transparent as possible to the emission wavelengths of the device. So, the essential
insulator requirements for use in ACTFEL devices are as follows [51]:
1. Sufficient dielectric breakdown electric field, FBD
2. High relative dielectric constant, Er
3. Small number of defects and pinholes
4. Good adhesion to phosphor and contacts
5. Transparency
6. Good thermal and chemical stability
7. Small dielectric loss factor, tan6
In order to have efficient device operation, as much of the applied voltage as
possible should be dropped across the phosphor layer. The proportions of the voltage
dropped across the phosphor and insulators are determined by the capacitance of the
phosphor, Cp, and the capacitance of the insulator, Ci. As discussed in section 2.4, the
capacitances of the layers are determined using
C EoEr
t
where So is the permittivity of free space, Er is the relative permittivity, and t is the
thickness of the layer.
In order to maximize the voltage drop across the phosphor, the capacitance of the
insulator should be much larger than the capacitance of the phosphor. Using the above
equation, either the insulator should be very thin or the relative dielectric constant of the
insulator should be large. Unfortunately, charge leakage has been shown to occur in
insulators that are thinner than 50 nm [52]. As noted above, high dielectric breakdown
strength is necessary for insulators because if the phosphor becomes a virtual short after