Lecture 12 Bipolar Junction Transistor (BJT) BJT 1-1
Course Info Lecture hours: 4 Two Lectures weekly (Saturdays and Wednesdays) Location: K2 Time: 1:40 pm Tutorial hours: 2 One tutorial class every week Lab hours: 1 One experiment every two weeks Course website: http://eng.alexu.edu.eg/~bmokhtar/courses/electronics_ee/spring_2018 /electronics_ee.htm BJT 1-2
Main References Lecture Notes (Presentation Slides and Whiteboard) Text Books R. Boylestad and L. Nashelsky, Electronic Devices and Circuit Theory, 7 th edition, Prentice Hall, 1998 B. Razavi, Fundamentals of Microelectronics, 1 st edition, 2006 BJT 1-3
Outline Introduction to Bipolar Junction Transistor (BJT) BJT principles and characteristics BJT configurations Currents in BJT BJT 1-4
Introduction to transistors The transistor is the main building block element of electronics It is a semiconductor device It is a three terminal device Two basic families of transistors are: Bipolar Junction Transistor (BJT) Field Effect Transistor (FET) Transistors are used in: Amplifiers Electronic switches Digital logic Memory circuits BJT 1-5
Bipolar Junction Transistor (BJT) There are two types of BJT: pnp npn The terminals are labeled: E - Emitter B - Base C - Collector npn and pnp BJT symbols BJT 1-6
npn BJT Structure The BJT is fabricated with three separately doped regions The BJT has two junctions (boundaries between the n and the p regions) Junctions are similar to the pn junctions (diodes) where they may be forward biased or reverse biased Emitter-base junction (EBJ) and collector-base junction (CBJ) These junctions have capacitance BJT 1-7
BJT Operation Depending on the biasing across each of the junctions, different modes of operation are obtained: cutoff, active, and saturation BJT in linear amplifier circuits: Active Operating range of the amplifier. Cutoff The amplifier is basically off. There is voltage, but little current. Saturation The amplifier is full on. There is current, but little voltage. BJT 1-8
BJT Operation (Active Mode) Two external voltage sources set the bias conditions for active mode The emitter-base junction (EBJ) is forward biased The base-collector junction (BCJ) is reverse biased BJT 1-9
Currents in BJT BJT operation in active mode Stronger DC voltage source. Why? BJT 1-10
Currents in BJT The current through EBJ is related to the B-E voltage as Due to the large differences in the doping concentrations of the emitter and the base regions, the electrons injected into the base region (from the emitter region) results in the emitter current I E BJT 1-11
Currents in BJT (cont d) The number of electrons injected into the collector region is directly related to the electrons injected into the base region from the emitter region So, the collector current is related to the emitter current BJT basic principle: The voltage between two terminals controls the current through the third terminal BJT 1-12
Currents in BJT (cont d) Emitter current is the sum of the collector and base currents: The collector current and the base current are related by β or h FE represents the amplification factor of transistor (β = 100 in transistors of interest) The relationship between the emitter and the base currents is The relationship between the emitter and the collector currents is Where is, called collector efficiency (α is between 0.9 and 0.998 in transistors of interest) BJT 1-13
Variation of β with I C for several temperatures BJT 1-14
Directions of currents and voltage polarities BJT 1-15
BJT Configurations Common-Base configuration The base is common to both input (emitter base) and output (collector base) of the transistor BJT 1-16
BJT Configurations Common-Base configuration Emitter (or input) Characteristics This curve shows the relationship between of input current (I E ) to input voltage (V BE ) for three output voltage (V CB ) levels BJT 1-17
BJT Configurations Common-Base configuration Collector (or output) Characteristics This graph demonstrates the output current (I C ) to an output voltage (V CB ) for various levels of input current (I E ) BJT 1-18
Lecture Summary Covered material Introduction to Bipolar Junction Transistor (BJT) BJT principles BJT structure BJT operation modes Currents in BJT Introduction to BJT configurations Material to be covered next lecture Continue BJT BJT configurations DC Biasing and analysis BJT 1-19