Environmental Engineering Reference
In-Depth Information
metal contact, making the CG layer negative and the metal contact
positive in polarity. Both models produce similar polarity, making
it unnecessary to think differently from the already assumed p-n
junctionmodel.SimilarI-Vcharacteristicswithrectifyingproperties
and PV activities can be produced by both device structures. The
only difference will be the higher performance from the second
devicestructureduetothecombinationoftwosupportingrectifying
junctions in one device and the additional contributions from the
impurity PV effect.
4.5.2
Capacitance-Voltage (C-V) Characteristics
According to both models, C-V measurements are possible due to
the presence of at least one active (rectifying) interface within each
device structure and, hence, a depletion region. If the device is fully
depleted due to low doping concentrations of CdTe, in the region of
10
15
cm
−
3
or below, capacitance values remain almost unchanged
with the applied bias voltage and approximately equal to the
geometric capacitance of the structure. If the doping concentration
is such that the depletion region covers only a part of the CdTe layer
thickness, then the capacitance varies according to the applied bias
voltage providing linear 1/C
2
versus bias voltage (Schottky-Mott)
plotsandtheestimateddopingconcentrationliesintherange10
16
-
10
17
cm
−
3
.TherearenumerousexamplesofsuchC-Vmeasurements
intheliterature,andthesedemonstratethisbehaviourgivingdoping
concentrations in the range 10
15
-10
17
cm
−
3
.Theworksreported
in publications by Raychaudhuri in 1987 [22], Chu
et al.
in 1988
[12], Morris
et al.
in 1991 [23], and Das
et al.
in 1993 [24, 25] are
good examples of devices with e
ciencies close to 10%. Similar C-V
observations could arise from both types of devices, making it more
di
cult to distinguish between the two models applicable to this
system.
4.5.3
Electron Beam-Induced Current Measurements
There are various reports in the literature giving electron beam-
induced current (EBIC) results for such devices with contradicting
conclusions (Mitchell
et al.
, 1977 [26], Nakayama
et al.
, 1980 [27],
