带隙变窄及多晶硅发射极

1 . Lecture #17 ANNOUNCEMENTS • Special Review Session: Wed. 3/19 @ 5 PM, 293 Cory • No Coffee Hour this week and next week OUTLINE The Bipolar Junction Transistor – Bandgap narrowing in emitter, base – Poly-Si emitter – Gummel plot Reading: Finish Chapter 11 Spring 2003 EE130 Lecture 17, Slide 1 NPN BJT Structure (cross-section) B E C P+polySi N+ polySi P+polySi + + p P base p Shallow N+ N collector trench Deep Deep trench N+ subcollector trench P− substrate Spring 2003 EE130 Lecture 17, Slide 2 1 

2 . Avalanche Multiplication • Avalanche may be important: 1.If it occurs before punchthrough 2.As an amplification mechanism in a phototransistor • Inject a photon into the CB depletion region to cause avalanche multiplication of it Spring 2003 EE130 Lecture 17, Slide 3 Emitter Bandgap Narrowing N E niB2 To raise β, NE is typically very large, so β∝ N B niE2 niE2 > ni2 (called the heavy doping effect). − E g / kT ni2 = N C NV e Since heavy doping can reduce EG, this effect is also known as band-gap narrowing. niE2 = ni2 e ∆E gE / kT ∆EGE is negligible for NE < 1018 cm-3 • 35 meV at 1018 cm-3 • 75 meV at 1019 cm-3 Spring 2003 EE130 Lecture 17, Slide 4 2 

3 . Narrow-Bandgap (SiGe) Base N E niB2 To further elevate β , we can raise niB by β∝ N B niE2 using an epitaxial Si1-ηGeη base. With η = 0.2, EGB is reduced by 0.1eV. Spring 2003 EE130 Lecture 17, Slide 5 EXAMPLE: Emitter Bandgap Narrowing Assume DB = 3DE , WE = 3WB , NB = 1018 cm-3, and niB2 = ni2. What is βdc for (a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1019 cm-3 and a SiGe base with ∆EgB = 60 meV ? (a) At NE = 1019 cm-3, ∆EgE ≈ 35 meV, ∆E gE / kT niE2 = ni2 e = ni2 e35 meV / 26 meV = 3.8ni2 DBWE N E ni2 9 ⋅1019 ⋅ ni2 β dc = ⋅ = = 23.6 DEWB N B niE2 1018 ⋅ 3.8ni2 (b) At NE = 1020cm-3, ∆EgE ≈ 150 meV niE2 = 320ni2 β dc = 3 (c) niB2 = ni2e ∆E gB / kT = ni2 e 60 meV / 26 meV = 10ni2 β F = 236 Spring 2003 EE130 Lecture 17, Slide 6 3 

4 . Polycrystalline-Silicon (Poly-Si) Emitter • βF is larger due to the large WE , mostly made of the N+ poly-Si. • Decreased mobility in poly-Si reduces pE slope at B-E edge, improving γ N+-poly-Si emitter SiO2 P-base N-collector Spring 2003 EE130 Lecture 17, Slide 7 Gummel Plot and βdc vs. IC 10-2 high level injection in base IC 10-4 βdc IC (A) 10-6 IB βF From top to bottom: 10-8 VBC = 2V, 1V, 0V 10-10 excess base current due to R-G in depletion regions 10-12 0.2 0.4 0.6 0.8 1.0 1.2 VBE Spring 2003 EE130 Lecture 17, Slide 8 4 

5 . Gummel Numbers Spring 2003 EE130 Lecture 17, Slide 9 Non-Ideal Effects at Low VEB • In the ideal transistor analysis, thermal R-G currents in the emitter and collector junctions were neglected. • Under active-mode operation with small VEB, the thermal recombination current is likely to be a dominant component of the base current ⇒ low emitter efficiency, hence lower gain Spring 2003 EE130 Lecture 17, Slide 10 5 

6 . Non-Ideal Effects at High VEB • Decrease in βF at high IC is caused by: – high-level injection – series resistance – current crowding Spring 2003 EE130 Lecture 17, Slide 11 6