1. EE 434 Lecture 22 Bipolar Device Models

2. Quiz 14 The collector current of a BJT was measured to be 20mA and the base current measured to be 0.1mA. What is the efficiency of injection of electrons coming from the emitter to the collector?

3. And the number is ….

5. Quiz 14 The collector current of a BJT was measured to be 20mA and the base current measured to be 0.1mA. What is the efficiency of injection of electrons coming from the emitter to the collector? Solution: Efficiency = α

6. –Bipolar device operation dependent upon how minority carriers in base contribute to collector current –Bipolar model (in high gain region) Diode model for BE junction, injection efficiency for I C α –Bipolar transistor is inherently a current amplifier with exponential relationship between collector current and V BE This property makes BJT very useful Review from Last Time

7. Bipolar Models Simple dc Model Small Signal Model Frequency-Dependent Small Signal Model Better Analytical dc Models Sophisticated Model for Computer Simulations Better Models for Predicting Device Operation

8. Bipolar Models Simple dc Model Small Signal Model Frequency-Dependent Small Signal Model Better Analytical dc Models Sophisticated Model for Computer Simulations Better Models for Predicting Device Operation

9. Bipolar Models Simple dc Model following convention, pick I C and I B as dependent variables and V BE and V CE as independent variables

10. Simple dc model From last time : This has the properties we are looking for but the variables we used in introducing these relationships are not standard It can be shown that is proportional to the emitter area A E Define and substitute this into the above equations

11. Simple dc model J S is termed the saturation current density Process Parameters : J S,β Design Parameters: A E Environmental parameters and physical constants: k,T,q At room temperature, V t is around 26mV J S very small – around.25fA/u 2

12. Transfer Characteristics J S =.25fA/u 2 A E =400u 2 V BE close to 0.6V for a two decade change in I C around 1mA

13. Transfer Characteristics J S =.25fA/u 2 A E =400u 2 V BE close to 0.6V for a four decade change in I C around 1mA

14. Simple dc model ICIC V CE V BE or I B Output Characteristics

15. Simple dc model ICIC V CE V BE or I B Better Model of Output Characteristics

16. Simple dc model V BE or I B Typical Output Characteristics ICIC V CE Forward Active Saturation Cutoff Forward Active region of BJT is analogous to Saturation region of MOSFET Saturation region of BJT is analogous to Triode region of MOSFET

17. Simple dc model V BE or I B Typical Output Characteristics ICIC V CE Projections of these tangential lines all intercept the –V CE axis at the same place and this is termed the Early voltage, V AF (actually –V AF is intercept) Typical values of V AF are in the 100V range

18. Simple dc model V BE or I B Improved Model ICIC V CE Valid only in Forward Active Region

19. Simple dc model V BE or I B Improved Model ICIC V CE Valid in All regions of operation Not mathematically easy to work with Note dependent variables changes Termed Ebers-Moll model V AF effects can be added Reduces to previous model in FA region

20. Simple dc model Ebers-Moll model Process Parameters: {J S, α F, α R } Design Parameters: {A E } α F is the parameter α discussed earlier α R is termed the “reverse α” J S ~10 -16 A/μ 2 β F ~100, β R ~0.4 Typical values for process parameters:

21. Completely dominant! Simple dc model Ebers-Moll model J S ~10 -16 A/μ 2 β F ~100, β R ~0.4 With typical values for process parameters in forward active region (V BE ~0.6V, V BC ~-3V), with V t =26mV and if A E =100μ 2 : Makes no sense to keep anything other than in forward active

22. Simple dc model Ebers-Moll model Alternate equivalent expressions for dependent variables {I C, I B } defined earlier for Ebers-Moll equations in terms of independent variables {V BE, V CE } No more useful than previous equation but in form consistent with notation Introduced earlier

23. Simple dc model Simplified Multi-Region Model Ebers-Moll Model Simplified Multi-Region Model V BE =0.7V V CE =0.2V I C =I B =0 Forward Active Saturation Cutoff

24. Simple dc model Simplified Multi-Region Model V BE =0.7V V CE =0.2V I C =I B =0 Forward Active Saturation Cutoff

25. Simple dc model Simplified Multi-Region Model V BE =0.7V V CE =0.2V I C =I B =0 Forward Active Saturation Cutoff V BE >0.4V V BC <0 IC<βIBIC<βIB V BE <0 V BC <0 A small portion of the operating region is missed with this model but seldom operate in the missing region

26. Simple dc model Equivalent Simplified Multi-Region Model V BE =0.7V V CE =0.2V I C =I B =0 Forward Active Saturation Cutoff V BE >0.4V V BC <0 IC<βIBIC<βIB V BE <0 V BC <0 A small portion of the operating region is missed with this model but seldom operate in the missing region

27. Bipolar Models Simple dc Model Small Signal Model Frequency-Dependent Small Signal Model Better Analytical dc Models Sophisticated Model for Computer Simulations Better Models for Predicting Device Operation