## 2 DBQW RTD Photo-Detectors
### Device Description  
An example of a surface-illuminated DBQW-RTD photo-detector consisting of InGaAs/AlAs semiconductor compounds is shown in Figure 1a. As exemplified in the conduction band diagram, since the applied external voltage drops almost entirely on the thick undoped InGaAs absorption layer, the photo-generated electrons and holes in it are separated by the electric field, the holes are accumulated at the barriers of the resonant structure whereas the electrons are collected by the external circuit. As a result, the applied electric field in the undoped layer is partially screened and that within the resonant structure is enhanced. Consequently, the NDR of the resonant tunneling device shifts to lower voltages, as exemplified in Figure 1b. If the accumulated holes have a long recombination time the sensitivity of the device is high but its frequency response is low whereas for fast hole recombination times the sensitivity is lower but high-speed can be achieved.  
Side-illuminated waveguide photo-detectors were proposed as a means to ease the trade-off between the speed and responsivity in order to achieve a large bandwidth-efficiency product [25,26]. This type of detector is generally edge-illuminated with light guided parallel to the plane of the intrinsic layer and where the light is absorbed. The photo-generated carriers and the light travel in perpendicular directions. In comparison with the surface transverse photodiode, the waveguide longitudinal photodiode responsivity is mainly determined by the waveguide length, and not by absorption layer thickness, therefore the waveguide detector can have a larger bandwidth-efficiency performance than the surface photo-detector.  
The optical waveguiding structure is similar to that of the double heterostructure lasers. Figure 2 shows the RTD-PD optical waveguide schematic, the wafer structure and the energy band diagram of the waveguide detector used in this work. The RTD component was a device fabricated from pre-existing material, the layer structure of which is given in [27]. The InGaAlAs RTD structure was grown by molecular bema epitaxy in a Varian Gen II system on a n\({}^{+}\) InP substrate. It essentially consisted of two 2-nm-thick AlAs barriers separated by a 6-nm-wide InGaAs quantum well, embedded in a 1-um thick InGaAs optical waveguide core, see epi-layer structure in the inset of Figure 2a. Besides the epi-layer structure shown in the inset of Figure 2a, a \(\delta\)-doped InGaAs cap layer was provided for formation of Au-Ge-Ni ohmic contacts. Ridges were fabricated by wet-etching after which ohmic contacts were deposited on top of the ridges. A SiO\({}_{2}\) layer was then deposited, and access contact windows were etched on the ridge electrodes making contact to high frequency bonding pads [coplanar waveguide (CPW) transmission line].  
The design described here was originally used to obtain a DBQW-RTD operating as an electro-absorption modulator (EAM), but the same epitaxial structure can be used for novel photo-detector devices and applications as discussed in this article. Light is edge-coupled to the waveguide and the responsivity is mainly limited by the coupling efficiency and the scattering loss  
Figure 1: **(a)** Schematic of the conduction band diagram of a surface illuminated InGaAs/AlAs DBQW-RTD. Inset is shown the schematic cross-section of the surface RTD-PD [22]. **(b)** Typical dark and illuminated I-V characteristics.  
along the waveguide. The coupling can be improved through the use of lensed fibers, antireflection coating at waveguide facet, or laterally tapered waveguide sections.