The photodiode is a device that operates in reverse bias, as shown in Figure 3–43(a), where Il is the reverse light current. The photodiode has a small transparent window that allows light to strike the pn junction. Some typical photodiodes are shown in Figure 3–43(b). An alternate photodiode symbol is shown in Figure 3–43(c).
OLED technology was developed by Eastman Kodak. It is beginning to replace LCD (liquid crystal display) technology in handheld devices such as PDAs and cellular phones. OLEDs are brighter, thinner, faster, and lighter than conventional LEDs or LCDs.
They also use less power and are cheaper to manufacture.
F Y I
Recall that when reverse-biased, a rectifier diode has a very small reverse leakage cur- rent. The same is true for a photodiode. The reverse-biased current is produced by thermally generated electron-hole pairs in the depletion region, which are swept across the pn junction by the electric field created by the reverse voltage. In a rectifier diode, the reverse leakage current increases with temperature due to an increase in the number of electron-hole pairs.
A photodiode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the light intensity. When there is no incident light, the reverse current, Il, is almost negligible and is called the dark current. An increase in the amount of light intensity, expressed as irradiance (mW/cm2), produces an increase in the reverse current, as shown by the graph in Figure 3–44(a).
From the graph in Figure 3–44(b), you can see that the reverse current for this particular device is approximately 1.4 mA at a reverse-bias voltage of 10 V with an irradiance of 0.5 mW/cm2. Therefore, the resistance of the device is
RR=VR
Il = 10 V
1.4 mA =7.14 MV
At 20 mW/cm2, the current is approximately 55 mA at VR=10 V. The resistance under this condition is
RR= VR
Il = 10 V
55 mA =182 kV
These calculations show that the photodiode can be used as a variable-resistance device controlled by light intensity.
I
(a) Reverse-bias operation using standard symbol
VR
(b) Typical devices (c) Alternate symbol
l
▲ FIGURE 3–43 Photodiode.
100
1 2 5 10 20 50
0 10 20 30 40 50 60 70 80 90 100
10 5
2 1 0.5
VR, reverse voltage (V) Irradiance, E
Reverse current, (Il)
0
(a) General graph of reverse current versus irradiance
(b) Example of a graph of reverse current versus reverse voltage for several values of irradiance
E = 20 mW/cm2
Il, reverse current (A)m
▲ FIGURE 3–44
Typical photodiode characteristics.
Figure 3–45 illustrates that the photodiode allows essentially no reverse current (except for a very small dark current) when there is no incident light. When a light beam strikes the photodiode, it conducts an amount of reverse current that is proportional to the light intensity (irradiance).
Light OFF
Il
VBIAS
+ –
Light ON
(a) No light, no current except negligible dark current
(b) Where there is incident light, resistance decreases and there is reverse current.
+ –
Il
VBIAS
+ –
+ –
▲ FIGURE 3–45
Operation of a photodiode showing current measurements.
For a TEMD1000 photodiode,
(a) Determine the maximum dark current for VR=10 V.
(b) Determine the reverse light current for an irradiance of 1 mW/cm2 at a wavelength of 850 nm if the device angle is oriented at 108 with respect to the maximum irra- diance and the reverse voltage is 5 V.
Solution (a) From Figure 3–46(a), the maximum dark current Iro =10 nA.
(b) From the graph in Figure 3–46(d), the reverse light current is 12 m A at 950 nm.
From Figure 3–46(b), the relative sensitivity is 0.6 at 850 nm. Therefore, the reverse light current is
Il= Ira=0.6(12 mA) =72 mA
For an angle of 108, the relative sensitivity is reduced to 0.92 of its value at 08.
Il=Ira=0.92(7.2 mA)=6.62 MA
Related Problem What is the reverse current if the wavelength is 1050 nm and the angle is 08?
EXAMPLE 3–13
Photodiode Datasheet Information
A partial datasheet for an TEMD1000 photodiode is shown in Figure 3–46. Notice that the maximum reverse voltage is 60 V and the dark current (reverse current with no light) is typically 1 nA for a reverse voltage of 10 V. The dark current increases with an increase in reverse voltage and also with an increase in temperature.
