# An Overview of Terahertz Imaging with Resonant Tunneling Diodes
## 3 RTD THz Technology  
### RTD Device  
RTDs are the highest frequency solid state electronic device with the fundamental frequency approaching 2 THz [13]. Its main limitation was low output power, typically in the microwatt range, but the technology is now producing THz powers in the milliwatt (mW) range, and therefore starting to enable practical applications in imaging, multi-gigabit wireless communication systems, and others [23; 24; 25; 26]. A good review of the state-of-the-art of RTD THz technology can be found in [27]. An RTD is a one-dimensional vertical transport unipolar two terminal semiconductor device. Figure 2a shows the typical layer structure of an RTD in the InP-based material system. The core of the device consists of a low bandgap semiconductor, in this case the indium gallium arsenide (InGaAs, E\({}_{\text{g}}\) = 0.738 meV) quantum well layer, sandwiched by high bandgap semiconductor layers, in this case the aluminum arsenide (AlAs, E\({}_{\text{g}}\) = 2.949 meV) layers, forming the so-called double barrier quantum well (DBQW) structure. The DBQW is nanometric in dimensions, typically under 10 nm, and so the energy states in the quantum well are quantized, i.e., only certain energy levels are allowed [28], and therefore also allowing quantum mechanical tunnelling of the electrons through the structure. The possibility of electrons tunnelling through the barriers is defined by the transmission coefficient, which is related to the alignment of the electron energies with the allowed energy states in the quantum well. Figure 2b shows the conduction band profile under forward bias corresponding to the peak resonant/current condition. In the illustration, the first and second quasi-bound resonant energy levels E\({}_{\text{1}}\) and E\({}_{\text{2}}\) are shown. The current through the DBQW increases with applied bias at low voltages up to the peak current/voltage corresponding to the resonant energy (E\({}_{\text{1}}\)) when the transmission coefficient is close to unity. Here, the electrons entering the DBQW have the same energy as the allowed energy level in the quantum well, and so the highest/peak current is reached. With further increase in bias, that transmission coefficient drops due to mismatch between the electron energies and E\({}_{\text{1}}\) and so the current drops with bias creating the negative differential resistance (NDR) region in the current-voltage _l-V_ characteristic. At higher bias still, corresponding to valley current/voltage, the electrons have higher energy compared to the barriers and therefore the current increases again with bias. Thus, the RTD is characterized by a highly non-linear _l-V_ characteristic comprising an NDR region and positive differential resistance (PDR) regions, as illustrated in Figure 2c. The precise shape of the _l-V_ characteristic depends on different elements such as the device size, material composition, epitaxial structure, and the temperature. Figure 2: (**a**) Typical InGaAs/AlAs RTD epilayer diagram. (**b**) The corresponding conduction band diagram under forward bias. The discrete quantized resonant energy levels E\({}_{\text{1}}\) and E\({}_{\text{2}}\) are denoted in the quantum well. (**c**) Typical RTD _l-V_ characteristic with the bias regions for use as a transmitter (Tx) or receiver (Rx) as shown. The span of the NDR region determines the maximum radio frequency (RF) power the diode can deliver to a load and this can be approximated by \((3/16)\Delta I\Delta V\)[29]. The actual RF output power would depend on various factors such as the operation frequency, device and circuit parasitic elements and impedance matching. In [30], the relation between the device output power with an optimal load in an oscillator circuit was derived. Using this relationship, simulation shows that 2 mW power at 300 GHz and 1 mW power at 800 GHz could be achieved for single RTD oscillator with an optimal load as shown in Figure 3. Recent experimental results show that this performance is becoming achievable [31; 32].