Bipolar Junction Transistors and Heterojunction Bipolar Transistors 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 IE ~ 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 (5) From Eqs. 3 and 4, since , Eq 5 becomes

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 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 Class3 companion

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 Eq. 7-81 of Streetman gives the ratio of electron current to hole current for a p-n heterojunction 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 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