Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

Nonetheless we do face a dilemma our treatment of

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Nonetheless, we do face a dilemma. Our treatment of device physics must contain enough depth to provide adequate understanding, but must also be sufficiently brief to allow quick entry into circuits. This chapter accomplishes this task. Our ultimate objective in this chapter is to study a fundamentally-important and versatile device called the “diode.” However, just as we need to eat our broccoli before having desert, we must develop a basic understanding of “semiconductor” materials and their current conduction mechanisms before attacking diodes. In this chapter, we begin with the concept of semiconductors and study the movement of charge (i.e., the flow of current) in them. Next, we deal with the the “ junction,” which also serves as diode, and formulate its behavior. Our ultimate goal is to represent the device by a circuit model (consisting of resistors, voltage or current sources, capacitors, etc.), so that a circuit using such a device can be analyzed easily. The outline is shown below. Charge Carriers Doping Transport of Carriers PN Junction Structure Reverse and Forward Bias Conditions I/V Characteristics Circuit Models Semiconductors It is important to note that the task of developing accurate models proves critical for all mi- croelectronic devices. The electronics industry continues to place greater demands on circuits, calling for aggressive designs that push semiconductor devices to their limits. Thus, a good un- derstanding of the internal operation of devices is necessary. As design managers often say, “If you do not push the devices and circuits to their limit but your competitor does, then you lose to your competitor.” 21
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 22 (1) 22 Chap. 2 Basic Physics of Semiconductors 2.1 Semiconductor Materials and Their Properties Since this section introduces a multitude of concepts, it is useful to bear a general outline in mind: Charge Carriers in Solids Crystal Structure Bandgap Energy Holes Modification of Carrier Densities Intrinsic Semiconductors Extrinsic Semiconductors Doping Transport of Carriers Diffusion Drift Figure 2.1 Outline of this section. This outline represents a logical thought process: (a) we identify charge carriers in solids and formulate their role in current flow; (b) we examine means of modifying the density of charge carriers to create desired current flow properties; (c) we determine current flow mechanisms. These steps naturally lead to the computation of the current/voltage (I/V) characteristics of actual diodes in the next section. 2.1.1 Charge Carriers in Solids Recall from basic chemistry that the electrons in an atom orbit the nucleus in different “shells.” The atom’s chemical activity is determined by the electrons in the outermost shell, called “va- lence” electrons, and how complete this shell is. For example, neon exhibits a complete out- ermost shell (with eight electrons) and hence no tendency for chemical reactions. On the other
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