Simulating, fabricating and characterising photoconductive microwave switches for RF applications

Photoconductive microwave switches can be used in place of traditional microwave switches to reconfigure antennas and RF circuits. The switch, which consists of a silicon die placed over a gap in transmission line, is controlled by illumination via a fibre optic cable. Hence there is no requirement to design electrical biasing lines which may affect RF performance. This benefit is the main motivation behind further developing and understanding the photoconductive switch. The second motivation is the growing demand for reconfigurable antennas which necessitate certain switching requirements; one specific area of interest is in cognitive radio applications. However, in order to use such a switch in RF circuitry, the photoconductive nature of the switch must be understood. This is addressed in this thesis presenting and applying analytical equations which dictate the material properties in photoconductive silicon. These equations are then used to generate a 3D EM simulation model to investigate transmission loss in the photoconductive switch. The concept of signal planarity is investigated so as to give some insight into the best way to package the switch. In order to potentially reduce loss and facilitate a packaged device, the fabrication of the switch is investigated. Namely, the treatment of the silicon and the addition of contacts on the silicon are discussed as possible methods to improve switch performance. Lastly, linearity, power handing and switching times are presented for the photoconductive switch. This characterisation is important with regards to understanding which types of application the switch can be used in. In particular the single tone and two tone linearity of the switch is measured these values have not previously been reported for this type of photoconductive switch. The results are encouraging and support further development of the switch into a packaged product to be used in reconfigurable antennas and circuitry

Design and operation influences regarding rise and fall time of a photoconductive microwave switch

This paper evaluates the effect switch design and control method have on the rise and fall time of a photoconductive microwave switch at 2GHz. The effects of switch dimensions, switch fabrication methods and light intensity of the control mechanism are investigated. Switch rise time is affected by switch dimension and optical illumination intensity. Switch fall time is dependent on passivation of the silicon - which is a fabrication step often used to improve the conductivity within photoconductive devices

Intermodulation distortion in a photoconductive microwave switch

Single-tone harmonic and two-tone intermodulation distortion measurements are presented for a photoconductive microwave switch. The switch consists of a lightly doped die of silicon mounted over a gap in a transmission line. The switch is controlled via near infra-red light delivered by a fiber optic cable. Whilst under constant illumination, the third order intercept for a 2 GHz CW signal is extrapolated to be 74 dBm. Under a 1 MHz spaced two-tone signal, the extrapolated intermodulation intercept point is 58 dBm

Investigating factors affecting photoconductive microwave switch performance using 3D em simulation

A series of 3D EM simulation models are presented in order to determine the effect that conductivity profile, passivation layer and connection method have on the transmission performance of a photoconductive microwave switch. The use of 3D EM simulation can help quantify the benefit and impact of different approaches before the manufacture stage. The aim is to find methods to reduce insertion loss of the switch to provide maximum efficiency when the device is integrated into reconfigurable applications. Results show improvement to the transmission is possible by altering passivation thickness and designing optical feed to maintain signal planarity

Power handling of a photoconductive microwave switch

The power handling performance of a photoconductive microwave switch up to an RF input power of 44dBm (25W) is presented. The switch consists of a lightly doped die of silicon mounted over a gap in a transmission line. A 2GHz signal is applied through the switch and the 1dB compression point is analysed

Characterising the linearity of an optically controlled photoconductive microwave switch

The linearity response of a photoconductive switch on microstrip line is presented at 2GHz. A silicon switch is exposed to incident signal power of up to 1W and controlled via illumination with a range of optical intensities at a wavelength of 980nm in order to characterise the linearity in terms of harmonic content. Reported single tone output third order intercept (TOI) was measured as 63dBm under 200mW of optical incident light. The study presents photoconductive switches as a promising alternative to conventional microwave switches in high power applications

Design and operation influences regarding rise and fall time of a photoconductive microwave switch

This paper evaluates the effect switch design and control method have on the rise and fall time of a photoconductive microwave switch at 2GHz. The effects of switch dimensions, switch fabrication methods and light intensity of the control mechanism are investigated. Switch rise time is affected by switch dimension and optical illumination intensity. Switch fall time is dependent on passivation of the silicon - which is a fabrication step often used to improve the conductivity within photoconductive devices

Intermodulation distortion in a photoconductive microwave switch

Single-tone harmonic and two-tone intermodulation distortion measurements are presented for a photoconductive microwave switch. The switch consists of a lightly doped die of silicon mounted over a gap in a transmission line. The switch is controlled via near infra-red light delivered by a fiber optic cable. Whilst under constant illumination, the third order intercept for a 2 GHz CW signal is extrapolated to be 74 dBm. Under a 1 MHz spaced two-tone signal, the extrapolated intermodulation intercept point is 58 dBm

Optimising the performance of an optically controlled microwave switch

Optical control of microwave switches is an appealing concept for use in reconfigurable antennas as it eliminates the need for metallic biasing lines which may affect the performance of the wireless system. The ultimate goal of this study is to minimise insertion loss of a photoconductive microwave switch in the ON state whilst maintaining high isolation in the OFF state. Firstly, a parameter simulation study using different substrate materials, thicknesses and gap widths is presented to obtain optimised S21 results. The best performance is from a 1.2mm line using a 0.3mm gap. Secondly, the effect of passivation and texturisation on the photoconductivity and microwave performance of the silicon die is investigated. Passivation of the sample decreases insertion loss, however texturing the surface increases loss

Optimising the performance of an optically controlled microwave switch

Optical control of microwave switches is an appealing concept for use in reconfigurable antennas as it eliminates the need for metallic biasing lines which may affect the performance of the wireless system. The ultimate goal of this study is to minimise insertion loss of a photoconductive microwave switch in the ON state whilst maintaining high isolation in the OFF state. Firstly, a parameter simulation study using different substrate materials, thicknesses and gap widths is presented to obtain optimised S21 results. The best performance is from a 1.2mm line using a 0.3mm gap. Secondly, the effect of passivation and texturisation on the photoconductivity and microwave performance of the silicon die is investigated. Passivation of the sample decreases insertion loss, however texturing the surface increases loss