A. The Traditional Thermionic Cathode

1. Principle of Thermal Electron Emission #6
a. Conduction electrons confined in potential energy well in solids

b. Electrons fill all energy states up to Fermi Energy at T=0K
c. In vacuum, minimum energy to remove electron is work function
d. Work function depends on material: metals (1eV to 5.6 eV)
e. At high T, some e-'s have enough energy to eject #7

f. Traditional cathode is hot filament of wire that "boils" off electrons
g. 1 to 30 KV between cathode, grid, screen accelerates electrons (F = eE)

2. Raster Scan of Electron Beam in Traditional CRT
a. Electric and magnetic fields sweep beam

b. Screen coated with fluorescent phosphors (ZnS, ZnO, etc.)
c. Phosphors excited by electrons emit R, G, or B light

B. The New Cold Micro-Cathode

1. Principle of Field-Emission
a. Electric fields strongest where minimum radius of curvature #11

b. Can microfabricate sharp pyramids with r ~ 200 Angstroms #12

c. Tips placed ~1 um from grid; get high E-field (> 10^8 V/cm) at low V
d. High electric fields distort potential energy well for electrons in tip
e. Electrons at Fermi Energy can tunnel out in a vacuum (L ~ 10 Ang)

f. These electrons are accelerated to grid and screen along the E-fields

2. Fowler-Nordheim Equation #12
a. Classic relation showing emission current's dependencies

b Typically, get 1-10 uA per pyramid with 140 V driving #12
c. 99% of current into this device is delivered to the phosphors
d. Maximize B with tip radius "r" and gate aperture "d"

C. The Thin-Film Diamond Cathode

1. IDEAL cathode since diamond naturally repels e-'s
2. Very low work function (0.2 to 0.3 eV)
3. Negative electron affinity and wide band gap #6

4. Donor e-'s need to have ~0.01 eV to eject (< KbT, 0.025 eV)
5. Hence, emits electrons at much lower E-fields than Si or metals
6. Simple, no need to microfabricate sharp tips
7. Can operate at higher pressures (1E-4 torr vs. 1E-7 torr)

  • Main Outline
  • III. Device Architecture
  • Sources