Breakdown of Bipolar-Junction-Transistors

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					Breakdown of Bipolar-Junction-Transistors




                 Breakdown of Bipolar-Junction-Transistors

                                             Daniel Kraft


                                            April 24th, 2008
Breakdown of Bipolar-Junction-Transistors




Outline

       Introduction to BJTs
           Usage of BJTs
           Mode of Operation
           Breakdown

       Derivation of Breakdown Condition
          Current Flow Analysis
          Solving the Equation
          Conclusion for Breakdown

       Experimental Results

       Summary
Breakdown of Bipolar-Junction-Transistors
   Introduction to BJTs
     Usage of BJTs



Usage of BJTs



          BJTs are a kind of transistors
          Used for current amplification
          Easily built on semiconductor wafers
                  Cheap and small
                  Good electrical properties and speed
                  Transistor-of-choice for most micro-chip
                  usages
Breakdown of Bipolar-Junction-Transistors
   Introduction to BJTs
     Mode of Operation



Semiconductor Diodes
               Junction of P-type and N-type semiconductors
               Done by different doping
                       Electrons in N region
                       Holes in P region
                       Recombination in depletion zone
               Outer electrical field changes width of depletion zone
               Current flow possible in one direction only
Breakdown of Bipolar-Junction-Transistors
   Introduction to BJTs
     Mode of Operation



Structure of BJTs
               Two diodes with one joined electrode
               NPN-type and PNP-type, respectively
               Potential between C-E: C-B reverse-bias
               Potential between B-E: Current flow
                       Very thin base
                       Most electrons get into C region
                       Amplified current flow between E and C
Breakdown of Bipolar-Junction-Transistors
   Introduction to BJTs
     Breakdown



Reason of Breakdown



       Too high potential on reverse-bias diode
       Electrons highly accelerated
       Impact-ionization creates new electron/hole
       pairs
       Avalanche-effect leads to breakdown current
       Can partly “escape” through base if not
       open
Breakdown of Bipolar-Junction-Transistors
   Derivation of Breakdown Condition
     Current Flow Analysis



Current Flow in a BJT
Breakdown of Bipolar-Junction-Transistors
   Derivation of Breakdown Condition
     Current Flow Analysis



Finding the Equation



                                                 UB                  R                1
                ⇒ InIN = IS exp                           exp InIN        (M − 1) −
                                                 UT                  UT               β

               Equation of the form x = a exp(bx) with
                                            UB
                       a = IS exp           UT
                              R                       1
                       b=     UT       (M − 1) −      β
Breakdown of Bipolar-Junction-Transistors
   Derivation of Breakdown Condition
     Solving the Equation



Solution Possibilities




       Two solutions
       One solution
       No solution possible
Breakdown of Bipolar-Junction-Transistors
   Derivation of Breakdown Condition
     Solving the Equation



Solution Possibilities




       Two solutions
       One solution
       No solution possible
Breakdown of Bipolar-Junction-Transistors
   Derivation of Breakdown Condition
     Solving the Equation



Solution Possibilities




       Two solutions
       One solution
       No solution possible
Breakdown of Bipolar-Junction-Transistors
   Derivation of Breakdown Condition
     Solving the Equation



Condition for One Solution


       a “shifts” graph
       Exponential function should “touch”
       straight line
       First derivative must be 1
       This leads to condition:

           x = a exp(bx) has exactly one solution
                                            ⇔
                                                1
                            ab = exp(−1) =      e
Breakdown of Bipolar-Junction-Transistors
   Derivation of Breakdown Condition
     Conclusion for Breakdown



Back to BJT Breakdown
               Possible current InIN is given by:
                                            UB               R                1
                       InIN = IS exp              exp InIN        (M − 1) −
                                            UT               UT               β
               No possible value for “no solution” case
               ⇒ this is the breakdown
               Breakdown condition:
                                                 1   UT      1
                                       M −1=       +
                                                 β   R I exp 1 +    UB
                                                        S           UT

               For R → ∞ this gives already known relation:
                                                             1
                                                  M −1=
                                                             β
Breakdown of Bipolar-Junction-Transistors
   Experimental Results




Calculated vs Measured Breakdown
Breakdown of Bipolar-Junction-Transistors
   Summary




Conclusion




               We discussed semiconductor diodes and BJTs
               Breakdown limits usage voltage
               For connected base we derived a formula for the BV
                       Open base: Reduces to known relation
                       Matches with measurements
Breakdown of Bipolar-Junction-Transistors
   Summary




Thanks for your attention!
Breakdown of Bipolar-Junction-Transistors
   Summary




Thanks for your attention!




                                            And don’t forget to switch on your
                                            mobile again now
Breakdown of Bipolar-Junction-Transistors
   Summary




Thanks for your attention!




                                            And don’t forget to switch on your
                                            mobile again now — it’s a lot of
                                            BJTs

				
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posted:12/18/2011
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