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OUTLINE BJT: Deviations from the Ideal Base-width modulation, Early voltage Punch-through Non-ideal effects at low | V EB |, high | V EB | Gummel plot Reading: Chapter 11.2. Lecture #25. Measured BJT Common-Emitter Output Characteristics:. Base-Width Modulation.

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## Lecture #25

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**OUTLINE**BJT: Deviations from the Ideal Base-width modulation, Early voltage Punch-through Non-ideal effects at low |VEB|, high |VEB| Gummel plot Reading: Chapter 11.2 Lecture #25 Measured BJT Common-Emitter Output Characteristics: EE130 Lecture 25, Slide 1**Base-Width Modulation**Common-Emitter Configuration, Active Mode Operation W IE IC P+ N P + VEB DpB(x) IC (VCB=0) x VEC W(VBC) 0 EE130 Lecture 25, Slide 2**The base-width modulation effect is reduced if we**(a) increase the base width, W, or (b) increase the base dopant concentration, NB, or (c) decrease the collector dopant concentration, NC . Which of the above is the most acceptable action? EE130 Lecture 25, Slide 3**Early Voltage, VA**Output resistance: A large VA (i.e. a large ro ) is desirable IC IB3 IB2 IB1 VEC 0 VA EE130 Lecture 25, Slide 4**Derivation of Formula for VA**Output conductance: for fixed VEB where xnC is the width of the collector-junction depletion region on the base side xnC P+ N P EE130 Lecture 25, Slide 5**BJT Breakdown Mechanisms**• In the common-emitter configuration, for high output voltage VCE, the output current IC will increase rapidly due to one of two mechanisms: • punch-through • avalanche EE130 Lecture 25, Slide 7**Punch-Through**E-B and E-B depletion regions in the base touch, so that W = 0 As |VCB| increases, the potential barrier to hole injection decreases and therefore IC increases EE130 Lecture 25, Slide 8**Holes are injected into the base [0], then collected by the**B-C junction Some holes in the B-C depletion region have enough energy to generate EHP [1] The generated electrons are swept into the base [3], then injected into the emitter [4] Each injected electron results in the injection of IEp/IEn holes from the emitter into the base [0] Avalanche Multiplication PNP BJT: • 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 C-B depletion region is internally amplified where VCB0 = reverse breakdown voltage of the C-B junction EE130 Lecture 25, 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 This limits the application of the BJT for amplification at low voltages. EE130 Lecture 25, Slide 10**Non-Ideal Effects at High VEB**• Decrease in bF at high IC is caused by: • high-level injection • series resistance • current crowding EE130 Lecture 25, Slide 11**Gummel Plot and bdcvs.IC**high level 10 -2 injection in base I 10 -4 C bdc 10 -6 I B From top to bottom: VBC = 2V, 1V, 0V b 10 -8 excess base current due to R-G in depletion region 10 -10 10 -12 0.2 0.4 0.6 0.8 1.0 1.2 V BE EE130 Lecture 25, Slide 12**Gummel Numbers**For a uniformly doped base with negligible band-gap narrowing, the base Gummel number is (= total integrated “dose” (#/cm2) of majority carriers in the base, divided by DB) Emitter efficiency GE is the emitter Gummel number EE130 Lecture 25, Slide 13**Notice that**In real BJTs, NB and NE are not uniform, i.e. they are functions of x The more general formulas for the Gummel numbers are EE130 Lecture 25, Slide 14**Summary: BJT Performance Requirements**• High gain (bdc >> 1) • One-sided emitter junction, so emitter efficiency g 1 • Emitter doped much more heavily than base (NE >> NB) • Narrow base, so base transport factor aT 1 • Quasi-neutral base width << minority-carrier diffusion length (W << LB) • IC determined only by IB (IC function of VCE,VCB) • One-sided collector junction, so quasi-neutral base width W does not change drastically with changes in VCE (VCB) • Based doped more heavily than collector (NB > NC) (W = WB – xnEB – xnCB for PNP BJT) EE130 Lecture 25, Slide 15**Review: Modes of Operation**Common-emitter output characteristics (ICvs.VCE) EE130 Lecture 25, Slide 16

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