e beam is creating the excited photocarriers which are accelerated by the time dependent electric field components of the incoming THz signal. The noise-free data-acquisition is obtained through a lock-in amplifier. It is to be noted that a PCA based THz receiver is always connected to an amplifier circuit. The THz detection using PCA is simple and reliable than other coherent detection techniques. 12.9 EQUIVALENT CIRCUITAL ANALYSIS OF PCA The working principle of PCA can be described through an electrical model using an equivalent circuit. The femtosecond laser beam is illuminated on the gap area between the electrodes. The photocarriers are created only if the incident photon energy is higher than the semiconductor band gap energy. Then the applied DC bias voltage 5G Wireless Communication System in Healthcare Informatics Vbias FIGURE 12.14 Dipole PCA equivalent circuit in dark state. Vs(t) 1c(t) Vr(t) FIGURE 12.15 Dipole PCA equivalent circuit with combined dark state and illuminated state. creates an electric field distribution and accelerates the electron-hole pair towards the corresponding electrodes which leads to the generation of a photocurrent. The terahertz is radiated from the antenna by the rate of change of this photocurrent. The left-over carrier without reaching the electrodes leads to the screening effect which is predicted by the equivalent circuit. As the electrode gap dimensions are less than the radiated wavelength, we can apply the lumped element approach along with the car- rier dynamics to model the equivalent circuit. The dark resistance R and capacitance C is due to the flow of dark current in semiconductor substrate material without laser illumination [Krotkus et al. (1995)] which is shown in Figure 12.14. Figure 12.15 represents the equivalent circuit of the dipole PCA by combining both the dark state and the optical illuminated state. The gap conductance (G(t)) and the gap capacitance (C(t)) is created when the antenna is optically illuminated, and it represents the gen- eration of pl