chapter6 - C HAPTER 6 Semiconductor Devices Most diodes are...

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CHAPTER 6 Semiconductor Devices Most diodes are essentially pn junctions fabricated by forming a contact between a p-type and an n-type semiconductor. The junction possesses rectifying properties in that a current in one direction can flow quite easily whereas in the other direction it is limited by a leakage current that is generally very small. A transistor is a three-terminal solid-state device in which a current flowing between two electrodes is controlled by the voltage between the third and one of the other terminals. Transistors are capable of providing current and voltage gains thereby enabling weak signals to be amplified. Transistors can also be used as switches just like electromagnetic relays. Indeed, the whole microcomputer industry is based on transistor switches. The majority of the tran- sistors in microelectronics are of essentially two types: bipolar juuction transistors (BJTs) and field effect transistors (FETs). The appreciation of the underlying princi- ples of the pn junction is essential to understanding the operation of not only the bipo- lar transistor but also a variety of related devices. The central fundamental concept is the minority carrier injection as purported by William Shockley in his explanations of the transistor operation. Field effect transistors operate on a totally different princi- ple than BJTs. Their characteristics arise from the effect of the applied field on a con- ducting channel between two terminals. The last two decades have seen enormous ad- vances and developments in optoelectronic and photonic devices which we now take for granted, the best examples being light emitting diodes (LEOs), semiconductor lasers, photodetectors, and solar cells. Nearly all these devices are based on pn junc- tion principles. The present chapter takes the semiconductor concepts developed in Chapter 5 to device level applications, from the basic pn junction to heterojunction laser diodes. 475
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I --- 476 CHAPTER 6 SEMICONDUCTOR DEVICES 6.1 IDEAL pn JUNCTION 6.1.1 No ApPLIED BIAs: OPEN CIRCUIT Consider what happens when one side of a sample of Si is doped n-type and the other p-type, as shown in Figure 6.1a. We assume that there is an abrupt discontinuity between the p- and n-regions, which we call the metallurgical junction and label as M in Figure 6.1 a, where the fixed (immobile) ionized donors and the free electrons (in the conduction band, CB) in the n-region and fixed ionized acceptors and holes (in the valence band, VB) in the p-region are also shown. Due to the hole concentration gradient from the p-side, where p = Ppo, to the n-side, where p = Pno, holes diffuse toward the right. Similarly the electron concentration EB (a) <B M Metallurgical junction 'E(x) --_ . 'mm~
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This note was uploaded on 11/16/2011 for the course EMA 3413 taught by Professor Xue during the Spring '11 term at University of Florida.

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chapter6 - C HAPTER 6 Semiconductor Devices Most diodes are...

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