# An Overview of Terahertz Imaging with Resonant Tunneling Diodes
## 3 RTD THz Technology
### RTD THz Detector  
Besides THz source or emitter, the other crucial component in the imaging system is the THz detector. Responsivity, noise equivalent power (NEP), response time, and capability for array integration are the key factors for imaging applications. In general, THz detectors can be classified into two categories: thermal detectors and rectification detectors. Thermal detectors include an acousto-optic detector, e.g., Golay cell, which consists of an enclosed chamber in which the changing of gas volume due to radiation heating is measured; and a bolometer detector that measures the changes of properties such as conductivity, temperature, or electrical polarization (pyroelectric detectors). Thermal detectors have the advantage of very broad bandwidth, but the response time is relatively slow (0.1-0.01 s) making them unsuitable for real-time imaging applications [43].  
Rectifying THz detectors normally utilize the non-linear characteristic of devices such as a Schottky barrier diode (SBD) detector [44; 45], field effect transistor (FET) detectors by utilizing stimulated plasma wave [46], or photoconductive antenna (PCA) detectors that introduce the transient voltage drop over the gap of antenna on photoconductive substate (e.g., LT-GaAs) [47] and metamaterial detectors due to artificial negative refractive index [48; 49]. The advantages of using rectifying THz detectors include the ability to provide coherent detection, enabling both amplitude and phase information to be extracted. They also show relatively high responsivity (kV/W), low noise equivalent power (NEP) of \(10^{-12}\) to \(10^{-11}\) W/\(\surd\)Hz, and fast response time of a few picoseconds [43; 45; 50; 51].  
RTD devices show strong non-linear characteristics as described in the previous section. There are two working modes for RTD detector: direct and indirect (coherent) mode. For direct detection, the RTD is biased in the non-linear region at zero bias (for triple barrier variant) or at peak voltage as depicted in Figure 2c. The input THz electromagnetic wave is strongly rectified following square law detection, i.e., the output current is proportional to the square of the input signal voltage [52]. The highest reported InP RTD detector responsivity is 4 kV/W at 0.35 THz [53] and 80 V/W at 0.76 THz, with the RTD biased near the peak-current voltage (Vp) [54]. In [55], it was reported that RTD detector showed four times higher responsivity compared to a commercial SBD detector at 300 GHz. It is worth  
Figure 4: (**a**) Structure of 1.92 THz RTD transmitter including the RTD chip was mounted on a silicon hemispherical lens (Reprinted with permission from [38]). (**b**) Structure of 1 mW 260 GHz RTD transmitter (Reprinted with permission from [40]).  
menting that the dynamic range of an RTD direct detector is limited since the input RF amplitude should not shift the RTD bias point into the NDR region, as that will cause DC instability. A triple barrier (TB) RTD has also been proposed as a high responsivity zero-bias detector. It has demonstrated strong non-linear characteristic near zero bias voltage and gave 66 kV/W responsivity at 280 GHz [56].  
For the coherent mode detection, RTD oscillator acts as a LO when the RTD is biased in the active NDR region. If the received signal has the frequency and amplitude within the locking range of the RTD LO, injection locking will take place, resulting in homodyne detection due to self-mixing [57; 58]. Coherent detection has shown up to a 40 dB improvement of the detected power compared with direct detection [58].