Development of asymmetric Fabry-Perot modulators and power HBT circuits for high-speed applications

Optical modulators are important components for long-haul, high-bit-rate optical communication systems. A normal incident reflectivity electro-absorption modulator can avoid the high insertion loss and the low fabrication yield in a waveguide modulator. We will demonstrate an asymmetric Fabry-Perot modulator (AFPM) operating at 1.55-μm wavelength range. In the first material growth run, with a multiple-quantum-well (MQW) absorption region of 900 nm (including barriers and wells), the extinction ratio is 8 dB with a voltage swing of ±5 V at a bias voltage of 20 V. The higher than expected bias voltage is attributed to the mismatch between Fabry-Perot (FP) cavity dip and optimum MQW wavelength. A large bias voltage is thus required to tilt the bandgap of MQWs in order to align with Fabry-Perot dip. In the second growth run, a more precise FP cavity length and MQW absorption wavelength were calibrated to reduce the bias and swing voltages. With the same absorption thickness, a DC extinction ratio 4 dB at a reverse voltage 5 V (and a sharp extinction ratio drop from 0 to 2 V) were obtained. The 3 dB electro/optical (E/O) frequency response is around 20 GHz. By comparing the RC time constant and the transit time across the p-n contacts, we conclude that the frequency response is limited by transit time across the absorption layers.; The desire for high-performance power amplifiers has driven the development of InGaP/GaAs heterojunction bipolar transistors (HBTs) due to their good high-temperature performance and power linearity. We will discuss the design issues of power InGaP/GaAs HBTs and the device performance associated with layer structures. We will then propose three power structures and describe the process flow. DC and RF performance will be discussed. Finally, a power HBT process technique, thermally shunted structure, will be explained.; We will also demonstrate AlGaN/GaN graded-emitter HBTs with common emitter (CE) current gains ranging from β∼5 to 10 and a CE offset voltage V_CEOFF = 4.1 V from the I_C − V_CE curves at room temperature. These values are observed both from the Gummel plots and the common-emitter I–V characteristics. The common emitter current gains and offset voltages are the best results reported to date for AlGaN/GaN HBTs.; To identify the limitation of DC performance of GaN HBTs, the CE I_C-V_CE modulation curves in AlGaN/GaN graded emitter HBTs at low temperature was studied in order to understand the limiting issues related to the low CE current gain of a GaN HBT. We have measured a CE current gain of 31.0 at 175 K and 11.3 at 295 K. The increase in collector current and CE current gain at lower temperature can be attributed to the reduced base recombination current, which is due to the carrier traps associated with defect (dislocation) centers in the base-emitter junction.