EE 330 Lecture 19

1. EE 330Lecture 19 Characteristics of Finer Feature Size Processes Bipolar Process

2. Basic Devices and Device Models • Resistor • Diode • Capacitor • MOSFET • BJT

3. Bipolar Junction Transistors • Operation • Modeling

4. Carriers in Doped Semiconductors n-type p-type

5. Carriers in Doped Semiconductors I V Current carriers are dominantly electrons Small number of holes are short-term carriers I V Current carriers are dominantly holes Small number of electrons are short-term carriers

6. Carriers in Doped Semiconductors

7. Carriers in MOS Transistors Consider n-channel MOSFET Saturation Region Channel Triode Region

8. Carriers in MOS Transistors Consider n-channel MOSFET Saturation Region Triode Region Carriers in electrically induced n-channel are electrons

9. Carriers in MOS Transistors Consider p-channel MOSFET Saturation Region Channel Triode Region

10. Carriers in MOS Transistors Consider p-channel MOSFET Saturation Region Triode Region Carriers in electrically induced p-channel are holes

11. Carriers in MOS Transistors Carriers in channel of MOS transistors are Majority carriers

12. E C B B C E pnp transistor npn transistor Bipolar Transistors E E B B npn stack pnp stack C C • Bipolar Devices Show Basic • Symmetry • Electrical Properties not • Symmetric • Designation of C and E critical With proper doping and device sizing these form Bipolar Transistors

13. E C B B C E pnp transistor npn transistor Bipolar Transistors p-channel MOSFET n-channel MOSFET In contrast to a MOSFET which has 4 terminals, a BJT only has 3 terminals

14. Bipolar Operation Consider npn transistor E B npn stack C Under forward bias current flow into base and out of emitter Current flow is governed by the diode equation Carriers in emitter are electrons (majority carriers) When electrons pass into the base they become minority carriers Quickly recombine with holes to create holes base region Dominant current flow in base is holes (majority carriers)

15. Bipolar Operation Consider npn transistor E B npn stack C Under forward BE bias and reverse BC bias current flows into base region Carriers in emitter are electrons (majority carriers) When electrons pass into the base they become minority carriers When minority carriers are present in the base they can be attracted to collector

16. Bipolar Operation Consider npn transistor E B npn stack C F1 If no force on electron is applied by collector, electron will contribute to base current

17. Bipolar Operation Consider npn transistor E B npn stack C F1 If no force on electron is applied by collector, electron will contribute to base current Electron will recombine with a hole so dominant current flow in base will be by majority carriers

18. Bipolar Operation Consider npn transistor E B npn stack C F1 F2 When minority carriers are present in the base they can be attracted to collector with reverse-bias of BC junction and can move across BC junction

19. Bipolar Operation Consider npn transistor E B npn stack C F1 F2 When minority carriers are present in the base they can be attracted to collector with reverse-bias of BC junction and can move across BC junction Will contribute to collector current flow as majority carriers

20. Bipolar Operation Consider npn transistor E B npn stack C F1 F2 So, what will happen?

21. Bipolar Operation Consider npn transistor E B npn stack C F1 F2 So, what will happen? Some will recombine with holes and contribute to base current and some will be attracted across BC junction and contribute to collector Size and thickness of base region and relative doping levels will play key role in percent of minority carriers injected into base contributing to collector current

22. Bipolar Operation Consider npn transistor E B npn stack C Under forward BE bias and reverse BC bias current flows into base region Carriers in emitter are electrons (majority carriers) When electrons pass into the base they become minority carriers When minority carriers are present in the base they can be attracted to collector Minority carriers either recombine with holes and contribute to base current or are attracted into collector region and contribute to collector current

23. Bipolar Operation Consider npn transistor E B npn stack C Under forward BE bias and reverse BC bias current flows into base region Efficiency at which minority carriers injected into base region and contribute to collector current is termed α α is always less than 1 but for a good transistor, it is very close to 1 For good transistors .99 < α < .999 Making the base region very thin makes α large

24. Bipolar Transistors E E B B npn stack pnp stack C C principle of operation of pnp and npn transistors are the same minority carriers in base of pnp are holes npn usually have modestly superior properties because mobility of electrons Is larger than mobility of holes

25. Bipolar Operation Consider npn transistor E B npn stack C In contrast to MOS devices where current flow in channel is by majority carriers, current flow in the critical base region of bipolar transistors is by minority carriers

26. Bipolar Operation E B C β is typically very large often 50<β<999

27. Bipolar Operation E B C β is typically very large Bipolar transistor can be thought of a current amplifier with a large current gain In contrast, MOS transistor is inherently a tramsconductance amplifier Current flow in base is governed by the diode equation Collector current thus varies exponentially with VBE

28. Bipolar Operation E B C β is typically very large Collector current thus varies exponentially with VBE This exponential relationship (in contrast to the square-law relationship for the MOSFET) provides a very large gain for the BJT and this property is very useful for many applications !!

