Comprehensive investigation on emitter ledge length of InGaP∕GaAs heterojunction bipolar transistors

Abstract

The influence of emitter ledge length on the performance of InGaP∕GaAs heterojunction bipolar transistors is comprehensively investigated. Due to the band-bending effect at the intersection of the emitter ledge edge with the exposed base surface, an undesired potential saddle point is formed. Moreover, emitter ledge passivations that are longer or shorter than an optimal length result in the deterioration of device performance. Based on the theoretical analysis and experimental results, the surface recombination effect of the device with an emitter ledge length of 0.8μm is negligible compared with the unpassivated device. Also, the device with the emitter ledge length of 0.8μm shows nearly the best dc characteristics and acceptable rf performance. Therefore, the optimum emitter ledge length in this study is near 0.8μm.

Quantitative analysis of dc characteristics of Ga0.5In0.5P/GaAs heterojunction bipolar devices

“…It may be attributed to the relatively higher space-charge-region width than at higher voltage and is directly proportional to the capture cross section r (¼0.1 Â 10 À15 cm 2 ), bulk trapping density n tb (¼0.5 Â 10 14 /cm 3 ), and trapping density at heterointerface, n ti (¼1.5 Â 10 13 /cm 2 ). [12][13][14] The thermionic diffusion model seems to be inadequate to predict I-V characteristics of a HBT in the complete range of base-emitter voltage. The third base current density component, injection current density from base to emitter J RE , primarily depends on hole diffusion coefficient d p (¼12 cm 2 /s) in quasineutral emitter.…”

“…Even the Gummel Plot has been compared only up to applied bias V BE $ 1.3 V where current begins to saturate. As a general observation, Atlas predicts good agreement only below the onset of current saturation and shows sharp discrepancy beyond V BE $ 1.3 V. 12,13 The base sheet resistance is a prime factor for the current gain of HBT and may thus be used for comparing its performance. Moreover, the high current operation beyond applied bias V BE $ 1.2 V in HBT plays an important role in device operation.…”

“…10,11 Using Atlas, many workers have studied HBT DC characteristics, but hardly, these are compared with experimental data at higher range of base to emitter voltage. 12,13 Although, the drift-diffusion model employed by Liou 14 for AlGaAs/GaAs HBT gives the correct trend of collector current characteristics of HBT in the lower base-emitter bias region but does not predict the onset of current saturation at higher bias. 12,13 Although, the drift-diffusion model employed by Liou 14 for AlGaAs/GaAs HBT gives the correct trend of collector current characteristics of HBT in the lower base-emitter bias region but does not predict the onset of current saturation at higher bias.…”