1. Bipolar Junction Transistors and Heterojunction Bipolar Transistors
Bipolar Junction Transistors and HBTs EE 3311/7322 and EE 5312/ (draft)SMU
Bipolar Junction Transistors and Heterojunction Bipolar Transistors
November 18, 2014

2. 391 San Antonio, Palo Alto, CA
May, 2006—original buildings stored vegetables from farms

3. 391 San Antonio, Palo Alto, CA
May, 2006—inside the quonset hut

4. 391 San Antonio, Palo Alto, CA, 2
~ 1958
~ 2006
Jaguar XK 140?

5. Playboy, August 1980 Issue (Bo Dereck)
PLAYBOY: What about your own children? How did they turn out?
SHOCKLEY: …my children represent a very significant regression. My first wife… Two of my three children graduated from college—my daughter from Radcliffe and my younger son from Stanford. He … has obtained a Ph.D. in physics. In some ways, I think the choice of physics may be unfortunate for him, because he has a name that he will probably be unlikely to live up to.

6. Playboy, August 1980 Issue (Bo Dereck)
PLAYBOY: What about your own children? How did they turn out?
SHOCKLEY: …my children represent a very significant regression. My first wife… Two of my three children graduated from college—my daughter from Radcliffe and my younger son from Stanford. He … has obtained a Ph.D. in physics. In some ways, I think the choice of physics may be unfortunate for him, because he has a name that he will probably be unlikely to live up to.

7. Excess Carriers and Quasi Fermi Levels
Recall:

8. Quasi Fermi Level Example
Consider a Si sample with
no = 1014 cm-3
nopo = ni2 so
po = 2.25x1020 cm-6/1014 cm-3
= 2.25x106 cm-3
By some means (optical generation or electrical injection), an additional 2x1013 cm-3 electron-hole pairs are generated
Now,
n = 1.2x1014 cm-3
p = 2x1013 cm-3 (107 increase)
np = 2x1027 cm-3 , not ni2

9. Quasi Fermi Level Example, cont’d 1

10. Bipolar Junction Transistors
Bipolar means that both positive and negative charge carriers contribute to current flow
FETs are unipolar—since only one type of charge carrier contributes to current flow
Consider a pn junction:

11. Illuminated pn Junction

12. Bipolar Junction Transistor Concept

13. BJT Concept, cont’d 3 Properties of a p+n Junction
If pp >> nn , then np is << pn
(since nopo = ni2 )
Therefore, the current across the depletion region is almost all hole current

14. BJT Concept, cont’d 4
Emitter (forward biased p+n junction is the source of holes
Reverse biased np junction “collects” holes injected into the base
Holes are swept across the np depletion region by drift ( )
Need to make base narrow (< 1µm) to reduce recombination in base

15. BJT Concept, cont’d 5 Heavy doping also decreases the bandgap
(not desirable), and
decreases resistance (desirable)

16. BJT Concept, cont’d 6 IE is mostly hole current
IC is mostly due to holes injected into the base
IE ~ IC
IB is small, determined by
- electrons injected across the p+n junction (5)
- recombination of injected holes and electrons in the base (1) and (4) [dominant term]
-electrons swept into the base from the collector (3) (thermal generation and electrons within a diffusion length of the CB junction edge)

