Environmental Engineering Reference
In-Depth Information
e
=
Electronic charge
φ
b
=
Potential barrier height
k
=
Boltzmann constant
n
=
Ideality factor of the diode
These types of potential barriers are formed when the semiconduc-
tor has moderate doping concentration of
10
15
-10
17
cm
−
3
. When
∼
the
0.40 eV — or the depletion region is narrow
— the electrical conduction through the interface will exhibit linear
(ohmic) behaviour due to the ease of electron transfer in both
directions. Heavy doping, in excess of 10
18
cm
−
3
, will form a thin
enough depletion layer, enabling charge carriers to tunnel through
the interface showing ohmic behaviour. Both rectifying and ohmic
electrical contacts can be incorporated in PV solar cells in order to
maximisethe internal electric field present in these devices.
φ
b
is less than
∼
1.4.6
Metal-Insulator-Semiconductor Interfaces
Ingeneral,the
φ
b
valuesobtainedatMSinterfacesaremuchsmaller
than those of p-n junctions for a given semiconductor. As indicated
in Fig. 1.8, the Schottky barrier represents approximately half of a
p-n junction and, hence, a lower
φ
b
.
Thisdisadvantagecanberemovedbyincorporatinganinsulating
layer, as shown in Fig. 1.9. The suitably thin insulating layer
decouplesthemetalfromthesemiconductorandincreasestheband
bending or the potential barrier at the interface. The rectification
property of this structure improves, and the interface behaviour is
described by Eq. 1.3 [15]:
δ
)exp
−
φ
b
kT
exp
eV
nkT
I
D
=
SA
∗
T
2
exp(
−
χ
1
/
2
(1.3)
χ
where
, in electron volts, is the mean barrier height presented by
the interfacial layer of thickness,
, in Angstroms (A).
As described in section 1.5.3, the open circuit voltage (
V
oc
)ofa
solarcell isafunctionofthe
φ
b
and, therefore, this isamethodused
to enhance the
V
oc
of PV solar cells based on Schottky diodes. The
insulating layer brings an additional advantage of minimising the
interactions between the metal and the semiconductor. In the case
δ
