Fig. 1. Diagrams of energy band structures of photodetectors^[32]. (a) UTC photodetector; (b) MUTC photodetector

Fig. 2. Monte Carlo simulation results of electron velocity^[35]. (a) Single electron velocity and average electron velocity; (b) average electron velocity for progressively doped and uniformly doped non-depleted absorber layers

Fig. 3. Optimization process and results for MUTC terahertz photodetector absorption layer^[37]. (a) Bandwidth optimization flow; (b) epitaxial layer structure of the device; (c) equivalent circuit diagram of the device; (d) simulation plots of transition bandwidth and transition time versus thickness of depleted absorbing layer; (e) distributions of photogenerated carrier velocity and electric field intensity; frequency responses of (f) PD-1 and (g) PD-2 at 3 V bias voltage under 5 mA and 10 mA conditions; (h) saturated output power in D-band

Fig. 4. Structure and test results of the MUTC photodetector^[12]. (a) Epitaxial layer structure of the device; (b) cross-section structure of the device; (c)(d) parasitic capacitance simulation results; (e) electrode inductance simulation results; (f) simulated frequency response of the device with different inductances; (g) measured frequency response of the device; (h) RF output power of the device

Fig. 5. High-power flip-chip bonded photodetector^[52]. (a) Schematic diagram of device structure; (b) comparison of device epitaxial layer structures; (c) front microscope image of the device; (d) side microscope image of the device; (e) frequency response of devices with different diameters; (f) saturated output power of devices with different diameters

Fig. 6. Terahertz near-ballistic transport photodetector^[48]. (a) Epitaxial layer structure of the device; (b) microscope images of the chip, aluminum nitride substrate, and flip-chip bonded device for structure A; (c) microscope images of the chip, aluminum nitride substrate, and flip-chip bonded device for structure B; (d) frequency response of the device; (e) saturated output power of the device at 285 GHz; (f) saturated output power of the device at 280 GHz using sinusoidal and pulsed signals

Fig. 7. Terahertz high-power photodetector prepared based on under-cut process^[53]. (a) Schematic diagram of the device structure; (b) front microscope image of the active area of the device; (c) microscopic image of the chip after fabrication; (d) microscopic image of the substrate; (e) microscopic image of the chip after flip-chip bonding; (f) frequency response of the device; (g) output power of the device at 235 GHz

Fig. 8. Terahertz high-power photodetector based on a flip-chip bonding process^[14]. (a) Band diagram of the device; (b) front view of the wafer; (c) scanning electron microscope (SEM) image of the device; (d) schematic diagram of the device structure; (e) frequency response of the device with a diameter of 4 µm; (f) frequency response of the device with a diameter of 6 µm; (g) frequency response of the device with a diameter of 8 µm; (h) output power of the device with a diameter of 10 µm; (i) output power of the device with a diameter of 8 µm; (j) output power of the device with a diameter of 4 µm

Fig. 9. Terahertz waveguide photodetector compatible with the indium phosphide monolithic integration platform. (a) Waveguide loss and series resistance^[54]; (b) quantum efficiencies of the devices with different lengths^[54]; (c) frequency responses of the devices with different sizes^[54]; (d) output power of the devices with different sizes^[54]; (e) photocurrent versus incident optical power^[55]; (f) frequency response of the device^[55]; (g) output powers of the device at different frequencies^[55]; (h) SEM image of the device^[55]

Fig. 10. Terahertz high-power waveguide photodetector^[56]. (a) Epitaxial layer structure of the device; (b) electric field distribution of the device; (c) 3D structure of the device; (d) responsivity of the device; (e) frequency response of the device; (f) output power of the device

Fig. 11. UTC photodetector based on GaAsSb-InP^[57]. (a) Band diagram of the device; (b) I-V curve of the device; (c) frequency responses of devices with different sizes

Fig. 12. Zero-bias terahertz photodetector. (a) Energy band diagram of the device at zero bias^[58]; (b) frequency response of the device at 2 mA photocurrent^[58]; (c) fequency response of the device at 3 mA photocurrent^[58]; (d) frequency response of the device at 5 mA photocurrent^[58]; (e) energy band diagram of the device at zero bias^[59]; (f) frequency response of the device at 3 mA photocurrent^[59]; (g) frequency response of the device at 5 mA photocurrent^[59]; (h) output power of the device^[59]

Fig. 13. Waveguide-type zero-bias sub-terahertz photodetector^[61]. (a) Epitaxial layer structure of the device at zero bias; (b) 3D structure of the device; (c) SEM image of the device; (d) responsivities of the devices with different sizes; (e) frequency responses of the devices with different sizes

Fig. 14. Germanium-silicon sub-terahertz photodetector with vertical PIN structure^[64]. (a) 3D structure of the device; (b) 2D cross-section of the device; (c) front view of the electrodes of the device; (d) front view of the electrodes of the reference device; (e) I-V curve of the device; (f) frequency response of the device

Fig. 15. Germanium-silicon terahertz photodetector with a biconcave germanium fin structure^[15]. (a)-(d) Process flow of the device; (e)(f) 2D cross-section views of the device; (g)(h) frequency response of the device with a 150 nm wide germanium absorption layer; (i) capacitance of the device; (j)(k) frequency response of the device with a 100 nm wide germanium absorption layer; (l) I-V curve of the device

Fig. 16. O₂ plasma-assisted wafer bonding^[76]. (a) Schematic of the O₂ plasma-assisted wafer bonding process; (b) schematic of the BCB-assisted die bonding process

Fig. 17. Silicon-based heterogeneous integrated UTC photodetector^[19]. (a) Epitaxial layer structure of the device; (b) responsivity of the device; (c) frequency response of the device; (d) I-V curves of the balanced detector; (e) differential/common-mode frequency responses of the balanced photodetector; (f) output power of the balanced photodetector

Fig. 18. Silicon nitride heterogeneous integrated terahertz photodetector^[80]. (a) Epitaxial layer structure of the device; (b) formation of the detector chip by etching; (c) etched release layer; (d) picking of the detector chip by PDMS; (e) structure of the silicon nitride chip; (f) fabrication of the silicon nitride waveguide; (g) transfer of the detector chip onto the silicon nitride waveguide; (h) fabrication of the detector electrode structure; (i) SEM image of photodetector after micro-transfer printing; (j) output RF power of the device; (k) bandwidth of the device

Fig. 19. Horizontally coupled silicon-based monolithic integrated photodetector^[81]. (a) 3D structural view of the device; (b) 2D side view of the InP and InGaAs laterally grown device; (c) 2D side view of the device perpendicular to the growth direction; (d) 2D front view of device with butt coupler; (e) 2D front view of device with taper coupler; (f) responsivity of the device; (g) frequency response of the device

Fig. 20. Thin-film lithium niobate heterogeneous integrated sub-terahertz photodetector^[99]. (a) 3D structure of the device; (b) epitaxial layer structure of the device; (c) cross-section view of the thin-film lithium niobate waveguide; (d) detailed microscope image of the device; (e) overall microscope image of the device; (f) I-V curve of the device; (g) frequency response of the device

Fig. 21. Thin-film lithium niobate heterogeneous integrated terahertz photodetector based on wafer-level fabrication process^[100]. (a)-(h) Heterogeneous integrated chip fabrication process flow; (i) I-V curves of the device; (j) responsivity of the device; (k)-(o) frequency responses of the devices with different sizes; (p) bandwidths of the devices with different sizes