Fundamentals
: Basically, a photodetector is a solid-state sensor that converts optical energy into electrical energy, typically generating a photocurrent at its output. The incidence of photons of a certain energy causes the transition of electrons from the valence band to the conduction band. To facilitate this generation, the minimum of the conduction band must be located directly above the maximum of the valence band in momentum space. These materials are called direct bandgap materials. Conversely, in the case of indirect bandgap materials, the absorption process requires the participation of a phonon (a particle associated with the vibrations of the structure that possesses low energy and high momentum compared to the photon). Due to the involvement of three different particles (electron, photon, and phonon), the probability of interaction in this second case is lower, so absorption in indirect bandgap materials is weaker than in the case of direct bandgap materials. As a result, thicker absorption layers are required, leading to longer transit times and lower bandwidths.

Unfortunately, silicon and germanium are among the materials characterized by an indirect bandgap. Despite this, they are two widely used materials in LAN applications. Silicon detectors are used in the 0.4 to 1.0 mm range, while germanium detectors cover longer wavelengths (up to 1.8 mm). For long-distance, high-performance applications, III-V semiconductors are commonly used. For example, an In0.53Ga0.47As on InP structure has a cutoff frequency of 1.65 mm, making it particularly useful for photodetectors in telecommunications systems, as it includes the two typical transmission windows (1.3 and 1.55 mm). Additionally, HBT (heterojunction bipolar transistors) and HEMT (high-electron mobility transistors) have also been demonstrated on InGaAs/InP, making it an ideal material for meeting the demands of long-distance communication systems with monolithic integration. Figure 1 shows the absorption coefficients as a function of wavelength for different types of materials, where the cutoff wavelength of each can be seen.
As previously mentioned, integrated photoreceptors consist of two components: a photodetector and an electronic amplifier. Monolithically integrated receptors offer the advantages of lower cost, reduced parasitic effects, and greater reliability compared to hybrid receptors. Regardless of the integration scheme, the choice of technology for photoreceptor design is based on performance criteria such as sensitivity, frequency response, and noise.

There are multiple types of photodetectors, and the most appropriate one is chosen according to its final application. In the case of high-bit-rate optical communication systems, avalanche, waveguide, PIN, or MSM (metal-semiconductor-metal) photodiodes are the most commonly used. Of these, MSM photodiodes are the least complex, although they have limited quantum efficiency for high-speed operation. On the other hand, avalanche photodiodes offer excellent sensitivity due to their internal gain, but are limited in bandwidth. Finally, PIN photodiodes are characterized by excellent bandwidth, good sensitivity, and linearity.
Bandwidth and Performance:
Conventional PIN photodiodes are illuminated from the top and are easy to align with optical fibers. However, there is a trade-off between bandwidth and responsiveness. This is generally common to all technologies, since increasing bandwidth requires reducing the carrier transit time, which is achieved by decreasing the size of the absorption region and, therefore, the device gain. To avoid this, waveguide photodiodes (WGPDs) were developed. This device is illuminated from one side through a waveguide, such that the light and carrier paths are perpendicular. This allows for more or less independent control of the absorption region's dimensions. The trade-off is increased difficulty in aligning the device with the optical fiber.

Both conventional PIN photodiodes and WGPDs are widely used in receivers for high-speed communication systems. From an industrial perspective, the PIN configuration is preferable due to its easier packaging. To improve the responsiveness of PIN devices, integrated circuits are manufactured using HBTs and HEMTs, resulting in receivers with high gain and high bandwidth. In fact, HBTs and HEMTs have been fabricated on InP with cutoff frequencies of 604 and 562 GHz, respectively. In terms of noise, HBTs have a lower 1/f noise figure than HEMTs, while HEMTs offer excellent noise performance at high frequencies. Pseudomorphic HEMTs, in turn, provide better 1/fy generation-recombination noise figures.

Manufacturing:
The monolithic integration of PINs and HEMTs consists of a stacked-layer structure. In general, for all HEMT-based photoreceptors, the HEMT layers are grown first, followed by the detector layers, as shown in Figure 2. This is because the performance of the upper device is often degraded by imperfections in the insulation quality of the lower layers. In the case of a photodetector, this manifests as a dark current, leading to increased shot noise. For HEMTs, parasitic effects cause degradation, significantly impacting the cutoff frequency of the gain response. The end result is a significant limitation for the design of low-noise, broadband amplifiers. A key advantage of PIN-HEMT photoreceptors is their flexibility in device design, as the heterostructures of the photodetector and the transistor/amplifier can be optimized independently.
Since the shot noise increase of the PINs due to dark current is negligible compared to the amplifier noise sources, the PIN layers are usually stacked on top of the HEMT layers. This is less critical for HBT-based designs, although the same fabrication method is generally used. The process of growing and stacking the HBT and PIN layers is complex because the resulting structure is not planar; therefore, the base and collector regions are often shared, as shown in Figure 3.

Commercial Devices.
Many companies manufacture broadband photodetectors. For example, one of them is Discovery Semiconductors. Figure 4 shows the frequency responses of some of their photodetectors. These are suitable for detecting signals of 10, 20, 40, and up to 80 Gbit/s. Another company specializing in this type of device is the German firm u2t Photonics. Figure 5 shows a photograph of the BPRV2123 photodetector, characterized by a differential gain of 2400 V/W and suitable for detecting OC-768/STM-256 signals up to 43 Gbit/s. Finally, New Focus also markets broadband photodetectors. Figure 6 shows a photograph of the 1014 model, which is characterized by a bandwidth of 45 GHz.

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Author:

Francisco Ramos Pascual. PhD in Telecommunications Engineering

Full Professor at the Polytechnic University of Valencia