note04 - Introduction to Microelectronics Chapter 4...

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Fig. 4.1. The original BJT by Bell Labs in 1947. Introduction to Microelectronics Chapter 4 P HYSICAL M ODELS FOR BJT AND MOSFET 4.1 BJT (Bipolar Junction Transistor) The bipolar junction transistor (BJT) was invented and experimentally demonstrated by Bardeen, Brattain and Shockley in 1947 in Bell Labs, Murray Hills, NJ, and is often regarded as the birth of modern microelectronics. Interestingly, the metal-oxide-semiconductor field-effect transistors (MOSFET), the dominant market leader today, was proposed earlier in 1930 without experimental demonstration, but it was predicted not to work due to the oxide-semiconductor interface concerns people have in those days. Before 1947, switching and amplification were done by vacuum tubes, which have successfully produced radios, radars and very expensive, large and hot computers with limited capabilities. Vacuum tubes dissipated a lot of heat due to the high voltage and current required to emit electron beams from a hot filament. They are also very difficult to scale in geometry. Although for large-power applications such as radar, vacuum tubes (or a more modern version of traveling-wave tubes (TWT)) are still in use. Engineers had dreamed for a replacement in the 1940’s, but was limited by a method that can reliably define a small geometry (on the order of 2 μ m, the reason of which will be clear later). In 1947, Bardeen and Brattain finally found that a sharp knife could create a wedge of that dimension, as shown in Fig. 1. The main amplification operation of BJT is by injecting carriers from a forward-bias junction to a reverse-biased region. We can use BJT as a good switch, because the carrier flow through the reverse-biased junction is not determined by the voltage across, but by how much is injected, which is controlled by the forward-bias part v IN ! Repeating Eq. (3.21), we know that BJT is a good small-signal amplifier as well!! OUT OUT IN OUT v i v i or 1 1 = = - v out m OUT OUT IN OUT A R g v i v i (4.1) There are two possible ways to implement BJT by the pn junction, NPN and PNP, denoted by the type of materials in concatenation. NPN is usually a bit more popular than PNP due to the higher electron mobility, but sometime we have to use both for the voltage polarities required. The terminal names, emitter (E), base (B) and collector (C) whose meaning will be further explained in the carrier operations, and circuit symbols are shown in Fig. 4.2. To Edwin C. Kan Page 4-1 9/9/2009
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Introduction to Microelectronics implement the generic two-port network in Figs. 1.17 and 1.18, we need three terminals, one at input, one at output and one at “common” as the reference. The output current in Eq. (4.1), most often the collector current I C , will be a function of all terminal voltages. From KVL, three- terminal devices will have two degrees of freedom in biasing, since the potential has to be taken as a difference. We will use the voltage convention, as V BE =V B – V
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note04 - Introduction to Microelectronics Chapter 4...

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