Optoelectronic Semiconductor Devices Principals and Characteristics

7.1.1 
pn-JUNCTION UNDER REVERSE BIAS AND REVERSE CURRENT
 
The diffusion current appears ideally under forward and reverse bias, and the diffusion current under 
reverse bias is called the saturation current. There are several kinds of current going through the pn-
junction in the forward and reverse directions. The main components of the additional currents are: 
surface leakage current, I
ds
, generation-recombination current, I
dg-r
, and tunneling current, I
dt
. The reverse 
current is not constant at the saturation current but gradually increases because of those additional 
currents. The total, called dark current, is given approximately by formula (47) 
(47) 
d
dd
ds
dg r
I
I
I
I
I
−
=
+
+
+
dt
where I
dd
is the diffusion current in the reverse direction and ideally corresponds to the saturation current. 

---

Important of these currents that they result in the dark current and noise, which strongly influence device 
reliability. 
It is important to remember that the dark current increases with increase of the temperature. The quantum-
mechanical tunneling is itself independent of the temperature, but increases as a result of the temperature 
dependence of the band-gap energy. The tunneling current is given by formula (48): 
(
)
(
)
1 2
1 2
3
2
2
1 2
2 3
1 2
2
exp
dt
pn
max
b
e
g
e
g
max
I
A
q F
V
h
m E
m
E
hqF
π
π


=
×
−


2
(48) 
where
F
max
is the maximum electric field in the pn-junction, 
m
e
is the effective electron mass, and
h is Plank's constant. 
In the photodiode mode the generation-recombination current and diffusion current are dominant and 
mainly determine the temperature dependence of the dark current. 
It is important to remember that the dark current has an influence on the noise behavior of the photodiode, 
increasing it at the higher temperature.