Photomixer

Fully Ballistic p-i-n diode-based photomixers

Scanning electron beam image (false color) of a p-i-n diode-based photomixer. / Image: S. Preu
Scanning electron beam image (false color) of a p-i-n diode-based photomixer. / Image: S. Preu

A photomixer device generates an AC photocurrent by absorbing the intensity beating of two slightly detuned (by the desired THz-frequency) laser beams. This (THz)-current can be fed into an antenna which emits the THz-radiation. The device has to be optimized with respect to the extremely high frequency. In particular, the capacitance of the device must be minimized since an RC roll-off results from the antenna (R)-device (C) system. Furthermore, the intrinsic “speed” has to be maximized by reducing the transit time of the optically generated charge carriers through the intrinsic layer. We demonstrated that a p-i-n diode shows ballistic electron motion under optimum field conditions and intrinsic layer lengths: For sufficiently low kinetic energy below the threshold of very efficient sidevalley scattering process, the electron motion is only perturbed by weak scatters such as phonons. We develop fully ballistic p-i-n diodes (FB-PIN) for efficient Terahertz generation where we optimize the intrinsic layer structure for best transport performance with transit-time 3 dB corner frequencies in the range of 1 THz.