**4.4. Responsivity, NEP, and imaging**

**4.3. Polarization sensitivity of photoresponse**

64 Design, Simulation and Construction of Field Effect Transistors

shows the devices at the zero-angle position.

contact.

It can be observed in **Figures 6(a)** and **7** that the obtained photoresponse is more intense under excitation at 0.3 than at 0.15 THz. This must be partly attributed to the higher power at 0.3 THz (~6 mW) than at 0.15 THz (~3 mW) and to the coupling of the THz radiation to the device that varies with frequency. Moreover, the bonding wires and the metallic pads could play an antenna role to couple the incoming terahertz radiation (linearly polarized) to the 2D electron channel [43–45]. To understand how radiation is coupled, devices were rotated in the plane perpendicular to the terahertz beam, and the photoresponse signal was measured for

**Figure 9.** Photoresponse versus rotation angle for all devices under excitation of 0.3 (a) and 0.15 THz (b). The inset figure

**Figure 8.** Photoresponse obtained from TCAD simulations versus VGS for different horizontal positions of the gate

Responsivity (*RV*) and noise equivalent power (NEP) are the two key parameters (figures of merit) that determine the performance of THz detectors. Responsivity is calculated according to the expression:

$$\mathbf{R}\_{\vee} = \frac{\Lambda \mathbf{U} \mathbf{S}\_{\vee}}{\mathbf{P}\_{\vee} \mathbf{S}\_{\vee}} \frac{\pi}{\sqrt{2}} \tag{10}$$

where ΔU is the photoresponse signal measured with the lock-in amplifier, *St* is the radiation beam spot area, *Sa* is the active area of the transistor, and *Pt* is the total incident power surrounding the detector. The radiation beam power and spot area were measured using a calibrated pyroelectric detector at the MODFET position (see **Figure 3**); the *Pt* values were *Pt* = 0.5 mW at 0.15 THz and *Pt* = 1 mW at 0.3 THz. The spot area is given by *πr<sup>2</sup>* where *r* is the radius of the beam spot (≈1.5 mm at 0.3 THz and 3.3 mm at 0.15 THz). The area of each single transistor, including the contact pads, is less than 0.05 mm2 (**Figure 2**), that is, it is much smaller than the diffraction limit area *Sλ = λ<sup>2</sup> /4*. Accordingly, to calculate *Rv* in Eq. (6), *Sa* was replaced by *Sλ* to avoid overestimation of the *Rv* as well as NEP. The factor π/√2 originates from the Fourier transform of the square wave-modulated THz signal detected as RMS value with a lock-in.

The NEP is given by *Nth*/*RV*, where *Nth* is the thermal noise of the transistor in V/Hz0.5 and *RV* is the responsivity in V/W. Since *RV* and the NEP were studied at zero drain current bias, the thermal noise *Nth* = (*4kTRds*) 0.5 is the only relevant source of noise of the transistor. Here, *Rds* is the drain-to-source resistance that can be extracted from the transfer characteristics measured at a low drain bias (20 mV) corresponding to the linear regime (i.e., **Figure 5(a)**).

**Figure 10** presents the responsivity and NEP curves for D1 with L<sup>G</sup> = 100 nm at 0.15 and 0.3 THz. **Table 2** summarizes the obtained NEP and Rv for the Si-MODFETs at 0.15 and at 0.3 THz. Device 1, with the shorter gate, exhibits the best performance at 0.3 THz with RV = 46.4 V/W and NEP ~0.12nW/Hz0.5 and Device 2 exhibits the best performance at 0.15 THz with RV = 74.5 V/W and NEP ~0.06 nW/Hz0.5. This must be attributed to the large photoresponse signal provided by the Si/SiGe MODFET and to a better coupling of the incoming terahertz radiation. The values obtained for the NEP and the responsivity are comparable to

was biased around its threshold voltage to obtain a maximum intensity of the signal. The clear terahertz image obtained of the hidden object confirms the suitability of strained-Si MODFETs to be used as detectors to obtain high-quality THz images. Better resolution could be obtained

Room-Temperature Terahertz Detection and Imaging by Using Strained-Silicon MODFETs

http://dx.doi.org/10.5772/intechopen.76290

67

The potential of submicron gate length strained-Si MODFETs as detectors of terahertz radiation was demonstrated. A broadband (non-resonant) THz detection was observed under excitation of the transistors by a continuous-wave source at 0.15 and 0.3 THz. TCAD results obtained using a HDM model were in good agreement with the experimental ones in terms of both the excitation frequency and the gate-to-source bias. When imposing a source-todrain current of 50 μA, both TCAD simulations and experiments show an increase of the photoresponse as compared to the photovoltaic mode. A theoretical study was performed to analyze the effect of gate's geometrical asymmetries on the THz detection. Coupling between THz radiation and strain-Si MODFETs channel was analyzed at 0.3 and 0.15 THz. It shows that the coupling is mainly performed by bonding wires at 0.15 THz. Finally, the strained-Si MODFET was used as a single pixel detector to obtain images of a concealed

We would like to acknowledge Thomas Hackbarth (Daimler AG) who fabricated the strained-Si MODFETs used in this work. Our work was financially supported by the Spanish Ministry of Economy and Commerce and FEDER (ERDF: European Regional Development Fund) under the Research Grant #TEC2015-65477-R and FEDER/Junta de Castilla y León Research Grant #SA045U16. Both Research Grants #TEC2015-65477-R and Salamanca University partly support Open Source publications. Y. M. Meziani acknowledges the financial support from

, Kristel Fobelets2

and Yahya Moubarak Meziani1

2 Department of Electrical and Electronic Engineering, Imperial College, South Kensington

,

at higher frequencies owing to its lower wavelength (λ <1 mm).

RIEC nation-wide Collaborative Reseach Project, Sendai, Japan.

1 USAL NanoLab, University of Salamanca, Salamanca, Spain

\*, Vito Clericò1

**5. Conclusions**

object at 0.3 THz.

**Author details**

Campus, London, UK

Juan Antonio Delgado-Notario1

Jesús Enrique Velázquez-Pérez1

\*Address all correspondence to: juanandn@usal.es

**Acknowledgements**

**Figure 10.** Responsivity (a) and NEP (b) measured under excitation of 0.15 (blue dots) and 0.3 THz (red squares) of the strained-Si MODFET with 100-nm gate length.


**Table 2.** Calculated NEPs and RV for the different devices under studio.

the ones of commercial terahertz detectors at room temperature like Golay cells, pyroelectric detectors, and Schottky diodes [47]. However, the Si/SiGe MODFET presents the advantage of working at higher modulation frequencies as compared to other detectors.

To test the ability of the strained-Si MODFETs as detectors in THz imaging, a single transistor (D1) was used as the sensor in the terahertz imaging system shown in **Figure 3**. **Figure 11** shows the visible image of a standard copper RT/duroid® laminate where the logo of the Nanotechnology Group at Salamanca University has been etched (a) and its terahertz image at 0.3 THz (b) when it was wrapped around with a paper. THz radiation passes through in the regions were the metal layer was etched off and it is reflected in the regions covered with copper. A pixel-by-pixel image was taken using D1 as the detector; the gate of the transistor

**Figure 11.** Visible (a) and 0.3 THz (b) images obtained at room temperature using the strained-Si MODFET with a shorter gate as sensor.

was biased around its threshold voltage to obtain a maximum intensity of the signal. The clear terahertz image obtained of the hidden object confirms the suitability of strained-Si MODFETs to be used as detectors to obtain high-quality THz images. Better resolution could be obtained at higher frequencies owing to its lower wavelength (λ <1 mm).
