Note 3 Semiconductor Electronic Devices Department of Mechanical Engineering, University Of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada 1
1. Semiconductor Materials Semiconductor materials comprise the Group IV elements in the periodic table. Most commonly used semiconductor materials are silicon (Si) and Germanium (Ge). Semiconductor materials have electrical conductivity property that is somewhere in between the conductors and insulators (nonconductors), hence the name semiconductor. A silicon atom has 14 electrons, and four of these electrons revolve in the outermost orbit around the nucleolus [Fig. 1 (a)]. In its pure form, silicon is not much of use as a semiconductor due to the fact that the crystalline structure of silicon is very stable [Fig. 1 (b) and (c)]. When a material from Group III of the periodic table (e.g., boron ()) is added to the silicon, there will be one missing electron for each added impurity atom in the crystal structure. One of the four connections around the Group III atom will be missing an electron [Fig. 1 (d)]. This effective positive charge, also called a hole, is loosely connected to the atom. Its ability to move around the crystal structure gives the material the ability to conduct electricity. Since the net added electrical charge is positive, such a semiconductor is called P-type semiconductor. Similarly, if the added material is from Group V of the periodic table (e.g., phosphorus (P)), there will be an extra electron around the added impurity atom. This provides the materials with the ability to conduct electricity. Since the net added electrical charge is negative, such a semiconductor is called n-type semiconductor. Fig. 1 Structures of semiconductors Department of Mechanical Engineering, University Of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada 2
2. Diodes A diode is two-terminal device made of a p-n junction [Fig. 2]. At a p-n junction, electrons from the n-type semiconductor jump to occupy the holes in the p-type semiconductor, creating what is called a depletion layer. The effect of the depletion layer is to inhibit current flow because charge carriers are tied up. However, if a voltage source is connected to the p-n junction with the anode connected to the p-type semiconductor and the cathode connected to the n-type semiconductor, the anode in effect becomes a source of holes and the cathode a source of electrons so that holes and electrons are continuously replenished. Current then flows easily and continuously, and we say the junction is forward biased. If, on the other hand, the anode is connected to the n-type silicon and the cathode to the p-type silicon, the depletion layer is enlarged and no current flows, and we say the junction is reverse biased. Thus, a diode works like an electronic switch that allows current flow only in one direction. p-type n-type Fig. 2 Diode (p-n junction) As shown in Fig. 2, the voltage and current relationship across a diode are: (1) if V D > VF (V F is called the forward bias voltage, V F = 0.3 V for germanium diode and V F = 0.7 V for silicon diode), the diode becomes a conductor and acts as a closed switch, (2) if V Z < VD < VF (V Z is the reverse breakdown voltage, V Z =5V - 100 V), the diode acts as an insulator or open switch, and (3) if V D < VZ, the diode operates in the breakdown region, and acts as a closed switch. The figure on the left shows one application of a diode for voltage surge protection due to inductive load. How does it work? Department of Mechanical Engineering, University Of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada 3
A light-emitting diode (LED) is one that gives out light with intensity that is proportional to the current; and a light-sensitive diode (LSD) is one that passes current proportional to LED LSD the received light intensity. LEDs and LSDs are used as optocouplers in electrical circuits in order to electrically isolate two circuits and couple them optically, as shown in the figure on the left. 3. Transistors A transistor is commonly used as an amplifier or electrically controlled switch. Shown in Fig. 3 are two types of bipolar junction transistor (JT): npn-type and pnptype, each type being a three-terminal device. These three terminals are collector (C), emittor (E), and base (). The voltage and current relationship across the C and E terminals of the transistor (V CE, i C ) is a function of the base current (i ), as shown in Fig. 3. There are three main design parameters of interest: (1) β : the current gain, or i C =β i (2) V F : the voltage drop between the base and the emitter when the transistor is conducting. V F 0.6 V - 0.8. (3) V SAT : the minimum voltage drop across the connector and emitter when the transistor is saturated or fully ON. V F 0.2 V - 0.5. Figure 3: ipolar junction transistor There are three main modes of operation of the JT in steady state: (1) Cut-off Region: V E < VF, then i = 0. The transistor acts like an OFF state switch. (2) Active Linear Region: V E = VF, then i 0 and i C =β i. The transistor acts like a current amplifier. Department of Mechanical Engineering, University Of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada 4
(3) Saturation Region: V E = VF, and i > i C,max / β, then V CE = VSAT. The transistor operates like a closed (ON) switch between C and E terminals. JT type transistor can support collector current in a range of 100 ma to 10 A. The JTs rated for above 500 ma are called power transistors and must be mounted on heat sinks. Example In the circuit shown in Figure 4, V CC2 = 25 V, R 1 = 100 KΩ, R 2 = 1 KΩ. Also, the values of the transistor parameters are known: β =100, V F = 0.7V, V SAT =0.2V. Determine the voltage across the C and E terminals, V CE, and the current i C for V CC1 = 0 V, 5.7 V, and 30.7 V, respectively. Figure 4 Department of Mechanical Engineering, University Of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada 5