Investigation of Base Active Region Design of Light-Emitting Transistors
Date Issued
2015
Date
2015
Author(s)
Chern, Yu-Wen
Abstract
In the thesis, the optical modulation bandwidths of light-emitting transistors (LETs) at different bias conditions are measured and compared to investigate the influence of the voltage-dependent charge-removing mechanism within the active region. The modulation bandwidth increases dramatically from 338 MHz (transistor saturation operation) to 1.338 GHz (forward-active operation) due to voltage modulation under a constant input current of 2 mA. We show that Giga-Hz spontaneous bandwidths of LETs depend not only on the input currents, which is similar to light-emitting diodes, but also are strongly related to the voltages of base-collector junction. By varying the reverse bias of collector terminal, different charge distribution profiles can be established to perform distinct optical modulation bandwidths. Moreover, we want to obtain the optimized structure which has high current gain, large light output, and fast optical response by changing the device structure before fabrication. The dc and microwave analysis of electrical and optical characteristics for light-emitting transistors are demonstrated by using software of technology computer-aided design, TCAD. We demonstrate the dc characteristics of heterojunction bipolar transistors and light-emitting transistors, i.e. current gain, optical gain, band diagram and optical distributions. The electrical current gain is decreased from 52.2 to 2.02 and the base radiative recombination rate is enhanced from 1.22×10^15 to 1.95×10^16 #/s when transistor changes from HBT to LET at the collector current of 12 mA. Based on microwave analysis and small signal equivalent circuit, corresponding base transit time is enhanced from 7.9 ps to 85.5 ps by incorporating QW in the HBT. Finally, different base active region design of the light-emitting transistors is presented and compared in order to obtain the optimized design, i.e. different base doping, compositions, QW widths, and QW positions. We focus on the effect of QW position design of light-emitting transistors on the electrical gain, optical output and microwave characteristics. Smaller electrical gain of 1.52, larger base radiative recombination rate of 1.52×10^16 #/s, larger base transit time of 87.5 ps, and smaller effective base recombination lifetime of 133 ps at the collector current of 12.4 mA can be obtained in the device with quantum well position closer to the emitter. These results can be discussed and organized through the dc, microwave and charge control model analyses.
Subjects
Light-emitting transistors
TCAD
optical communication
charge-removing mechanism
tilted-charge distribution
base active region design
Type
thesis
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