| Summary: | 博士 === 國立中央大學 === 電機工程學系 === 106 === In this thesis, we demonstrated several kinds of uni-traveling-carrier photodiodes (UTC-PDs) for THz operation. One among such is near-ballistic uni-traveling-carrier photodiode (NBUTC-PD) with P-type charge layer in order to sustain electrons overshot drift-velocity for high-speed bandwidth. The optimum bias for THz operation of this device is usually located at −2 V. However, it has high probability of suffering from higher thermal failure under high output power due to high bias voltage.
For the sake of solving this issue, we report another kind of novel collector design by inserting N-type charge layer, N-UTCPD, which optimizes the profile of electric-field distribution in order to further suppress serious Space charge Screening (SCS) effect. The electrons will drift across the collector layer with overshoot velocity even under lower electric-field operation at −1V. Overall, for new structure N-UTC-PD, it still has superiorly high bandwidth (~315GHz) performance, but the new structure also leads to degradation of output power(-3dBm@280GHz); for NBUTC-PD, it has better output power (-1.8dBm) and bandwidth (~325GHz), but this kind of photodiode must need better heat dissipation to avoid thermal failure.
However, by using thin absorption layer for ultra-speed bandwidth operation, low responsivity performance in these ultrafast PDs is a big issue. For this reason, we demonstrate a GaAs0.5Sb0.5/In0.53Ga0.47As type-II hybrid absorbers UTC-PD by improving the epi-layer structure to get higher responsivity. According to the measurement results, the flip-chip bonding packaged device with active diameter of 3μm shows moderate responsivity of 0.11A/W (NBUTC-PD:0.08A/W, N-UTC-PD:0.09A/W) along with the recorded, really wide 3-dB optical-to-electrical bandwidth reaching 0.33 THz, among all those reported for long wavelength (1.3–1.55μm) PDs. A 13-mA saturation current and a continuous wave output with power as high as -3 dBm at −1 V are successfully demonstrated at an operating radio frequency (RF) of 0.32 THz under an optical signal with a sinusoidal envelope and ∼63% modulation depth for PD excitation.
By use of such kind of PDs, the broadband, integrated photonic transmitter front-end with a novel waveguide-coupled (WR6) is demonstrated. According to the measurement results with the novel design in high gain dual-ridge horn antenna and planar circuit for waveguide excitation, such transmitter achieves recorded-wide fractional bandwidth (100%; 0.1 to 0.3 THz) and high detected power in the receiving-end (31.6 W) at 0.24 THz through wireless transmission.
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