Device Elements and Circuits for Image Sensors - Essential Principles of Image Sensors - page 16

formed in the pn -junction. The p -type region is connected to ground level and voltage
source V J is applied to the n -type region. Figure 2.7b shows that the n -type region and
p -type region exist separately in energy space. Ionized donors and electrons as carriers
are equally distributed in the n -type region and ionized acceptors and holes are equally
distributed in the p -type region. All areas are electrically neutral. As the Fermi level is
situated in the vicinity of the maximum energy of the electron distribution, it is near the
lower level of the conductive band in the n -type region and near the upper level of the
valence band in the p -type region. Then, if the n -type region and p -type region are con-
nected, electrons in the vicinity of the junction in the n -type region and holes in the vicin-
ity of the junction in the p -type region diffuse toward the opposite areas, according to the
diffusion voltage caused by the concentration difference between both sides. Diffusion
of electrons and holes continues till the electric field formed by the remaining ionized
impurities increases to balance out the diffusion voltage. Consequently, as Figure 2.7c
shows by the potential distribution, ionized donors remain in the n -type region near the
junction and ionized acceptors remain in the p -type region, that is, only spatially fixed
charges remain, and no carrier exists continuously in this area. Therefore, this area is
called the depletion layer.
The appearance of diffusion stops and an equilibrium is reached. Figure 2.7d shows
this situation by the spatial distribution, while Figure 2.7c shows it in energy space. In
the depletion layer, the total number of positive charges in the n -type region and nega-
tive charges in the p -type region are equal. Outside the depletion layer in both the n -type
region and p -type region, mobile and fixed charges exist equally so that they are electri-
cally neutral. The difference in potential between the n -type region and p -type layer is
called the built-in potential and is decided by the impurity concentrations of both sides
and temperature. It is about 0.6 V in silicon at room temperature.
Bias condition is discussed next. While the p -type region in the pn -junction is grounded,
positive voltage V J is applied to the n -type region, as shown in Figure 2.8a as potential and
charge distribution and in Figure 2.8b as distribution in space.
In the n -type area, electrons drain and decrease until the voltage increases to V J . It is
also believed that the decrease of negatively charged electrons creates a positive voltage
in the n -type region. As a result, the width of the depletion layer increases on both sides
of the junction, and no current flows through the pn -junction. When positive voltage is
applied to the n -type region, the state of the pn -junction is called reverse-biased condi-
tion. As will be mentioned later, as photodiodes in image sensors, pn -junctions are used
by being made electrically separate and floating in a reverse-biased condition, to inte-
grate signal charges generated by incident light. At the instant of electrical separation,
it is in nonequilibrium, and it makes use of a phenomenon returning to an equilibrium
situation.
In contrast to the above case, when negative voltage is applied to the n -type region, V J
as applied voltage decreases from 0 V and when the absolute value becomes the same
as the built-in potential, that is, V J = −|ϕ B |, the band energies of the n -type and p -type
regions become equal. At the point when V J becomes large in the negative direction, elec-
tron energy in the n -type region becomes higher than in the p -type region, as Figure 2.8c
shows; then the flow of electrons starts from the n -type region to p -type region. At the
same time, holes are now in the same situation and flow from the p -region to n -region.
Thus, electric current flows in this bias condition. This bias state is called forward bias.
Although electric current flows through the pn -junction under forward bias, it does not
occur under reverse bias, that is, pn -junctions have a rectifying action. As discussed, the
names of reverse and forward biases come from this function.