Lecture 26 OUTLINE The BJT contd Breakdown mechanisms

Lecture 26 OUTLINE The BJT (cont’d) • • • Breakdown mechanisms Non-ideal effects Gummel plot & Gummel numbers Modern BJT structures Base transit time Reading: Pierret 11. 2 -11. 3, 12. 2. 2; Hu 8. 4, 8. 7

BJT Breakdown Mechanisms • In the common-emitter configuration, for high output voltage VEC, the output current IC will increase rapidly due to one of two mechanisms: – punch-through – avalanche EE 130/230 M Spring 2013 Lecture 26, Slide 2

Punch-Through E-B and E-B depletion regions in the base touch W = 0 As |VCB| increases, the potential barrier to hole injection decreases and hence IC increases EE 130/230 M Spring 2013 Lecture 26, Slide 3

Avalanche Multiplication • Holes are injected into the base [0], then PNP BJT: collected by the B-C junction – Some holes in the B-C depletion region have enough energy to generate EHP [1] • Generated electrons are swept into the base [3], then injected into emitter [4] – Each injected electron results in the injection of IEp/IEn holes from the emitter into the base [0] ® For each EHP created in the C-B depletion region by impact ionization, (IEp/IEn)+1 > bdc additional holes flow into the collector i. e. carrier multiplication in the C-B depletion region is internally amplified where VCB 0 = reverse breakdown voltage of the C-B junction EE 130/230 M Spring 2013 Lecture 26, Slide 4

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, thermal recombination current is likely to be a dominant component of the base current Þ low emitter efficiency, hence lower gain This limits the application of the BJT for amplification at low voltages. EE 130/230 M Spring 2013 Lecture 26, Slide 5

Non-Ideal Effects at High VEB • Decrease in bdc at high IC is caused by: – high-level injection – series resistance – current crowding EE 130/230 M Spring 2013 Lecture 26, Slide 6

Gummel Plot and bdc vs. IC bdc EE 130/230 M Spring 2013 From top to bottom: VBC = 2 V, 1 V, 0 V Lecture 26, Slide 7

Gummel Numbers For a uniformly doped base with negligible band-gap narrowing, the base Gummel number is (total integrated “dose” (#/cm 2) of majority carriers in the base, divided by DB) Emitter efficiency GE is the emitter Gummel number EE 130/230 M Spring 2013 Lecture 26, Slide 8

Notice that In practice, NB and NE are not uniform, i. e. they are functions of x The more general formulas for the Gummel numbers are EE 130/230 M Spring 2013 Lecture 26, Slide 9

Modern NPN BJT Structure Features: • 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 EE 130/230 M Spring 2013 Lecture 26, Slide 10

Poly-Si Emitter • bdc is larger for a poly-Si emitter BJT as compared with an allcrystalline emitter BJT, due to reduced dp. E(x)/dx at the edge of the emitter depletion region Continuity of hole current in emitter: (1 poly-Si; 2 crystalline Si) EE 130/230 M Spring 2013 Lecture 26, Slide 11

Emitter Gummel Number w/ Poly-Si Emitter where Sp DEpoly/WEpoly is the surface recombination velocity For a uniformly doped emitter, EE 130/230 M Spring 2013 Lecture 26, Slide 12

Emitter Band Gap Narrowing To achieve large bdc, NE is typically very large, so that band gap narrowing is significant (ref. Lecture 3, Slide 20). 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 EE 130/230 M Spring 2013 Lecture 26, Slide 13

Narrow Band Gap (Si 1 -x. Gex) 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 This allows a large bdc to be achieved with large NB (even >NE), which is advantageous for • reducing base resistance • increasing Early voltage (VA) EE 130/230 M Spring 2013 Lecture 26, Slide 14 courtesy of J. D. Cressler (GATech)

Heterojunction Bipolar Transistors a) Uniform Ge concentration in base b) Linearly graded Ge concentration in base built-in E-field EE 130/230 M Spring 2013 Lecture 26, Slide 15

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 DEGB = 60 me. V (a) For NE = 1019 cm-3, DEGE 35 me. V (b) For NE = 1020 cm-3, DEg. E 160 me. V: (c) EE 130/230 M Spring 2013 Lecture 26, Slide 16

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, EE 130/230 M Spring 2013 Lecture 26, Slide 17

Base Transit Time, tt • time required for minority carriers to diffuse across the base • sets the switching speed limit of the transistor EE 130/230 M Spring 2013 Lecture 26, Slide 18

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. EE 130/230 M Spring 2013 Lecture 26, Slide 19

Built-in Base E-Field to Reducett The base transit time can be reduced by building into the base an electric field that aids the flow of minority carriers. 1. Fixed EGB , NB decreases from emitter to collector: E B - C Ec Ef Ev 2. Fixed NB , EGB decreases from emitter to collector: E - B C Ec Ef Ev EE 130/230 M Spring 2013 Lecture 26, Slide 20 E

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 EE 130/230 M Spring 2013 Lecture 26, Slide 21