Applied Electronics chapter 3, BJT Adama University, Dept of Electrical Engineering 1 CHAPTER THREE BIPOLAR JUNCTION TRANSISTORS 3.1 INTRODUCTION Transistors are three terminal, three-layered, two-junction electronic devices whose voltage-current relationship is controlled by a third voltage or current. We may regard a transistor as a controlled voltage or current source. They were demonstrated by a team of scientists at Bell laboratories in 1947 and their introduction brought an end to the era of vacuum tube devices . Advantages of transistors over vacuum tubes: • Smaller size, light weight • No heating elements required • Low power consumption • Low operating voltages Areas of application: Used in applications such as signal amplifiers, electronic switches, oscillators, design of digital logics, memory circuits etc. Physical structure of transistors: According to the physics of the device, we can classify transistors into two main classes: 1. Bipolar junction transistors (BJT) 2. Field effect transistors (FET) (to be discussed in the next chapter). BJT: definition, construction, normal operating condition & symbolic representation BJT is a diode-based transistor which is usually blocked unless the control terminals are forward biased. So, the controlling agent is a current and BJT is a current amplifier by nature. Two types of construction exist namely: • Thin layer of n-type material sandwiched between two p-type materials (called a PNP transistor). • Thin layer of p-type material sandwiched between two n-type materials (called NPN transistor) Fig 3.1 physical structure and the terminals of BJT Emitter (E) is heavily doped – supplies charge carriers. Base (B) is lightly doped – allows most of the charge carriers to pass through it. Collector is moderately doped(C) – collects the charge carriers. We can also see that there are two junctions shared between the three terminals, the Emitter-base junction and Collector-base junction .
Applied Electronics chapter 3, BJT Adama University, Dept of Electrical Engineering 2 The normal operating condition prevails if E-B junction is forward biased and C-B junction is reverse biased. The following figure shows the symbolic representation of the PNP and NPN transistors. In each case arrow head represents the direction of current through emitter. Fig. 3.2: Transistor symbolic representation 3.2 PRINCIPLE OF OPERATION AND CHARACTERISTICS The working principle of NPN transistor is discussed here and that of PNP transistor is similar except the fact that roles of free electrons and holes are interchanged and current directions are reversed. Principle of operation of an NPN BJT transistor • EB diode is forward biased. So, depletion region at EB junction is narrow. • CB diode is reverse biased. So, depletion region at CB junction is wide.
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- Winter '19
- Abebaw Dagu
- Transistor, Bipolar junction transistor, APPLIED ELECTRONICS, Dept of Electrical Engineering, adama university