors, refer to figure 2. A doped semiconductor is also called as an extrinsic semiconductor. Silicon doped with donors is also called n-type semiconductor because of the abundance of electrons. Department of EECS EE100/42-43 Spring 2007 Rev. 1 Figure 2. Silicon doped with phosporous There is another kind of conduction that can take place in semiconductors. Suppose we dope silicon with elements from group III of the periodic table, say boron. This is shown in figure 3. Now we have some missing orbitals or places where electrons can go. Thus the electrons are moving to the left. The vacancy caused by the departure of a free electron is called a hole. Note that whenever a hole is present, we have, in effect a positive charge. The positive charges also contribute to the conduction process, in the sense that if an electron “jumps” to fill a neighboring hole, thereby neutralizing a positive charge, it correspondingly creates a new hole at a neighboring location. Because elements from group III of the periodic table “accept electrons”, they are also called acceptors. Silicon doped with acceptors is also called a p-type semiconductor because of the abundance of holes. Figure 3. Silicon doped with boron. The movement of an electron to the left is equivalent to a hole moving to the right. It is important to point out here that mobility – that is, the ease with which charge carriers move across the lattice differs greatly for electrons and holes. In the case of silicon doped with phosphorous, we have free electrons in the lattice that can move easily. In contrast, when silicon is doped with boron, holes move only if a neighboring electron jumps to fill the empty bond. Department of EECS EE100/42-43 Spring 2007 Rev. 1 Now that we have learned about p-type and n-type semiconductors, we come to the central concept in semiconductor devices: the pn junction. When a p-type and n-type material are brought in contact, a number of interesting properties arise. Figure 4 shows the pn junction. Figure 4. The pn junction In the interface layer between the p-type and n-type material, a nonconducting layer called the depletion region occurs. This is because the electrical charge carriers in doped n-type silicon (electrons) and p-type silicon (holes) attract and eliminate each other in a process called recombination. By manipulating this nonconductive layer, p-n junctions are commonly used as diodes: electrical switches that allow a flow of electricity in one direction but not in the other (opposite) direction. This property is explained in terms of the forward-bias and reverse-bias effects, where the term bias refers to an application of electric voltage to the p-n junction. This leads to our discussion of diodes, in the next section. 4. Diodes [2] In simple terms, a diode is a device that restricts the direction of flow of charge carriers (electrons in this class) [1]. Essentially, it allows an electric current to flow in one direction, but blocks it in the opposite direction. Thus, the diode can be thought of as an electronic version of a check valve. Circuits that require current flow in only one direction typically include one or more diodes in the circuit design. Today the most common diodes are made from semiconductor materials such as silicon or germanium. There are a variety of diodes; A few important ones are described below. Normal (p-n) diodes The operation of these diodes is the subject of this document. Usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop (typically 1.4–1.7 V per "cell," with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode's metal substrate), much larger than a silicon diode of the same current ratings would require. The vast majority of all diodes are the p-n diodes found in CMOS integrated circuits, which include 2 diodes per pin and many other internal diodes. Switching diodes Switching diodes, sometimes also called small signal diodes, are single diodes in a discrete package. A switching diode provides essentially the same function as a switch. Below the specified applied voltage it has high resistance similar to an Department of EECS EE100/42-43 Spring 2007 Rev. 1 open switch, while above that voltage it suddenly changes to the low resistance of a closed switch. Schottky diodes Schottky diodes are constructed from a metal to semiconductor contact. They have a lower forward voltage drop than a standard diode. Their forward voltage drop at forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation. Varicap or varactor diodes These are used as voltage-controlled capacitors. These are important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock.. Zener diodes Diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. Light-emitting diodes (LEDs) In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to violet.

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