Bipolar Junction Transistor

Transcription

1 ROCHESTER INSTITUTE OF TECHNOLOGY MICROELECTRONIC ENGINEERING Bipolar Junction Transistor Dr. Lynn Fuller Webpage: 82 Lomb Memorial Drive Rochester, NY Tel (585) Department webpage: BJT.ppt Page 1

2 OUTLINE Definitions Schematic Symbols Theory Integrated BJT Structure Modes of Operation IC-VCE Family of Curves Modifications References Homework Questions Page 2

3 DEFINITIONS Bipolar Junction Transistor - (BJT) Both holes and electrons participate in the conduction of current, hence the name bipolar. Minority carrier - In a p-type semiconductor electrons are the minority carrier type, in an n-type semiconductor holes are the minority carrier type. Emitter - Emits minority carriers into the base region of a BJT. For example, in an NPN BJT the n-type emitter, emits electrons into the p-type base. The emitter usually has the highest doping levels of the three regions of a BJT. Base - Thin region which is used to control the flow of minority carriers from the emitter to the collector Collector -Collects the minority carriers that make it through the base from the emitter. The collector usually has the lightest doping concentrations of the three regions. DC Beta ( β dc ) - The ratio of the collector current to the base current. β dc = I C / I B AC Beta ( β ac ) - The ratio of the change in the collector current to the change in the base current. β ac = I C / I B Page 3

4 BJT - BIPOLAR JUNCTION TRANSISTOR Label 2N3904 Flat Emitter Base Collector Page 4

5 SCHEMATIC SYMBOLS npn Collector pnp Collector Base n p n Base p n p Emitter Emitter Collector Collector Base Base Emitter Emitter The arrow on the emitter is in the direction that current will flow in the Base Emitter pn junction Page 5

6 IDEALIZED STRUCTURE p-type Emitter N SC P SC N Collector n-type Base n-type Page 6

7 ELECTRON CONCENTRATIONS IN AN NPN BJT Emitter Base Collector n~nde n~very small but not zero ~ni2/nab BE Space Charge Layer E BC n~ndc x With the B-E junction forward biased, and B-C junction reverse biased. There is a concentration gradient in the base that forces electrons to flow toward the collector. Page 7

8 COMMENTS 1. The concentration of electrons in n-type silicon is ~ doping concentration in that region. 2. In p-type silicon the number of electrons is almost zero 3. A forward biased pn junction means more carriers of both types can cross the potential barrier. So a forward biased base-emitter junction (in an npn BJT) means more electrons on the base side than in equilibrium (no bias). 4. A reverse biased pn junction means less carriers of both types can cross the potential barrier. So a reverse biased base-collector junction (in an npn BJT) means less electrons on the base side than in equilibrium (no bias). Even closer to zero electrons in p-type base at the edge of the B-C space charge layer. 5. The base is so narrow that few electrons a lost as they diffuse across the base width. Diffusion is driven by a concentration gradient. So electrons move towards the collector and current flows in the opposite direction. Page 8

9 INTEGRATED BJT STRUCTURE Collector (n) Base (p) Emitter (n + ) ~ cm -3 ~ cm -3 p-type ~ cm -3 electrons current Lightly doped (~10 15 cm -3 ) n-type silicon wafer Since the emitter is more heavily doped compared to the base than the collector, the emitter-base junction has a lower breakdown voltage than the base-collector junction. n + means heavily doped n-type n - means lightly doped n-type p + means heavily doped p-type p - means lightly doped p-type Page 9

10 CHARACTERISTICS OF TWO TERMINAL DEVICES I I + V - V Page 10

11 BE JUNCTION, BC JUNCTION, CE I I I V V V Base I + V - Emitter Base I V + - Collector I V + Collector - Emitter Page 11

12 CHARACTERISTICS OF THREE TERMINAL DEVICES Iin Iout Iin Vin + Vin - Common Each trace if for a different value for Iout or Vout + Vout - Iout Vout Each trace is for a different value for Iin or Vin Page 12

13 BJT IC-VCE FAMILY OF CURVES I C 10 ma 9 ma 8 ma 7 ma 6 ma 5 ma 4 ma 3 ma 2 ma 1 ma DI C = 2.5 ma Steps of base current - I B b dc b ac I B = 30 ma 10 ma increments I B = 20 ma DI B = 10 ma I B = 10 ma V CE Beta (b ac ) = Beta (b dc ) = DI C DI B I C I B 2.5x10 = -3 = x x10 = -3 = x10-6 The two Beta values are not always the same! Page 13

14 NPN COMMON EMITTER IC-VCE CHARACTERISTICS Forward Active Mode Base-Emitter junction is forward biased Base-Collector junction is reverse biased V a > 0 is a forward biased junction V a < 0 is a reverse biased junction Va is defined as the voltage from p to n npn I C base current steps I B B-C junction is reverse biased base - p V bc 0.7 V negative + + V be positive B-E junction is forward biased V Collector - n Emitter - n 0 V Sweep from 0 to 5 Volts Inverse Active Mode Base-Emitter junction is reverse biased Base-Collector junction is forward biased Switch the connections to the March emitter 2, 2009 Dr. and Lynn collector Fuller, Professor leads Page 14

15 TRIPLE DIFFUSED BJT STRUCTURE Collector Emitter Base p-wafer n+ n+ n-well collector p-base Doping Conc n+ emitter 1-D Doping Profile Simple BJT structure Large collector series resistance Large dimensions Isolation issues n-well collector p-base depth W B Base Width W B ~ 0.5mm Page 15

16 SHALLOW TRENCH ISOLATION Collector Emitter Base SiO 2 n+ plug SiO2 n+ p-base SiO 2 n-type epitaxial silicon p-wafer n+ buried layer Process Enhancements Oxide-plug isolation Patterned buried sub-collector Epitaxial silicon Performance Improvements Low collector series resistance Improved collector/emitter isolation smaller geometries Page 16

17 DESIGN A BJT TESTER Page 17

18 DESIGN A BJT COMMON EMITTER AMPLIFIER Page 18

19 REFERENCES 1. Sedra and Smith, 2. Device Electronics for Integrated Circuits, 2nd Edition, Kamins and Muller, John Wiley and Sons, The Bipolar Junction Transistor, 2nd Edition, Gerold Neudeck, Addison-Wesley, Page 19

20 HOMEWORK - BJT S 1. Why won t two back to back diodes behave like a BJT? 2. Sketch a figure like that on page 7 showing the hole concentration for a pnp transistor with B-E junction forward biased and B-C junction reverse biased. Show direction of current flow. 3. Design Page 20