Fig. 1. Circuitry diagram of the setup, device properties, concept, and performance of our avalanche transistor THz modulator. (a) The THz-TDS system schematic and the experimental setup. (b) Schematic illustration of the device. (c) Top electrode, bottom electrode of the avalanche transistor. (d) Scanning electron micrograph (SEM) of the device section. (e) The I–V characteristic curve of the avalanche transistor in the linear and avalanche modes. (f) The corresponding I–G curve.

Fig. 2. Performance of the avalanche transistor THz switcher and its comparison with devices in the linear mode or using THz-FET effect. (a) Time-domain THz signals under different currents in the avalanche mode. (b) The calculated frequency-domain transmission of THz waves at currents of 0.1 A, 0.3 A, 0.4 A, 0.5 A, and 0.9 A in the avalanche mode. (c) The corresponding MD of the avalanche transistor under different currents in the avalanche mode at 0.7 THz. (d) The transmissions of our device in the linear mode and a conventional monolayer graphene field-effect transistor (FET) with the same thickness of p-type silicon and silicon dioxide as the avalanche transistor under different voltages.

Fig. 3. Calculated electrical properties of p-silicon in the avalanche mode. (a), (b) The real and imaginary parts of permittivity of p-silicon under different gate currents in the frequency domain. (c), (d) The derived conductivity/carrier concentration as well as the plasma frequency.

Fig. 4. Modulation speed of the avalanche transistor device. (a) The experimental setup. The normalized MD of CW THz wave driven by a square-wave modulated illumination laser (b) at the rate of 1 kHz and (c) at the rate of 12 kHz. (d) Normalized MD magnitude at different modulation frequencies, showing a 3 dB operation bandwidth about 12 kHz.

Fig. 5. Device preparation lithography flow chart.

Fig. 6. (a) Schematic to obtain the I–V characteristic of bare p-Si. (b) The measured I–V curve of p-Si with a thickness of 300 μm.

Fig. 7. Photo of experimental setup for the avalanche transistor THz modulator.

Fig. 8. Time-domain THz signals under different currents in the avalanche mode.

Fig. 9. (a) Forth-back I–G curve of the avalanche breakdown transistor (ABT) in the avalanche breakdown mode. (b) The forth-back modulation depth of the THz-ABT device.

Fig. 10. Behaviors of the THz-ABT device in the reflection configuration. (a) The measurement schematic for reflection configuration. (b) The time-domain signals under different biased currents. (c) The zoomed-in THz waveforms of the peak area of E_r1 in (b). (d), (e) The measured and simulated reflection coefficients in the frequency domain under different biased currents.

Fig. 11. Conductivities of the THz-ABT device under different currents by setting different carrier collision frequency.

Fig. 12. (a) Measured (circle) and theoretical (solid curve) values of the transmission spectra of the THz-ABT at 0.1 A, 0.3 A, 0.4 A, 0.5 A, and 0.9 A in the avalanche mode. (b) The corresponding modulation depth of theoretical values. The behavior of the THz-ABT is well explained by the built theoretical model.

Fig. 13. (a), (b) Real and imaginary parts of permittivity of p-silicon under different gate currents in the frequency range 0.2–4 THz. (c) Theoretical transmission spectra of the THz-ABT at 0.1 A, 0.3 A, 0.4 A, 0.5 A, and 0.9 A in the avalanche mode at 0.2–4 THz. (d) The corresponding modulation depth of theoretical values.

Fig. 14. Schematic of the experimental configuration for dynamic modulation measurements.

Fig. 15. (a) Normalized modulation magnitude at different modulation frequencies, showing a −3  dB operation bandwidth at the current of 0.2 A. (b) The −3  dB operation bandwidth at the current of 0.4 A.

Table 1. Comparison of the THz Devices Performance of Amplitude Modulation Depth