Sensitivity From the graph in part (b), you can see that the maximum sensitivity for this device occurs at a wavelength of 950 nm. The angular response graph in part (c) shows an area of response measured as relative sensitivity. At 108 on either side of the maximum orientation, the sensitivity drops to approximately 82% of maximum.
Absolute Maximum Ratings
Tamb = 258C, unless otherwise specified
Basic Characteristics
Tamb = 258C, un less otherwise specified Tamb = 258C, un less otherwise specified
Parameter Test condition Symbol Value Unit
Reverse Voltage VR 60 V
Power Dissipation Tamb # 258C PV 75 mW
Junction Temperature Tj 100 8C
Storage Temperature Range Tstg - 40 to + 100 8C
Operating Temperature Range Tstg - 40 to + 85 8C
Soldering Temperature t # 5 s Tsd < 260 8C
Parameter Test condition Symbol Min Ty p. Max Unit
Forward Voltage IF = 50 mA VF 1.0 1.3 V
Breakdown Voltage IR = 100 mA, E = 0 V(BR) 60 V
Reverse Dark Current VR = 10 V, E = 0 Iro 1 10 nA
Diode capacitance VR = 5 V, f = 1 MHz, E = 0 CD 1.8 pF
Reverse Light Current Ee = 1 mW/cm2, l = 870 nm, VR = 5 V
Ira 10 mA
Ee = 1 mW/cm2, l = 950 nm, VR = 5 V
Ira 5 12 mA
Temp. Coefficient of Ira VR = 5 V, = 870 nm TKIra 0.2 %/K
Absolute Spectral Sensitivity VR = 5 V, = 870 nm s( l ) 0.60 A/W
VR = 5 V, = 950 nm
f
s(l ) 0.55 A/W
Angle of Half Sensitivity ? 615 deg
Wavelength of Peak Sensitivity l p 900 nm
Range of Spectral Bandwidth l0.5 840 to 1050 nm
ns
ns Rise Time VR = 10 V, RL = 50,
l = 820 nm
tr 4
Fall Time VR = 10 V, RL = 50,V V
l = 820 nm
tf 4
Parameter Test condition Symbol Min Ty p. Max Unit
0.1 1.0 10 100
0.01 0.1 1
I-ReverseLightCurrent(A)ra
Ee- Irradiance ( mW/cm2) 10 (d)
m
V = 5 V l= 950 nmCE
750 850 950 1050
0 0.2 0.4 0.6 0.8 1.2
S()-RelativeSpectralSensitivityrel
l- Wavelength ( nm ) 1150 (b)
(a)
1.0
0.4 0.2 0 0.2 0.4 S-RelativeSensitivityrel
(c) 0.6 0.6 0.9 0.8
08
308 108 208
408
508 608 708 808 0.7
1.0
l
▲ FIGURE 3–46
Partial datasheet for the TEMD1000 photodiode. Datasheet courtesy of Vishay Intertechnology, Inc.
1. Name two types of LEDs in terms of their light-emission spectrum.
2. Which has the greater wavelength, visible light or infrared?
3. In what bias condition is an LED normally operated?
4. What happens to the light emission of an LED as the forward current increases?
5. The forward voltage drop of an LED is 0.7 V. (true or false) 6. What is a pixel?
SECTION 3–4 CHECKUP
The Solar Cell Structure Although there are other types of solar cells and continuing research promises new developments in the future, the crystalline silicon solar cell is by far the most widely used. A silicon solar cell consists of a thin layer or wafer of silicon that has been doped to create a pn junction. The depth and distribution of impurity atoms can be controlled very precisely during the doping process.
Thin circular-shaped wafers are sliced from an ingot of ultra-pure silicon and then are polished and trimmed to an octagonal, hexagonal, or rectangular shape for maximum cov- erage when fitted into an array. The silicon wafer is doped so that the n region is much thinner than the p region to permit light penetration, as shown in Figure 3–47.
A gridwork of very thin conductive contact strips are deposited on top of the wafer by methods such as photoresist or silk-screen, as shown in Figure 3–47(b). The contact grid must maximize the surface area of the silicon wafer that be exposed to the sunlight in order to collect as much light energy as possible.
7. In what bias condition is a photodiode normally operated?
8. When the intensity of the incident light (irradiance) on a photodiode increases, what happens to its internal reverse resistance?
9. What is dark current?