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ELECTRONIC DEVICES AND CIRCUIT THEORY

BIPOLAR JUNCTION

TRANSISTORS

ELECTRONIC DEVICES AND CIRCUIT THEORY

INTRODUCTION TO TRANSISTORS ❑ Beside diodes, the most popular semiconductor devices is transistors. ❑ Transistors are often said to be the most significant invention of the 20 th Century. ❑ Transistors are the building blocks of the digital revolution. ❑ Without transistors, the technological wonders you use everyday – cellphones, computers, cars – would be vastly different, if they existed at all. ❑ Transistors are more complex and can be used in many ways. ❑ Most important feature: can amplify signals ad act as switch. ❑ Amplification can make weak signal strong (make sound louder and signal levels greater). ❑ In general, they provide a function called Gain.

INTRODUCTION TO TRANSISTORS ❑ HISTORY OF TRANSISTORS

• In the mid 1940s, a team of scientists working for Bell Technology Labs were working to discover a device to replace the then present vacuum tube technology.
• Vacuum tubes were the only technology available at that time to amplify signals or serve as switch devices in electronics.
• CONS: very expensive, consume a lot of power, gave off too much heat and unreliable
• In 1947, John Bardeen and Walter Brattain devised the first “point contact” transistor.

INTRODUCTION TO TRANSISTORS ❑ Two primary classes of transistor:

1. Bipolar Junction Transistors (BJTs)
2. Field-Effect Transistors (FETs) ❑ Difference between NPN/PNP and NMOS/PMOS is the type of semiconductor (N-type/P-type) used at each terminal.

BIPOLAR JUNCTION TRANSISTORS ❑ BJTs are essentially two back-to-back PN junctions. ❑ Two types:

1. NPN 2. PNP

TRANSISTOR CONSTRUCTION ❑ BJT is bipolar because both holes (+) and electrons (-) will take part in the current flow through the device. ❑ N-type regions contains free electrons (negative carriers) ❑ P-type regions contains free holes (positive carriers) ❑ The transistor regions are: ❑ Emitter (E) – send the carriers into the base region and then on to the collector ❑ Base (B) – acts as control region. It can allow none, some or many carriers to flow ❑ Collector (C) – collects the carrier

BJT MODES OF OPERATION ❑ There are two junction in bipolar junction transistor.

**1. Emitter-Base Junction

2. Collector-Base Junction** ❑ Each junction can be forward or reverse biased independently. ❑ Thus, there are different modes of operation: ❑ Forward Active ❑ Cut off ❑ Saturation

OPERATION OF PNP TRANSISTOR IN ACTIVE MODE ❑ With the external sources, VEE and VCC, connected: ❑ The EBJ is forward biased. ❑ The BCJ is reverse biased.

CURRENTS IN A TRANSISTOR ❑ The minority current is called the leakage current and is given by ICO (IC with emitter terminal open)

COMMON BASE CONFIGURATION ❑ INPUT CHARACTERISTICS ❑ This curve shows the relationship between input current (IE) and input voltage (VBE) for three output voltage levels (VCB).

COMMON BASE CONFIGURATION ❑ OUTPUT CHARACTERISTICS ❑ This graph demonstrates the output current (IC) to an output voltage VCB for various levels of input current (IE).

APPROXIMATIONS ❑ EMITTER AND COLLECTOR CURRENTS: IC ≅ IE ❑ BASE-EMITTER VOLTAGE: VBE = 0. 7 V (For Silicon)

ALPHA ❑ ALPHA ( 𝜶) is the ratio of IC to IE: ❑ Ideally, 𝜶 = 1. ❑ In reality, 𝜶 is between 0. 9 and 0. 998. ❑ ALPHA ( 𝜶) in the AC mode: