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Bipolar Junction Transistors and HBTs EE 3311/7322 and EE 5312/7312 (draft)SMU Bipolar Junction Transistors and Heterojunction Bipolar Transistors November 18, 2014 391 San Antonio, Palo Alto, CA May, 2006—original buildings stored vegetables from farms 391 San Antonio, Palo Alto, CA May, 2006—inside the quonset hut 391 San Antonio, Palo Alto, CA, 2 ~ 1958 ~ 2006 Jaguar XK 140? Playboy, August 1980 Issue (Bo Dereck) PLAYBOY: What about your own children? How did they turn out? SHOCKLEY: …my children represent a very significant regression. My first wife… Two of my three children graduated from college—my daughter from Radcliffe and my younger son from Stanford. He … has obtained a Ph.D. in physics. In some ways, I think the choice of physics may be unfortunate for him, because he has a name that he will probably be unlikely to live up to. Playboy, August 1980 Issue (Bo Dereck) PLAYBOY: What about your own children? How did they turn out? SHOCKLEY: …my children represent a very significant regression. My first wife… Two of my three children graduated from college—my daughter from Radcliffe and my younger son from Stanford. He … has obtained a Ph.D. in physics. In some ways, I think the choice of physics may be unfortunate for him, because he has a name that he will probably be unlikely to live up to. Excess Carriers and Quasi Fermi Levels Recall: Quasi Fermi Level Example Consider a Si sample with no = 1014 cm-3 nopo = ni2 so po = 2.25x1020 cm-6/1014 cm-3 = 2.25x106 cm-3 By some means (optical generation or electrical injection), an additional 2x1013 cm-3 electron-hole pairs are generated Now, n = 1.2x1014 cm-3 p = 2x1013 cm-3 (107 increase) np = 2x1027 cm-3 , not ni2 Quasi Fermi Level Example, cont’d 1 Bipolar Junction Transistors Bipolar means that both positive and negative charge carriers contribute to current flow FETs are unipolar—since only one type of charge carrier contributes to current flow Consider a pn junction: Illuminated pn Junction Bipolar Junction Transistor Concept BJT Concept, cont’d 3 Properties of a p+n Junction • If pp >> nn , then np is << pn (since nopo = ni2 ) • Therefore, the current across the depletion region is almost all hole current BJT Concept, cont’d 4 • • • • Emitter (forward biased p+n junction is the source of holes Reverse biased np junction “collects” holes injected into the base Holes are swept across the np depletion region by drift ( ) Need to make base narrow (< 1µm) to reduce recombination in base BJT Concept, cont’d 5 Heavy doping also decreases the bandgap (not desirable), and decreases resistance (desirable) BJT Concept, cont’d 6 • IE is mostly hole current • IC is mostly due to holes injected into the base • I E ~ IC • IB is small, determined by - electrons injected across the p+n junction (5) - recombination of injected holes and electrons in the base (1) and (4) [dominant term] -electrons swept into the base from the collector (3) (thermal generation and electrons within a diffusion length of the CB junction edge) BJT Concept, cont’d 7 BJT—no bias BJT– with bias BJT Concept, cont’d 8 BJT Concept, cont’d 9 Amplification in BJTs Dominant component of base current is due to EHP recombination in the base Amplification in BJTs, cont’d 1 ~ 100 is a typical number Amplification in BJTs, cont’d 2 BJT as a Circuit Element Why Heterojunction BJTs? From J. Singh, Semiconductor Devices Band Gap Shrinkage with Doping Heterojunctions and Homojunctions From J. Singh, Semiconductor Devices Heterojunctions and Homojunctions, cont’d Stop Here for 3311/7322 Heterojunction Conduction Band Offsets AlGaAs/GaAs Heterojunction Conduction Band Offsets InGaAs/GaAs Heterojunction Conduction Band Offsets InGaAs/InAlAs/InP Heterojunction Conduction Band Offsets Si/SiGe Heterojunction Conduction Band Offsets Band Gap Shrinkage with Doping Heterojunction Energy Band Diagram Heterojunction Energy Band Diagram, cont’d 1 Heterojunction Energy Band Diagram, cont’d 2 Band Lineups: InP/InGaAs, InAlAs/InGaAs Go to Schubert’s slides... • Mention homework problems, too! Emitter Injection Efficiency Recall the IV equation for a pn junction: Consider an n+p emitter base junction In (4), accounts for the finite length of the base Emitter Injection Efficiency, cont’d 1 or From Eqs. 3 and 4, since , Eq 5 becomes (5) Heterojunction Correction to Junction Current For high doping, the bandgap of Si shrinks Eq 7.11, Singh Previously we determined, so + + • Emitter bandgap shrinks: exponential increase in base carriers injected into emitter • Emitter bandgap increases: exponential decrease in base carriers emitted into emitter Band Gap Shrinkage with Doping For Silicon: Heterojunction Junction Current, cont’d 1 So the emitter injection efficiency increases or decreases: from and using we have • Emitter bandgap shrinks: exponential decrease in performance • Emitter bandgap increases: exponential increase in performance Al0.3Ga0.7As/GaAs HBT Eq. 7-81 of Streetman gives the ratio of electron current to hole current for a p-n heterojunction Eq. 7-81 applies to a homojunction BJT if the emitter is heavily doped, which causes the bandgap of Si to shrink For a p-n heterojunction, the bandgap difference between the p and n region is 0.416 eV. Such a bandgap difference gives a factor of ~ 107 compared to a homojunction BJT Si Bandgap Dependence on Doping http://ecee.colorado.edu/~bart/book/eband6.htm Abrupt and Graded Interfaces BJT and HBT Energy Band Diagram BJT • • • • HBT ne and pb are majority carrier densities in the emitter and base vn is the electron velocity in the base vp is the hole velocity in the base DEg is the bandgap difference • Want high base doping (low resistance) for high speed—but lowers gain • HBT wider bandgap solves problem by adding exp(DEg/(kT)) term to the gain And Another Eg vs Lattice Constant Chart EE7312 IntelliEPI HBT Typical InGaAs HBT Graded Base HBT • Gradual change in bandgap induces an electric field due to the bandgap gradient • Adds a drift term in addition to diffusion current Double Heterojunction HBT (DHBT) Add Wide Band Gap Collector • Eliminates the injection of holes into the collector • Reduces saturation stored-charge density • Speeds up device turnoff Silicon-Germanium HBTs • addition of Ge reduces the bandgap of Si • lower effective electron mass in SiGe AlGaAs/GaAs HBTs InGaAs/InP HBTs • Electron mobility in InGaAs is 1.6 times that of GaAs and 9 times higher than in Si • highest ft values in InP based HBTs Typical InGaAs/InP HBT • • • • InGaAs has a smaller bandgap than Si or GaAs requires lower voltage power supplies better thermal properties than GaAs surface recombination velocities smaller than GaAs InGaAs/InP HBT Frequency Performance Si/Si BJT This slide is unfinished—meant to calculate the degradation due to highly doping the collector with respect to the base for a Si-Si BJT and then on another slide show how to compensate with a SeGe base HBT Eq. 7-81 of Streetman gives the ratio of electron current to hole current for a p-n heterojunction Eq. 7-81 applies to a homojunction BJT if the emitter is heavily doped, which causes the bandgap of Si to shrink For a p-n heterojunction, the bandgap difference between the p and n region is 0.416 eV. Such a bandgap difference gives a factor of ~ 107 compared to a homojunction BJT Ballistic Collection HBTs ft ~ 105 GHz