Analysis and design of silicon/silicon-germanium terahertz diodes.

Abstract

This thesis investigated the potential and limitations of transit-time diodes as power sources in the millimeter-wave frequency range. A full band Monte Carlo program was developed to study charge transport dynamics on device performance. The program models charge transport based on detailed scattering mechanisms. It calculates diode DC, small-signal RF and large-signal RF properties by using different boundary conditions. Important device physics that limits the diode high frequency performance were illustrated by comparing the DC and small-signal results with the solutions from a simpler drift-diffusion method. The power generation capability of Si and SiGe mixed tunneling avalanche transit-time (MITATT) diodes were predicted based on the large-signal results. Simulation showed that charge non-equilibrium transport phenomena, velocity overshoot and dead-space for impact ionization, degrade the diode negative conductance. The velocity overshoot effect increases the power consumption by the diode itself, and the dead-space increases the effective generation region width. A SiGe generation region was designed to improve the charge injection conditions. The large-signal results showed that the SiGe MITATT diode improves the power generation by 63% at 200 GHz and 20% at 250 GHz. For frequencies beyond 300 GHz, both Si and SiGe MITATT diodes showed great promise for power generation. They produce more than 8 mW at 300 GHz and more than 1 mW at 400 GHz. The experimental method is critical in this frequency range. A good experimental method with excellent thermal handling capability extends the diode operating frequency up to 500 GHz with 1 mW of RF power. An integrated fabrication process was developed to achieve the desired electrical and thermal requirements based on the theoretical calculations. The traditional experimental method is no longer suitable for high frequency circuit applications due to the wire bonding approach used. This thesis developed a process which integrates the matching circuit into the device fabrication. As a result, diodes smaller than 10 mum were achieved which is a key requirement for high frequency circuits. A fully integrated oscillator circuit can be further developed based on this process.