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Bipolar Junction Transistors (1) Dr. Wojciech Jadwisienczak EE314

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Introduction Your goal is to explain the transistor. It is assumed that EE314 students to which this presentation is aimed, have not a clue to how these little Buggers work and/or how to use them. A real problem with previous explanations: for the sake of "fidelity" authors' include confusing details until the concept, or thread--of how they actually work & how to use them--is lost. The following presentation is comprised of several different explanations. You should read chapter 13 and this presentation several times, because any insight gained from one will help in understanding another.

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Chapter 13: Bipolar Junction Transistors pp History of BJT 2.First BJT 3.Basic symbols and features 4.A little bit of physics… 5.Currents in BJT’ 6.Basic configurations 7.Characteristics

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The transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research. First - BJTs Reference: Bell Labs Museum B. G. Streetman & S. Banerjee ‘Solid State Electronic Devices’, Prentice Hall 1999.

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Picture from previous slide shows the workbench of John Bardeen and Walter Brattain at Bell Laboratories. They were supposed to be doing fundamental research about crystal surfaces. The experimental results hadn't been very good, though, and there's a rumor that their boss, William Shockley, came near to canceling the project. But in 1947, working alone, they switched to using tremendously pure materials. It dawned on them that they could build the circuit in the picture. It was a working amplifier! John and Walter submitted a patent for the first working point contact transistor. Shockley was furious and took their work and invented the junction transistor and submitted a patent for it 9 days later. The three shared a Nobel Prize. Bardeen and Brattain continued in research (and Bardeen later won another Nobel). Shockley quit to start a semiconductor company in Palo Alto. It folded, but its staff went on to invent the integrated circuit (the "chip") and to found Intel Corporation. By 1960, all important computers used transistors for logic, and ferrite cores for memory. Interesting story…

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Point-Contact Transistor – first transistor ever made

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Qualitative basic operation of point-contact transistor Problems with first transistor…

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First Bipolar Junction Transistors W. Shockley invented the p-n junction transistor The physically relevant region is moved to the bulk of the material

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Understanding of BJT force – voltage/current water flow – current - amplification

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Basic models of BJT Diode npn transistor pnp transistor

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Qualitative basic operation of BJTs

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Basic models of BJT

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BJTs – Basic Configurations Fluid Flow Analogy Difference between FET (field effect transistor) and BJT Technology of BJTs pnp BJT npn BJT

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BJTs – Practical Aspects Heat sink

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BJTs – Testing

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A little bit of physics…

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More accurate physical description… pnp BJT 1.Injected h + current from E to B 2. e - injected across the forward-biased EB junction (current from B to E) 3. e - supplied by the B contact for recombination with h + (recombination current) 4. h + reaching the reverse-biased C junction 5,6.Thermally generated e - & h + making up the reverse saturation current of the C junction iEiE -i B -i C -V CE

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Now, you can try… npn BJT

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BJTs – Basic configurations

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npn BJTs – Operation Modes Forward & reverse polarized pn junctions Different operation modes:

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npn BJTs – Operation Modes When there is no I B current almost no I C flows When I B current flows, I C can flow The device is then a current controlled current device Operational modes can be defined based on V BE and V BC

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BJT-Basic operation npn BJTpnp BJT (n + ), (p + ) – heavy doped regions; Doping in E>B>C

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Operation mode: v BE is forward & v BC is reverse The Shockley equation I ES –saturation I ( A); V T =kT/q -thermal V (26meV) D – diffusion coefficient [cm 2 /s] – carrier mobility [cm 2 /Vs] The Kirchhoff’s laws It is true regardless of the bias conditions of the junction Useful parameter the common-emitter current gain for ideal BJT is infinite BJTs – Current & Voltage Relationships Einstein relation

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Useful parameter the common-base current gain for typical BJT is ~0.99 The Shockley equation once more If we define the scale current A little bit of math… search for i B Finally… BJTs – Current & Voltage Relationships

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BJTs – Characteristics Schematic Common-Emitter Input Output V BC V BE If V CE < V BE the B-C junction is forward bias and I C decreases Remember V BE has to be greater than V Example 13.1

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BJTs – Load line analysis Common-Emitter Amplifier Input loop if i B =0if v BE =0 smaller v in (t)

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BJTs – Load line analysis Output loop Common-Emitter Amplifier Example 13.2

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Circuit with BJTs Our approach: Operating point - dc operating point Analysis of the signals - the signals to be amplified Circuit is divided into: model for large-signal dc analysis of BJT circuit bias circuits for BJT amplifier small-signal models used to analyze circuits for signals being amplified Remember !

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Large-Signal dc Analysis: Active-Region Model Important: a current-controlled current source models the dependence of the collector current on the base current The constrains for I B and V CE must be satisfy to keep BJT in the active-mode V BE forward bias V CB reverse bias ? ?

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Large-Signal dc Analysis: Saturation-Region Model V BE forward bias V CB forward bias ? ?

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Large-Signal dc Analysis: Cutoff-Region Model V CB reverse bias V BE reverse bias ? ? If small forward-bias voltage of up to about 0.5 V are applied, the currents are often negligible and we use the cutoff-region model.

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Large-Signal dc Analysis: characteristics of an npn BJT

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Large-Signal dc Analysis Procedure: (1) select the operation mode of the BJT (2) use selected model for the device to solve the circuit and determine I C, I B, V BE, and V CE (3) check to see if the solution satisfies the constrains for the region, if so the analysis is done (4) if not, assume operation in a different region and repeat until a valid solution is found This procedure is very important in the analysis and design of the bias circuit for BJT amplifier. The objective of the bias circuit is to place the operating point in the active region. Bias point – it is important to select I C, I B, V BE, and V CE independent of the and operation temperature. Example 13.4, 13.5, 13.6

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Large-Signal dc Analysis: Bias Circuit From Example 13.6 Remember: that the Q point should be independent of the stability issue) V BB & V CC provide this stability, however this impractical solution Other approach is necessary to solve this problem-resistor network V BB acts as a short circuit for ac signals

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Large-Signal dc Analysis: Four-Resistor Bias Circuit Thevenin equivalent Equivalent circuit for active-region model Solution of the bias problem: Input Output

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npn BJTs – Practical Aspects

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