The Design of SiGe pnp HBTs

1. The Design of SiGe pnp HBTs • At present, SiGe technology development is almost exclusively centered on npn SiGe HBTs. • However, for high-speed analog and mixed-signal circuit applications, a complementary (npn+pnp) bipolar technology offers signification performance advantages over an npn-only technology . for example : Push-pull circuits

2. Push-pull circuits Class B output stage

3. NPN and PNP of Si BJTs • Performance : npn Si BJTs > pnp Si BJTs

4. NPN SiGe HBT • Valance band offset in SiGe strained layers translates into an induced conduction band offset

5. PNP SiGe HBT • The valance band offset directly results in a valance band barrier , even at low injection. strongly degrades minority hole transport and limits the frequece response.

6. Simplistic hypothetical npn and pnp SiGe profiles • With const. E,B,C doping, and a Ge content not subject to thermodynamic stability constraint. This artificial assumption on constant doping yields ac performance numbers (eg. fT) that are lower than what would be expected for a real complementary.

7. Profile Optimization Issues • Npn without any Ge retrograding into the collector(i.e. an abrupt transition from the peak Ge content to zero Ge content in the CB junction).

8. Profile Optimization Issues • pnp without any Ge retrograding into the collector An obvious valence band barrier even for low Ge content

9. Valence band barrier in pnp • acts to block minority holes transiting the base. • The pileup of accumulated holes produces a retarding electric field in the base , which compensates the Ge-grading-induced drift field.

10. Retrograding Ge into the collector • retrograding of the Ge edge into the collector can “smooth” this valence band offset in the pnp SiGe HBT, although at the expense of film stability. • For an increase of the Ge retrograde from 0 to 40nm

11. FTand Current gain

12. Ge retrograding in PNP “smooth” valence band barrier

13. The box Ge retrograde (back side) on PNP • The box Ge retrograde is not effective in improving the pnp SiGe HBT performance , since it does not smooth the Ge barrier, but rather only pushes it deeper into collector

14. The effects of Ge retrograding on NPN • The effects of Ge retrograde on the npn SiGe HBT performance ------- minor, while the film stability is worse due to the additional Ge content. So, we know using one Ge profile design for both npn and pnp SiGe HBTs is not optimum for high peak Ge content values.

15. Stability Constraints in PNP SiGe HBTs • The total amount of Ge that can be put into a given SiGe HBT -----limited by the thermodynamic stability. • Above the critical thickness the strain in the SiGe film relaxes generating defects. • The empirical critical thickness of a SiGe multilayer with a top-layer Si cap ----- approximately 4x the theoretical stability result of Matthews and Blakeslee

16. Stability Constraints in PNP SiGe HBTs The peak Ge content The Ge retrograde distance trade off

17. The best design point in PNP

18. Similar exercise for the npn • For npn SiGe HBT , the ac performance is not sensitive to the SiGe profile shapes used. So the same Ge profile may be used for both pnp and npn SiGe HBTs. being advantageous from a fabrication viewpoint.

19. Low-Injection Theory • Minority carrier transport in an npn SiGe HBT • The minority carrier base transit time ----- determined by the net force acting on electron resulting from the induced electric field.

20. The net force on the electrons • Two components : (1). The quasi-electric field due to the gradient induced by conduction band offset. (2). The built-in electric field. • The hole current density with a nonuniform bandgap :

21. The net force on the electrons

22. The net force on the electrons • The net force acting on the electrons becomes finally,

23. The net force on the electrons • So the base transit time is determined only by the total band offset across the neutral base. --------and is independent of its distribution between conduction and valence bands for low-injection operation(i.e. p=Nab).

24. Impact of High Injection • To shed light on this issue, we consider the following four representative band offset distributions(band alignments) :

25. Four representative band offsets (with the same total band offset)

26. Simulation • 2-D dc and ac simulations were performed for a 0.5µm emitter widthnpn SiGe HBT.

27. FT V.S. JC

28. The conduction band edge of simulation

29. The explantation of the physics This case gives the highest(best) FT and the highest(best) Jcritical , because it has the largest valence band offset , which acts to effectively prevent hole injection into the collector.

30. The explantation of the physics This case gives the lowest FT and Jcritical, because it has the largest conduction band offset which serves as a barrier to electron, and thus results in excess charge storage.

31. The explantation of the physics • For FT and Jcritical Because this conduction band barrier height resulting from the pileup of holes this coduction band barrier height

32. SiGe HBTs under High –Current density operation • From the viewpoint of improving the ac characteristics of SiGe HBTs under high-current density operation, a large positive valence band offset together with a negative conduction band offset is the most desirable bandgap offset distribution.(as condition (4) ). • It is worth noting that these offsets(condition (1) ) are in fact different from those produced by strained SiGe on Si (i.e. mostly EV, with a small EC.)

33. Profile Optimization Issues • One way to minimize high-injection barrier effects in SiGe HBTs is to retrograde the mole fraction deep into the collector.

34. Low-C-content SiGeC • Low-C-content SiGeC layers can provide better thermodynamic stability than strained SiGe - So we can allow a higher average Ge mole fraction for a deeper grading.

35. Ge-Induced Collector-Base Field Effects • The specifics of the backside Ge profile(i.e. on the CB side of the neutral base) strongly influence high-injection heterojunction barrier effects, which produce premature roll-off of and FT at high current density. • Here we will show that the backside Ge profile also alters the electric field distribution in the CB space-charge region, and thereby indirectly affects impact ionization in SiGe HBTs.

36. Influence on Impact Ionization • For a strained SiGe layer on Si, the band offset in the SiGe film predominantly resides in the valence band and its value is proportional to the Ge content. - according to ΔEV=0.74x (eV), where x is Ge fraction (i.e. 10% Ge=0.10) .

37. Valence band edge for Strained SiGe on Si (NPN) This SiGe “control” profile is labeled “0nm Ge”. (i.e. the location of the SiGe-Si heterointerface is referenced to the metallurgical CB junction)

38. Heterojunction-Induced Quasi-Electric Field • This change in the valence band creates a heterojunction-induced quasi-electric field • For the present SiGe control profile Which is larger than the peak field formed by the doping-induced charge in the CB space charge region.

39. Heterojunction-Induced Quasi-Electric Field It is clear thatas thebackside Ge retrograde location moves toward the neutral collector, the peak electric field moves in the same direction and the magnitude of the peak electric field drops. 絕對值

40. Avalanche multiplication factor • This decrease of the peak field reduces the impact ionization rate , as reflected in the avalanche multiplication factor (M-1) .