MTL Annual Research Report 2012

Cathode for X-ray Generation with Arrays of Individually Addressable Field Emitters Controlled by Vertical Ungated FETs

This work focuses on the design and fabrication of a cathode for a portable x-ray source. The cathode is made of an array of individually addressable electron guns, each containing double-gated field emitters. Compared to thermionic cathodes, field emission arrays operate at lower vacuum and lower temperatures, use less power and are more portable. The electron beam from each gun is extracted by a proximal gate and collimated using a distal gate before it hits an anode in a micron-sized spot that generates Bremsstrahlung x-rays. The architecture of the cathode is shown in Figure 1. Each field emitter is fabricated on top of a vertical ungated field-effect transistor (FET) ^{[1] [2]} that acts as a current source due to the velocity saturation of electrons in silicon when the voltage across the FET is above a saturation voltage. Current source-like behavior provides spatial and temporal uniformity of the output current across the emitter array; it also protects against emitter burnout and current surges. Individual addressability is achieved by fabricating the structure on SOI wafers to create electrically isolate strips of silicon. The extractor and focus gates are monolithically integrated with the cathode chip. They are patterned in strips that are orthogonal to the silicon strips, so that a single electron gun can be turned on at once. Each vertical ungated FET is a 25-μm-tall column with a 0.5-μm diameter, and emitter tip radius is in the range of 20 nm. The saturation current and saturation voltage of the silicon columns are plotted as a function of doping density in Figure 2. Wafer doping of 10-20 Ω cm provides a saturation current of 0.5 μA and an output impedance of 2×10⁹ Ω. With 100 emitters per chip, the total output current per chip is 50 μA, corresponding to a current density of 139 μA/cm².

1. L. F. Velásquez-García, S. Guerrera, Y. Niu, and A. I. Akinwande, “Uniform high-current cathodes using massive arrays of Si field emitters individually controlled by vertical Si ungated FETs—Part 1: Device fabrication and characterization,” IEEE Transactions in Electron Devices, vol. 58, pp. 1783-1791, June 2011. [↩]
2. L. F. Velásquez-García, S. Guerrera, Y. Niu, and A. I. Akinwande, “Uniform high-current cathodes using massive arrays of Si field emitters individually controlled by vertical Si ungated FETs—Part 2: Device design and simulation,” IEEE Transactions in Electron Devices, vol. 58, pp. 1775-1782, June 2011. [↩]