2.7.6 Capacitance versus Voltage
A capacitance versus voltage (C-V) plot allows the measurement of the dynamic
capacitance of an ACTFEL device against the voltage across the terminal during the
rising edge (segments AC and FH) of the voltage pulse. C-V plots are derived from the
data obtained during measurements for Q-V; the slope of the Q-V plot during the rising
edges of both pulses is plotted. The capacitance is calculated using
C(v) =i(t)
dv(t)
dt
where i(t) is the current derived using the inverse of the previous equation for dqext(t),
dqex, (t)
i(t) =
dt
These reduce to the following formula,
dqex, (t)
C(v) =
dv(t)
An example of a C-V plot is shown in figure 2-13. The total physical capacitance,
Ctcv, is the capacitance of the whole device below the turn-on voltage and is usually in
good agreement with the capacitance calculated from dielectric constant and film
thickness measurements, Ctphy. Ciphys, the capacitance above turn on is simply that of the
insulators, since the phosphor is shorted (broken down) at higher voltages. Cicv, is the
measurement of the capacitance above turn-on and, in the ideal case, should equal the
calculated insulator capacitance. Usually, however, the value of Cicv is either above or
below the value of the ideal case. Cicv less than the calculated value occurs when the
phosphor does not completely short, resulting in some remnant capacitance. Cicv can be
larger than Ciphys if there is dynamic space charge built up in the phosphor that decreases