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Unformatted text preview: ECE 654 Solid State Devices II Prof. S. Mohammadi - 122 - Chapter 8 Homojunction and Heterojunction Bipolar Transistors Bipolar junction transistors (BJTs) and hereojunction bipolar transistors (HBTs) represent two of the most interesting devices that have been introduced over the past five decades as each of the two devices account for a noble prize in Physics (Brattain and Shockley, 1956 and Kroemer 2000). Although bipolar transistors transfer characteristics are exponential, they are extremely linear when fed with an input current signal. As a result, they are often preferred in linear power amplifiers. Their main advantage of CMOS and other unipolar transistors such as MESFETs and HEMTs is that they do not require sub-micron technology to operate in multi-GHz frequency range. In this chapter, we first introduce basic operation of bipolar junction transistors. Then we discuss the physics of uniformly doped BJTs and briefly discuss high current effects in these devices. We then discuss the promises of HBTs and why they are so much advantageous over their homojunction counterparts. We then discuss different material systems that are used to build HBTs and noise and power performance of HBTs. We conclude this chapter by presenting advantages of HBTs over MESFETs, HEMTS and CMOS transistors. A Simplified Picture of BJT Operation BJT uses a small input current (Base Current) to control a larger output current (Collector current). BJT comes in both NPN and PNP configurations each with three terminals known as base, emitter and collector. Direction of the arrows in the symbols show the direction of positive current. Under normal operation condition, the base-emitter junction is forward biased while the collector-base junction is reversed biased. Note that if you have two separate diodes one forward and one Fig. 8.1. NPN and PNP transistor Symbols. A simple PNP transistor cross section. Collector NPN Emitter Base Collector PNP Emitter Base E B C P+ N P I E I B I C PNP transistor Collector NPN Emitter Base Collector PNP Emitter Base E B C P+ N P I E I B I C PNP transistor ECE 654 Solid State Devices II Prof. S. Mohammadi - 123 - reverse biased back to back, you will not get any current through. In BJTs the two closely placed PN junctions have current because the reversed biased junction steals the current of the forward biased junction. The following explains what really happens. Imagine that you have an N + P junction. When you forward bias this diode, current that is formed is mainly due to electrons being injected from the N+ region to P region. Only small part of the current is due to holes being injected to N+ region. In 1954, Brattain and Shockley decided to steal some of the electron currents and send it to a third terminal using a structure similar to the one shown in Fig....
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This note was uploaded on 02/19/2012 for the course ECE 654 taught by Professor Mohammadi during the Spring '08 term at Purdue University-West Lafayette.
- Spring '08