Dense Array of Field Emitters Using Vertical Ballasting Structures

Applications

Field emitter arrays are used as small-scale, ultra-bright electron beam sources. This opens up possibilities for nano-emissive displays, microwave generators, radio frequency communications, and X-ray tube cold cathodes. These arrays are superior to conventional anodes (e.g., photocathodes, thermionic sources) due to low power consumption, instantaneous switching, current/voltage independence, and compatibility for operation at low vacuum.

Problem Addressed

Electrons are field emitted from the surface of metals and semiconductors when the potential barrier that holds electrons within the material is narrowed and deformed by high electrostatic fields, which are typically obtained by the application of a bias voltage between a gate structure and a nanometer scale, high-aspect-ratio structure ended in a nanosharp tip. Due to their exponential dependence on tip radius, emission currents are extremely sensitive to tip radii variation. Consequently, only a small fraction of the tips in an array emit electrons when a voltage is applied between the gate structure and the emitters. Attempts to increase the emission current by increasing the voltage results in burnout and on-voltage up-shifting. Spatial uniformity can be maximized if the emitters are ballasted. Previous ballasting approaches used large resistors in series with field emitters, but this resulted in power dissipation in the resistors and low emission currents from the array.

Technology

The field emitter structure includes vertical pillar structures to define un-gated transistor structures. This emitter structure provides high dynamic resistance with large saturation currents. The un-gated transistor structure is formed on a conducting substrate made of vertical pillar structures. This substrate is etched to fill the gap between the vertical pillars with a series of dielectric layers. This technology is inherently scalable, so a plurality of vertical un-gated transistor structures may be formed on a conducting substrate, e.g., where the substrate is made of a plurality of vertical pillar structures to define the un-gated transistor structures. In this case, a series of emitter structures are formed on the vertical ungated structures and positioned in a ballasted fashion to provide high dynamic resistance with large saturation currents.

Advantages

  • Accomplishes uniform and high currents from field emitter arrays
  • Provides high dynamic resistance with large saturation currents
  • Obtains current source-like behavior at high fields
  • Protects electron sources from current surcharge, lower risk of damage