(C, + C )
q(t) = q,Q) Cp [v2 (t)-3(t)]
C,
where q(t) is the internal charge, Ci and Cp are the insulator and phosphor capacitances,
respectively, qext is the external charge, and v2 and v3 are the voltages measured on each
side of the device. The phosphor field is obtained using
1 qqt (t) ) v3(t)]}
f p = d C,- [v2 3(
where dp is the phosphor thickness. These equations basically use the raw data from a Q-
V curve, remove the capacitive displacement charge, and remove the voltage drop across
the insulators to calculate the field across the phosphor. The equations are developed
from the equations used to describe an ACTFELD with a phosphor layer free from space
charge [89]. A typical graph of Qint-Fp is shown in figure 2-14. Unlike a Q-V plot the
Qint-Fp loop goes in a clockwise direction. A Qint-Fp plot shows several of the same
quantities as in a Q-V plot, but these are only charges and fields in the phosphor. The
charge information shown is Qcond, the conduction charge transported across the
phosphor, Qpo,, the polarization charge stored at the phosphor/insulator interface, Qleak,
the leakage charge between the voltage pulses, Qrelax, the relaxation charge flowing
during the voltage pulse plateau, and Qmax, the maximum charge across the phosphor
(Figure 2-14). The other information available is the steady state field, Fss. Field
clamping, when the charge flow through the device is sufficient to counteract the
increasing field generated by increasing the applied voltage, can be determined by
comparing Fss at different voltages above the threshold voltage. If there is field
clamping, then Fss will be independent of voltage above threshold. Some devices