1. Section 8.1 Heithem Souissi Dina Miqdadi Mohammad Butt Ahmad Elmardini Wyatt Sullivan Fares Alnajjar

2. Beta rolloff & Avalanche breakdown HEITHEM SOUISSI DINA MIQDADI

3. Beta Rolloff

4. β Rolloff Beta= Current gain for a BJT Beta is commonly referred to as a constant, although it varies considerably depending on the collector current.

5. β Rolloff (con’t) High current beta rolloff is caused by high- level injection. Low current beta rolloff results from recombination, and shallow emitter effect.

7. β Rolloff (con’t) The lateral PNP beta curve is different from that of an NPN for several reasons 1. Due to its light doping,PNP exhibits lower emitter injection efficiency that reduces its beta peak. 2. The flow of carriers near the surface of the PNP increases surface recombination, causing low-current beta rolloff. 3. The lightly doped base of the PNP causes high level injections to begin at relatively low current levels, therefore high current beta rolloff.

8. Avalanche Breakdown

9. What is Breakdown? Deleterious effect that occurs in the presence of high electric field. Causes high resistance elements to allow flow of high current. Typically an irreversible effect permanently damaging the element.

10. Avalanche/Zener Breakdown 'Zener diode' and 'avalanche diode' are terms often used interchangeably. Both refer to breakdown of a diode under reverse bias.

11. Avalanche occurs with lightly doped PN junctions. (Multiplication effect). Zener occurs in highly doped junctions (quantum tunneling effect). Avalanche/Zener Breakdown (con’t)

12. Reverse bias = Very little current flow = Open circuit As Reverse voltage a point is reached where current dramatically, therefore dynamic resistance.

13. Avalanche Breakdown Avalanche breakdown causes high flow of current under reverse bias condition! The question is: How does this happen?

14. Avalanche Breakdown (con’t) Reverse bias Thick depletion region causes high electric field and tremendous acceleration Very few electrons make it through depletion region with high velocity These electrons collide with atoms in the depletion region and free more electrons ( Process called Multiplication). Results in higher and higher current flow

15. An empirical relationship used to describe avalanche breakdown: M=Multiplication factor= 1/(1-|V/Vbr|^n) Avalanche Breakdown (con’t)

16. By analogy, the process is named because a single carrier can spawn literally thousands of additional carriers through collisions, just as a single snowball can cause an avalanche. Avalanche Breakdown (con’t)

18. Thermal Runaway, Secondary Breakdown and Saturation in NPN Transistors Mohammad Butt Ahmad Elmardini

19. Topics in this section (8.1.3-4) Thermal runaway Hot spots Secondary breakdown Forward-bias safe operating area(FBSOA) Saturation in NPN transistors Current hogging Prevention schemes for current hogging

20. Thermal Runaway: Increase in temperature leads to higher current and power dissipation, which in turn increases the temperature further until the device is destroyed. Hot Spot: The region of the transistor that conducts current and steadily shrinks as it grows hotter is called a hot spot. Temp. increases β increases I CB0 increases V BE decreases

21. Unstable Hot Spot: If the temperature in the hot spot reaches 350 o C to 450 o C, the transistor shorts out. Stable Hot Spot: If the increased current density causes beta to roll off far enough at a high but not destructive temperature, it stabilizes the hot spot. A transistor with stable hot spot self- destructs during turn-off due to avalanche of the collector-base junction at a voltage below the V CEO

22. Secondary Breakdown: High values of V CE and I C cause burnout of a transistor junction area. It is not necessary for the average junction temp. to exceed the maximum rating. Secondary breakdown can also occur in transistors which have not experienced thermal runaway. Thermal runaway and secondary breakdown can be avoided by restricting the operating conditions of the transistor.

23. Forward-bias Safe Operating Area(FBSOA)

24. Which transistor loses less FBSOA due to second breakdown? Transistor 1: A very robust transistor without a heat sink. Transistor 2: A properly heat sunk but poorly designed transistor. Answer: Transistor 1

25. Saturation in NPN Transistors Occurs when V BE > 0 and V BC >0 Useful in power transistors to reduce V CE(sat) and to minimize power dissipation Unintentional saturation is the biggest device- related design flaw Affects discrete and integrated transistors in different ways

26. Integrated Bipolar Transistors

27. Current Hogging Caused by saturation due to flow of base current through base-collector junction rather than base emitter junction Reduces the base-emitter voltage Base current of saturating transistor increases at the expense of other transistors

28. Current Mirror Transistor

29. Prevention of Current Hogging Base-side Ballasting: Insertion of matched resistors into base leads of each transistor. Resistors must ratio inversely to the emitter areas of their respective transistors. Schottky clamps: Clamping diode is connected across base-collector junction to prevent saturation.

30. Base-side Ballasting

31. Schottky-clamped Transistor

32. Lateral PNP Transistors Wyatt Sullivan Fares Alnajjar

33. Lateral Parasitic Transistance Emitter -> P-substrate Major cause of collector efficiency NBL required to compensate Collector -> BOI Creates saturation currents Determines Saturation voltages

34. NBL Explained N-type Buried Layer (NBL) Acts as a minority carrier repellant Holes are “repelled” back into the N-epi Drift currents cause a slight potential

35. Sidewall Transistance High collector voltages create PNP Collector -> base -> sidewall (P-Type) Narrow base in high voltages Emitter continues to inject current Excess collector current flows to substrate

36. PNP Lateral Transistor

37. Models for Parasitics of BJTs Major Parasitics Sidewall saturation Avalanche breakdown Leakage Capacitance between junctions

38. Models for Transistors

39. Transistor Models Explained Diodes characterize Capacitance Avalanche breakdown Resistors show internal resistances R b is normally quite large R e is negligible

40. Design Considerations Low R b –Higher frequency operation Low R c –Higher saturation voltages –Power applications for high current

41. Bibliography “The Art of analog layout” by Alan Hastings. “Principles of semiconductor devices” by Bart Van.