Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2000-12-05
2003-07-15
Munson, Gene M. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S458000, C257S464000, C257S466000
Reexamination Certificate
active
06593636
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to the field of photodiodes. More particularly, it is related to silicon photodiodes useable in high-speed telecommunications applications.
BACKGROUND OF THE INVENTION
Silicon photodiodes are semiconductor devices responsive to high-energy particles and photons. Photodiodes operate by absorption of photons or charged particles and generate a flow of current in an external circuit, proportional to the incident power. Photodiodes can be used to detect the presence or absence of minute quantities of light. Planar diffused silicon photodiodes (also known as PIN diodes) are simply P-N junction diodes. A P-N junction can be formed by diffusing either a P-type impurity (anode), such as Boron, into a N-type bulk silicon wafer, or a N-type impurity (cathode), such as phosphorous, into a P-type bulk silicon wafer. The diffused area defines the photodiode active area—that region of the photodiode sensitive to incident radiation.
To form an ohmic contact another impurity diffusion into the backside of the silicon wafer is necessary. The impurity is an N-type for a diode with a P-type active area and the impurity is P-type for a diode with an N-type active area. The contact pads are typically deposited on the front active area in defined areas, and on the backside, completely covering the device. The active area is covered with an anti-reflection coating to reduce the reflection of the light for a specified predefined wavelength. The non-active area on the top is covered with a thick layer of silicon oxide (SiO
2
). By controlling the thickness of the bulk substrate, the speed and response of the photo diode can be controlled. Photo diodes, when biased, must be operated in the reverse bias mode, i.e., a relatively negative voltage applied to the anode and a relatively positive voltage applied to the cathode.
FIG. 1
illustrates a conventional silicon photodiode
10
built on an N-type substrate
12
. The front side
14
of the diode
10
includes an active photo receptor area
16
coated with an anti-reflection coating
18
and a non-active area
20
coated with a layer of SiO
2
22
. A P+ diffusion region
24
is formed in the substrate in the area of the active photoreceptor area. Between the P+ diffusion region
24
and the substrate
12
exists a depletion region
26
. A metallization layer
28
is formed in contact with an N+ diffusion region
30
on the backside
32
of the silicon wafer
34
. A first contact
35
forms an electrical contact with P+ diffusion region
24
. Metal layer
28
forms a second contact.
FIG. 2
illustrates a conventional photodiode
36
built on a P-type substrate
38
. The structure is similar to that of the diode of
FIG. 1
except that current flow is reversed.
Silicon is a semiconductor with a band gap energy of 1.12 eV at room temperature. This is the gap between the valence band and the conduction band. At absolute zero temperature the valence band is completely filled and the conduction band is vacant. As the temperature increases, the electrons become excited and escalate from the valence band to the conduction band by thermal energy. The electrons can also be escalated to the conduction band by particles or photons with energies greater than 1.12 eV, which corresponds to wavelengths shorter than 1100 nm. The resulting electrons in the conduction band are free to conduct current.
Due to the concentration gradient, the diffusion of electrons from the N-type region to the P-type region and the diffusion of holes from the P-type region to the N-type region, develops a built-in voltage across the junction. The inter-diffusion of electrons and holes between the N and P regions across the junction results in a region with no free carriers. This is the depletion region. The built-in voltage across the depletion region results in an electric field with a maximum at the junction and no field outside of the depletion region. Any applied reverse bias adds to the built-in voltage and results in a wider depletion region. The electron-hole pairs generated by light are swept away by the electric field of the depleted region. The current generated is proportional to the incident light or radiation power. The light is absorbed exponentially down with distance and is proportional to the absorption coefficient. The absorption coefficient is very high for shorter wavelengths in the UV region and is small for longer wavelengths (FIG.
3
). Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while silicon becomes transparent to light wavelengths longer than 1200 nm. Moreover, photons with energies smaller than the band gap are not absorbed at all.
A silicon photodiode can be represented by a current source in parallel with an ideal diode (FIG.
4
). The current source represents the current generated by the incident radiation, and the diode represents the p-n junction. In addition, a junction capacitance (C
j
) and a shunt resistance (R
SH
) are in parallel with the other components. Series resistance (R
S
) is connected in series with all components in this model.
Shunt Resistance, R
SH
Shunt resistance is the slope of the current-voltage curve of the photodiode at the origin, i.e. V=0. Although an ideal photodiode should have a shunt resistance of infinite, actual values range from 10 s to 1000 s of Mega ohms. Experimentally it is obtained by applying ±10 mV, measuring the current and calculating the resistance. Shunt resistance is used to determine the noise current in the photodiode with no bias (photovoltaic mode). For best photodiode performance the highest shunt resistance is desired.
Series Resistance, R
S
Series resistance of a photodiode arises from the resistance of the contacts and the resistance of the undepleted silicon (FIG.
1
). It is given by:
R
s
=
(
W
s
-
W
d
)
ρ
A
+
R
c
(
EQ
.
1
)
where W
s
is the thickness of the substrate, W
d
is the width of the depleted region, A is the diffused area of the junction, &rgr; is the resistivity of the substrate and R
c
is the contact resistance. Series resistance is used to determine the linearity of the photodiode in photovoltaic mode (no bias, V=0). Although an ideal photodiode should have no series resistance, typical values ranging from 10 to 1000 ohms are measured.
Junction Capacitance, C
j
The boundaries of the depletion region act as the plates of a parallel plate capacitor (FIG.
1
). The junction capacitance is directly proportional to the diffused area and inversely proportional to the width of the depletion region. In addition, higher resistivity substrates have lower junction capacitance. Furthermore, the capacitance is dependent on the reverse bias as follows:
C
j
=
ϵ
Si
ϵ
o
A
2
ϵ
Si
ϵ
o
μ
ρ
(
V
A
+
V
bi
)
(
EQ
.
2
)
where ∈
o
=8.854×10
14
F/cm, is the permitivity of free space, ∈
Si
=11.9 is the silicon dielectric constant, &mgr;=1400 cm
2
/Vs is the mobility of the electrons at 300·K, &rgr; is the resistivity of the silicon, V
bi
is the built-in voltage of silicon and V
A
is the applied bias.
FIG. 5
shows the dependence of the capacitance on the applied reverse bias voltage. Junction capacitance is used to determine the speed of the response of the photodiode.
Rise/Fall Time and Frequency Response, t/t/f
3dB
The rise time and fall time of a photodiode is defined as the time for the signal to rise or fall from 10% to 90% or 90% to 10% of the final value respectively. This parameter can be also expressed as frequency response, which is the frequency at which the photodiode output decreases by 3 dB. It is roughly approximated by:
t
r
=
0.35
f
3
dB
(
EQ
.
3
)
There are three factors defining the response time of a photodiode:
1. t
DRIFT
, the charge collection time of the carriers in the depleted region of the photodiode.
2. t
DIFFUSED
, the charge collection time of the carri
Bui Peter Steven
Taneja Narayan Dass
Munson Gene M.
Ritchie David B.
Thelen Reid & Priest LLP
UDT Sensors, Inc.
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