THz power detection in CMOS for imaging applications

THz imaging is a promising field with numerous applications, going from quality control over security to medical imaging. THz radiation (0.3-3THz) is non-ionizing and can be used to distinguish between different dielectrics. CMOS brings the well-known benefits of low cost, integration with digital logic, and high yield. Unfortunately, the fmax in CMOS is limited, making efficient power generation at THz frequencies challenging. On top of this, the free-space path loss is very high. 

A THz power detector must be as sensitive as possible to handle the extremely low input power levels. The figure of merit is the Noise Equivalent Power (NEP), which must be minimized. Using Schottky barrier diodes and self-mixing transistors, NEP’s of around 10pW/√Hz have been demonstrated in CMOS for frequencies above 500GHz. The presented work reaches an even lower NEP of 2.3pW/√Hz at 605GHz, while consuming very low area and DC power. 

An oscillator is not only sensitive to perturbations around its natural frequency, but also to (sub)harmonics. To exploit this property, a 200GHz injection-locked oscillator was realized. The oscillator’s inductor facilitates oscillation at 200GHz, but it also serves as a 600GHz folded dipole antenna. Incoming radiation at 600GHz inter-modulates with the fundamental oscillation. If the resultant intermodulation product falls into the oscillator’s lock range, the oscillation amplitude increases. By measuring the oscillation amplitude with an envelope detector, the incoming THz power can thus be determined.

To better understand and predict the behaviour of the presented topology, a compact and intuitive model based on the equivalent diagram in the attached figure has been developed. It uses inter-harmonic translation factors to predict the change in fundamental and harmonic oscillation voltage in response to a harmonic interferer.