Effect of thin emitter set-back layer on GaAs delta-doped emitter bipolar junction transistor

GaAs delta-doped emitter bipolar junction transistors ( δ - BJT ) with different emitter set-back layer thicknesses of 10 to 50 nm were fabricated to study the emitter set-back layer thickness effect on device dc performance. We found that the current gain decreases following decrease in the emitter set-back layer thickness. A detailed analysis was performed to explain this phenomenon, which is believed to be caused by reduction of the effective barrier height in the δ - BJT . This is due to change in the electric-field distribution in the delta-doped structure caused by the built-in potential of the base-emitter ( B - E ) junction. Considering the recombination and barrier height reduction effects, the thickness of the emitter set-back layer should be designed according to the B - E junction depletion width with a tolerance of ± 5 nm . The dc performance of a δ - BJT designed based on this criteria is compared to that of a Al 0.25 Ga 0.75 As ∕ Ga As heterojunction bipolar transistor (HBT). Both devices employed base doping of 2 × 10 19 cm − 3 and base-to-emitter doping ratio of 40. Large emitter area ( A E ≈ 1.6 × 10 − 5 cm − 2 ) and small emitter area ( A E ≈ 1.35 × 10 − 6 cm − 2 ) device current gains of 40 and 20, respectively, were obtained in both types of transistors passivated by ( N H 4 ) 2 S treatment. The measured current gain of the GaAs δ - BJT is the highest reported for a homojunction device with such high base-to-emitter doping ratio normally used in HBT devices.