17. BJT Concept, cont’d 7
BJT—no bias
BJT– with bias

18. BJT Concept, cont’d 8

19. BJT Concept, cont’d 9

20. Amplification in BJTs
Dominant component of base current is due to EHP recombination in the base

21. Amplification in BJTs, cont’d 1
~ 100 is a typical number

22. Amplification in BJTs, cont’d 2

23. BJT as a Circuit Element

24. Why Heterojunction BJTs?
From J. Singh, Semiconductor Devices

25. Band Gap Shrinkage with Doping

26. Heterojunctions and Homojunctions
From J. Singh, Semiconductor Devices

27. Heterojunctions and Homojunctions, cont’d

28. Stop Here for 3311/7322

29. Heterojunction Conduction Band Offsets

30. AlGaAs/GaAs Heterojunction Conduction Band Offsets

31. InGaAs/GaAs Heterojunction Conduction Band Offsets

32. InGaAs/InAlAs/InP Heterojunction Conduction Band Offsets

33. Si/SiGe Heterojunction Conduction Band Offsets

34. Band Gap Shrinkage with Doping

35. Heterojunction Energy Band Diagram

36. Heterojunction Energy Band Diagram, cont’d 1

37. Heterojunction Energy Band Diagram, cont’d 2

38. Band Lineups: InP/InGaAs, InAlAs/InGaAs

39. Go to Schubert’s slides...
Mention homework problems, too!

40. Emitter Injection Efficiency
Recall the IV equation for a pn junction:
Consider an n+p emitter base junction
In (4), accounts for the finite length of the base

41. Emitter Injection Efficiency, cont’d 1or
(5)
From Eqs. 3 and 4,
since
, Eq 5 becomes

42. Heterojunction Correction to Junction Current
For high doping, the bandgap of Si shrinks
Eq 7.11, Singh
Previously we determined, so + +
Emitter bandgap shrinks: exponential increase in base carriers injected into emitter
Emitter bandgap increases: exponential decrease in base carriers emitted into emitter

43. Band Gap Shrinkage with Doping
For Silicon:

44. Heterojunction Junction Current, cont’d 1
So the emitter injection efficiency increases or decreases:
from
and using
we have
Emitter bandgap shrinks: exponential decrease in performance
Emitter bandgap increases: exponential increase in performance

46. Al0.3Ga0.7As/GaAs HBT
Eq of Streetman gives the ratio of electron current to hole current for a p-n heterojunction
Eq applies to a homojunction BJT if the emitter is heavily doped, which causes the bandgap of Si to shrink
For a p-n heterojunction, the bandgap difference between the p and n region is eV.
Such a bandgap difference gives a factor of ~ 107 compared to a homojunction BJT

47. Si Bandgap Dependence on Doping

48. Abrupt and Graded Interfaces

49. BJT and HBT Energy Band Diagram
ne and pb are majority carrier densities in the emitter and base
vn is the electron velocity in the base
vp is the hole velocity in the base
DEg is the bandgap difference
Want high base doping (low resistance) for high speed—but lowers gain
HBT wider bandgap solves problem by adding exp(DEg/(kT)) term to the gain

50. And Another Eg vs Lattice Constant Chart
EE7312 Class3 companion

51. IntelliEPI HBT

52. Typical InGaAs HBT

53. Graded Base HBT
Gradual change in bandgap induces an electric field due to the bandgap gradient
Adds a drift term in addition to diffusion current

54. Double Heterojunction HBT (DHBT)
Add Wide Band Gap Collector
Eliminates the injection of holes into the collector
Reduces saturation stored-charge density
Speeds up device turnoff

55. Silicon-Germanium HBTs
addition of Ge reduces the bandgap of Si
lower effective electron mass in SiGe

56. AlGaAs/GaAs HBTs

57. InGaAs/InP HBTs
Electron mobility in InGaAs is 1.6 times that of GaAs and 9 times higher than in Si
highest ft values in InP based HBTs

58. Typical InGaAs/InP HBT
InGaAs has a smaller bandgap than Si or GaAs
requires lower voltage power supplies
better thermal properties than GaAs
surface recombination velocities smaller than GaAs

59. InGaAs/InP HBT Frequency Performance

60. Si/Si BJT
Eq of Streetman gives the ratio of electron current to hole current for a p-n heterojunction
This slide is unfinished—meant to calculate the degradation due to highly doping the collector with respect to the base for a Si-Si BJT and then on another slide show how to compensate with a SeGe base HBT
Eq applies to a homojunction BJT if the emitter is heavily doped, which causes the bandgap of Si to shrink
For a p-n heterojunction, the bandgap difference between the p and n region is eV.
Such a bandgap difference gives a factor of ~ 107 compared to a homojunction BJT

61. Ballistic Collection HBTs
ft ~ 105 GHz