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Measurement of bandgap narrowing effects in p-GaAs and implications for AlGaAs/GaAs HBT performance

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5 Author(s)
Klausmeier-Brown, M.E. ; Sch. of Electr. Eng., Purdue Univ., West Lafayette, IN ; DeMoulin, P.D. ; Lundstrom, M.S. ; Melloch, M.R.
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The authors report the first electrical measurements of bandgap narrowing effects in p+-GaAs and explore their influence on the performance of AlGaAs/GaAs heterojunction bipolar transistors (HBTs). A sequential etch technique was used to measure electron injection currents in MOCVD (metalorganic vapor-phase epitaxy)-grown p +-n GaAs diodes with Zn dopant concentrations ranging from 6.3×1017 to 1.3×1019 cm-3. The measurement consisted essentially of characterizing the n=1 electron injection current as a function of thickness of the p-type layer. From the measured n=1 saturation current density, the product, nie2Dn was determined, where nie is the effective intrinsic carrier concentration and Dn is the minority-carrier diffusion coefficient. This product was observed to increase greatly with doping and was still a factor of two to three greater than expected, even when bandgap shrinkage was accounted for. This affects AlGaAs/GaAs n-p-n HBTs by increasing the electron current injected into the base. Bandgap narrowing effects increase the collector current by more than one order of magnitude when the base doping exceeds 1019 cm-3. These results demonstrate the importance of treating bandgap narrowing effects when modeling and designing HBTs

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Electron Devices, IEEE Transactions on  (Volume:35 ,  Issue: 12 )