High-speed, high-responsivity, and high-power performance of near-ballistic uni-traveling-carrier photodiode at 1.55-μm wavelength

We demonstrate a novel photodiode at a 1.55-μm wavelength: the near-ballistic uni-traveling-carrier photodiode (UTC-PD). After a p/sup +/ delta-doped layer was inserted into the collector of a UTC-PD, near-ballistic transport of photogenerated electrons under high reverse bias voltage (-5 V) and a high output photocurrent (/spl sim/30 mA) was observed. The demonstrated device has been combined with an evanescently coupled optical waveguide to attain high responsivity and high saturation power performance. Extremely high responsivity (1.14 A/W), a high electrical bandwidth (around 40 GHz), and a high saturation current-bandwidth product (over 1280 mA/spl middot/GHz, at 40 GHz) with high saturation radio-frequency power (over 12 dBm at 40 GHz) have been achieved simultaneously at a 1.55-μm wavelength.

The performance of microwave and millimeter-wave photonic systems would benefit from the use of photodiodes (PDs) with high saturation power, high-speed performance, and high responsivity [1], [2]. Fig. 1. (a) Cross-sectional view, (b) top view, and (c) conceptual band diagram of the demonstrated NBUTC-PD, which incorporates with an evanescently coupled optical waveguide. In order to meet these three requirements simultaneously, several technologies have been demonstrated, such as evanescently coupled waveguide PDs [3], [4], partially depleted absorber PDs [5], [6], and uni-traveling-carrier PDs (UTC-PDs) [1], [2]. The structure of UTC-PD has attracted much attention due to its excellent speed and output power performance [1]. However, such a device usually suffers from the problem of bandwidth degradation under high dc external bias voltage [7] due to both the high electric field existing at the junction of the collector (C) and photoabsorption (P) layer, and the decrease in the overshoot drift-velocity of photogenerated electrons [7]. One possible way to enhance the speed performance of UTC-PD is to reduce the externally applied bias voltage and let the value of the electric field in the C-P junction approximately for the critical field, which will enhance the drift-velocity of photogenerated electrons [7]. However, under such low reverse bias voltage (around −1 V for 200-nm collector), the field-screening effect, which originates from the difference in polarity between the output ac voltage across the standard 50- load and the dc bias voltage, will seriously limit the maximum output photocurrent of the UTC-PD [7]. In this work, we demonstrate the state-of-the-art performance of a novel photodiode: the near-ballistic UTC-PD (NBUTC-PD). By inserting an additional p⁺ delta-doped layer in the collector of a traditional UTC-PD, we can achieve near-ballistic transport under high reverse bias voltage (−5 V) and a high output photocurrent (30 mA). The demonstrated novel device has been combined with the evanescently coupled waveguide structure [3], [4] to attain very high responsivity performance.