29. Bipolar Models Simple dc Model following convention, pick IC and IB as dependent variables and VBE and VCE as independent variables

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

31. Simple dc model JS is termed the saturation current density Process Parameters : JS,β Design Parameters: AE Environmental parameters and physical constants: k,T,q At room temperature, Vt is around 26mV JS very small – around .25fA/u2

32. Transfer Characteristics JS=.25fA/u2 AE=400u2 VBE close to 0.6V for a two decade change in IC around 1mA

33. Transfer Characteristics JS=.25fA/u2 AE=400u2 VBE close to 0.6V for a four decade change in IC around 1mA

34. IC VBE or IB VCE Simple dc model Output Characteristics

35. Simple dc model Better Model of Output Characteristics IC VBE or IB VCE

36. Simple dc model Typical Output Characteristics Saturation IC Forward Active VBE or IB VCE Cutoff Forward Active region of BJT is analogous to Saturation region of MOSFET Saturation region of BJT is analogous to Triode region of MOSFET

38. Simple dc model Typical Output Characteristics IC VBE or IB VCE Projections of these tangential lines all intercept the –VCE axis at the same place and this is termed the Early voltage, VAF (actually –VAF is intercept) Typical values of VAF are in the 100V range

39. Simple dc model Improved Model IC VBE or IB VCE Valid only in Forward Active Region

40. Simple dc model Improved Model IC VBE or IB VCE Valid in All regions of operation VAF effects can be added Not mathematically easy to work with Note dependent variables changes Termed Ebers-Moll model Reduces to previous model in FA region

41. Simple dc model Ebers-Moll model Process Parameters: {JS, αF, αR} Design Parameters: {AE} αF is the parameter α discussed earlier αR is termed the “reverse α” Typical values for process parameters: JS ~10-16A/μ2 βF~100, βR~0.4

42. Simple dc model Ebers-Moll model With typical values for process parameters in forward active region (VBE~0.6V, VBC~-3V), with Vt=26mV and if AE=100μ2: JS ~10-16A/μ2 βF~100, βR~0.4 Completely dominant! in forward active Makes no sense to keep anything other than

43. Simple dc model Ebes-Moll model Alternate equivalent expressions for dependent variables {IC, IB} defined earlier for Ebers-Moll equations in terms of independent variables {VBE, VCE} after dropping the “-1” terms No more useful than previous equation but in form consistent with notation Introduced earlier

44. Simple dc model Simplified Multi-Region Model Ebers-Moll Model Simplified Multi-Region Model • Observe VCE around 0.2V when saturated • VBE around 0.6V when saturated • In most applications, exact VCE and VBE • voltage in saturation not critical VBE=0.7V VCE=0.2V Saturation

45. Simple dc model Simplified Multi-Region Model Ebers-Moll Model Simplified Multi-Region Model Forward Active VBE=0.7V VCE=0.2V Saturation Cutoff IC=IB=0

46. Simple dc model Simplified Multi-Region Model Forward Active VBE=0.7V VCE=0.2V Saturation IC=IB=0 Cutoff

47. Simple dc model Simplified Multi-Region Model VBE>0.4V VBC<0 Forward Active VBE=0.7V VCE=0.2V IC<βIB Saturation VBE<0 VBC<0 IC=IB=0 Cutoff A small portion of the operating region is missed with this model but seldom operate in the missing region

48. Simple dc model Equivalent Simplified Multi-Region Model VBE>0.4V VBC<0 Forward Active VBE=0.7V VCE=0.2V IC<βIB Saturation VBE<0 VBC<0 IC=IB=0 Cutoff A small portion of the operating region is missed with this model but seldom operate in the missing region

49. Simplified dc model Forward Active Adequate when it makes little difference whether VBE=0.6V or VBE=0.7V

50. Simplified dc model Forward Active Mathematically VBE=0.6V IC=βIB Or, if want to show slope in IC-VCE characteristics VBE=0.6V IC=βIB(1+VCE/VAF)