Image Processing Reference
In-Depth Information
ermally excited charge
(dark current)
Conduction
band
SiO
2
-Si interface
Interface state
SiO
2
Valence
band
n
p
Depletion layer
n
-type
p
-type
Depletion layer
(a)
(b)
FIGURE 5.24
pn
-Junction PD: (a) cross-sectional view of structure; (b) energy diagram along interface including depletion layer.
are generated even under dark conditions, especially in the depletion layer, as mentioned
below. Therefore, they are called dark current.
Expressing
n
and
p
as electron and hole density, respectively, there is a rule in semicon-
ductors that the product
np
is a constant value decided by the absolute temperature under
equilibrium conditions as a result of the balance between generation and recombination.
Since this does not depend on
n
/
p
-type, it is the same with the square of intrinsic car-
rier density
n
i
. The value of
n
i
is 1.5 × 10
10
/c m
3
in the case of silicon at room temperature.
Therefore, phenomena tend toward the thermal equilibrium in which the product
np
is
2.3 × 10
20
/c m
3
. The impurity concentration of a nondepleted region is at the level of 10
14
-
10
17
/c m
3
, and carrier density is at the same degree. But in a depleted region, the densities of
electrons in the conduction band and holes in the valence band are very low. The product
np
is near zero. Because this situation is the farthest one from equilibrium, the speed of
change toward equilibrium is very high. Therefore, charge generation rate is very high. In
nondepleted regions under nonequilibrium, charges are also excited toward equilibrium
by using the interface state as stepping stones. But the rate is not so high because there are
charges with the density of impurity concentration mentioned above. On the other hand,
the dark current by way of interface state in depleted regions is predominant. Therefore,
in the case of a PD fully depleted to the whole surface, many charges are thermally gener-
ated, that is, there is a very high level of dark current.
From the above discussion, it can be said that it is effective for dark current suppres-
sion to bring about near-equilibrium conditions at the surface to decrease the density of
additional electrons by increasing the carrier density around the interface. Theoretically,
either electrons or holes are possible. But it is necessary to increase hole density rather than
electron density to be consistent with realization of the depleted PD and completely cover
the depletion layer at the interface.
A high-concentration
p
-type layer in the range of 10
17
−10
19
/c m
3
is introduced at the sur-
face of the PD in buried/pinned PDs, as shown in Figure 5.25. Therefore, almost the same
density of holes exists in this space. Hence, the density of electrons that can exist around
the interface is reduced to the level of 10−10
3
/c m
3
. Thus, the thermal excitation probability
by way of interface state is very low, and generation of dark current is drastically sup-
pressed. It is desirable that the impurity concentration of the
p
+
layer is high from the view-
point of dark current suppression. And the thickness should be thin to avoid sensitivity
loss caused by recombination of signal electrons with high density holes. To compare dark
