22.3 Photodiodes
605
Fig. 22.20 Band diagram
of a MSM structure with an
n-type semiconductor in
thermal equilibrium. In the
general case, two different
metals cause two different
Schottky barrier heights
and related depletion layer
widths. Adapted
from [1840]
Fig. 22.21 a Band
diagram for a MSM
structure under bias
(V < V RT ), b electric field
distribution. Adapted
from [1840]
(a)
(b)
In the general case two different metals with two different barriers φ n1 , φ n2 and built-in voltage V D1 ,
V D2 are considered. If a voltage is applied across the MSM diode, one of the junctions is biased in
the forward, the other in the reverse direction. We assume in Fig. 22.21 that the voltage biases the first
contact in the reverse direction, i.e. the ‘+’ pole is on the left contact. The applied voltage V is split
between the two contacts, the larger voltage will drop at the reverse-biased contact (here: V 1 > V 2 )
V = V 1 + V 2 .
(22.36)
The electron current arises from thermionic emission at contact 2. Due to current continuity (without
recombination since we inject majority charge carriers) this is also the current through contact 1, i.e.
j n1 = j n2 .
(22.37)