for them are rather stringent. However, the most important requirements for them are:

1. High dielectric constant

 Below threshold, the applied voltage is capacitively divided by the phosphor and

the insulators as following


 V = 1 V

 CP
 CP
 V = P V
 SC + CP

where Va is the applied voltage and Vi and Vp are the voltages across the insulators and

the phosphor layer, respectively. CI and Cp are the insulator and phosphor capacitance,

respectively.

 To maximize the portion of the applied voltage dropped across the phosphor layer,

the capacitance of the insulators should be maximized while that of the phosphor layer

should be minimized. To optimize the insulator capacitance, either the dielectric constant

should be maximized or the layer thickness should be minimized. Since the layer

thickness can't be set too low due to processing and breakdown issues, a high dielectric

constant is desirable.

2. High electric field strength

 Because of the clamp effect, excessive voltage above the threshold voltage will

drop across the insulator layer. So, high electric field strength of the material is required

to avoid dielectric breakdown.

 However, it should be noted that as the dielectric constant increases, the electric

field strength decreases because these properties are usually inversely proportional to

each other. A figure of merit for insulator materials was introduced by Howard [35]. The