Lecture 26 OUTLINE Modern BJT Structures PolySi emitter

Lecture #26 OUTLINE • Modern BJT Structures – Poly-Si emitter – Heterojunction bipolar transistor (HBT) • Charge control model • Base transit time Reading: Finish Chapter 11, 12. 2 1 Spring 2007 EE 130 Lecture 26, Slide 1

Modern BJT Structure • Narrow base • n+ poly-Si emitter • Self-aligned p+ poly-Si base contacts • Lightly-doped collector • Heavily-doped epitaxial subcollector • Shallow trenches and deep trenches filled with Si. O 2 for electrical isolation 2 Spring 2007 EE 130 Lecture 26, Slide 2

Polycrystalline-Silicon (Poly-Si) Emitter • bdc is larger for a poly-Si emitter BJT as compared with an all-crystalline emitter BJT, due to reduced dp. E(x)/dx at the edge of the emitter depletion region Continuity of hole current in emitter 3 Spring 2007 EE 130 Lecture 26, Slide 3

Emitter Gummel Number w/ Poly-Si Emitter where Sp DEpoly/WEpoly is the surface recombination velocity For a uniformly doped emitter, 4 Spring 2007 EE 130 Lecture 26, Slide 4

Emitter Band Gap Narrowing To achieve large bdc, NE is typically very large, so that band gap narrowing (Lecture 8, Slide 5) is significant. DEGE is negligible for NE < 1 E 18/cm 3 N = 1018 cm-3: DEG = 35 me. V N = 1019 cm-3: DEG = 75 me. V 5 Spring 2007 EE 130 Lecture 26, Slide 5

Narrow Band Gap (Si. Ge) Base To improve bdc, we can increase ni. B by using a base material (Si 1 -x. Gex) that has a smaller band gap • for x = 0. 2, DEGB is 0. 1 e. V Note that this allows a large bdc to be achieved with large NB (even >NE), which is advantageous for • reducing base resistance • increasing Early voltage (VA) 6 Spring 2007 EE 130 Lecture 26, Slide 6

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EXAMPLE: Emitter Band Gap Narrowing If DB = 3 DE , WE = 3 WB , NB = 1018 cm-3, and ni. B 2 = ni 2, find bdc for (a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1019 cm-3 and a Si 1 -x. Gex base with DEg. B = 60 me. V (a) At NE = 1019 cm-3, DEg. E 35 me. V (b) At NE = 1020 cm-3, DEg. E 160 me. V: (c) Spring 2007 8 EE 130 Lecture 26, Slide 8

Charge Control Model A PNP BJT biased in the forward-active mode has excess minority-carrier charge QB stored in the quasi-neutral base: In steady state, 9 Spring 2007 EE 130 Lecture 26, Slide 9

Base Transit Time, tt • time required for minority carriers to diffuse across the base • sets the switching speed limit of the transistor 10 Spring 2007 EE 130 Lecture 26, Slide 10

Relationship between t. B and tt • The time required for one minority carrier to recombine in the base is much longer than the time it takes for a minority carrier to cross the quasi-neutral base region. 11 Spring 2007 EE 130 Lecture 26, Slide 11

Drift Transistor: Built-in Base Field The base transit time can be reduced by building into the base an electric field that aids the flow of minority carriers. • Fixed Eg. B , NB decreases from emitter end to collector end. E B - C Ec Ef Ev • Fixed NB , Eg. B decreases from emitter end to collector end. E - B C Ec Ef Ev Spring 2007 1 d. EC E = q dx 12 EE 130 Lecture 26, Slide 12

EXAMPLE: Drift Transistor • Given an npn BJT with W=0. 1 mm and NB=1017 cm-3 (mn=800 cm 2/V s), find tt and estimate the base electric field required to reduce tt 13 Spring 2007 EE 130 Lecture 26, Slide 13