# Silicon Field Effect Transistor as the Nonlinear Detector for Terahertz Autocorellators

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## Abstract

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## 1. Introduction

## 2. Si CMOS FET Detector

## 3. Experiment Setup

## 4. Nonlinearity of the TeraFET Detector

## 5. Nonlinear Autocorrelation Measurements

## 6. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**(

**a**) the micrograph image of the bow-tie detector on a CMOS crystal is used in this paper; (

**b**) a simplified schematic view of the detector (not to scale).

**Figure 2.**The simulated current responsivity for a field-effect-transistor (

**a**) and a zero-bias Schottky barrier diode (

**b**). Excitation power is proportional to the amplitude squared. Both devices exhibit three different regimes: a linear regime (the response is linearly proportional to the input power), a super-linear regime and a saturation regime. FET threshold voltage is 0.48 V and the diode non-ideality factor is 1.18.

**Figure 3.**The THz autocorrelation setup. The Ti:sapphire oscillator generates 30 fs-long infrared pulses. The THz radiation power, emitted by a GaAs photoconductive emitter, is modulated through the rectangular-waveform bias. The response of THz detector is measured using a lock-in technique (not shown on diagram).

**Figure 4.**The average power of THz pulses measured with the Golay cell as a function of the photoconductive THz emitter bias voltage. The dashed line is the guide to an eye, a square law ${P}_{\mathrm{THz}}\sim {V}^{2}$.

**Figure 5.**Rectified voltage (

**a**) and responsivity (

**b**) of the CMOS TeraFET detector exposed to a broadband pulsed THz radiation at various gate bias voltages. The threshold voltage of the FET is 480 mV. The noise data shown in panel (

**a**) is a measurement result taken at 50 mV gate bias. The dashed lines are a guide to the eye—represent a law ${V}_{\mathrm{det}}\sim {P}_{\mathrm{THz}}^{n}$.

**Figure 6.**Modeled autocorrelation signal according to Equation (6) for two cases: a linear (n = 1) and a nonlinear (n = 1.5). The inset is the THz pulse measured with a photoconductive antenna.

**Figure 7.**Measured autocorrelation traces of the CMOS TeraFET at different gate-voltages (solid lines) and the Golay cell (symbols) response. The sub-threshold bias regimes of the TeraFET are below 450 mV. The n numbers indicate the modeled exponent of relation between the detector response and the radiation power. The average power of a THz source is 0.87 $\mathsf{\mu}$W.

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**MDPI and ACS Style**

Ikamas, K.; Nevinskas, I.; Krotkus, A.; Lisauskas, A.
Silicon Field Effect Transistor as the Nonlinear Detector for Terahertz Autocorellators. *Sensors* **2018**, *18*, 3735.
https://doi.org/10.3390/s18113735

**AMA Style**

Ikamas K, Nevinskas I, Krotkus A, Lisauskas A.
Silicon Field Effect Transistor as the Nonlinear Detector for Terahertz Autocorellators. *Sensors*. 2018; 18(11):3735.
https://doi.org/10.3390/s18113735

**Chicago/Turabian Style**

Ikamas, Kęstutis, Ignas Nevinskas, Arūnas Krotkus, and Alvydas Lisauskas.
2018. "Silicon Field Effect Transistor as the Nonlinear Detector for Terahertz Autocorellators" *Sensors* 18, no. 11: 3735.
https://doi.org/10.3390/s18113735