Lab VIII

Photodetectors
ECE 476

I. Purpose

The electrical and optical properties of various photodetectors will be investigated.

II. Background

Photodiode
A photodiode is a standard diode packaged so that light can shine on the p-n junction.
The standard diode equation is:

⎡ ⎛ qV ⎞ ⎤
I = Io ⎢exp⎜ ⎟ −1⎥ (1)
⎣ ⎝ kT ⎠ ⎦

where I0, is the saturation current and V is the applied voltage (negative for reverse bias
and positive for forward bias). The current-voltage characteristic for the diode is shown
in figure 1. The current that flows when no light is shining is called the dark current.

Figure 1: I-V curve of diode with no light.

The presence of light being absorbed by the p-n junction produces the current-voltage
characteristic shown in figure 2. The equation describing the current-voltage
characteristics includes both a dark current term and a light generated term.

Figure 2: Photodiode characteristic.

This equation is
⎡ ⎛ qV ⎞ ⎤
I = Io ⎢exp⎜ ⎟ −1⎥ − Iλ (2)
⎣ ⎝ kT ⎠ ⎦
where Iλ is the current due to the light. This light current is constant versus voltage, so the
light produced curve in Figure 2 is simply the dark current curve shifted straight down.
The light current Iλ is linear with respect to the light intensity absorbed.

Three regions of operation can be identified in Fig. 2. Region one is the photovoltaic
mode of photodetector operation where the diode is operated as an open circuit (I = 0).
The voltage measured is related to the intensity of the light. To determine this
relationship the current is set to zero in Eq. 2 and the voltage is solved for giving

kT ⎛ Iλ ⎞
V= ln⎜ + 1⎟ (3)
q ⎝ Io ⎠
where V=VOC is called the open circuit voltage. Region two is the photoconductive mode
(Quadrant III) where the diode is reversed biased. In this mode typically the photocurrent
Iλ is larger in magnitude than the dark current I0 so the current measured is directly
proportional to the light intensity. Region three is the solar cell mode (Quadrant IV in
figure 2) where the diode generates a net power out.

Photoconductor
A photoconductor is simply a piece of semiconductor as shown in Fig. 3.

Figure 3: Photoconductor.

The resistance of the semiconductor is

ρd
R= (4)
A
where ρ is the resistivity, d is the length and A is the cross-sectional area. The resistivity
for a semiconductor is given by

1
= σ = q(μn n + μ p p) (5)
ρ
where σ is called the conductivity. The other terms include the mobilities of the electron
and the hole carriers (μn and μp), the electron density (n) and the hole density (p). The
quantities n and p increase linearly with increases in the light absorbed. Hence the
photoconductor resistance is expected to be related to light intensity as
R ~ l/(light intensity).

III. Procedure

Preliminary
1. Measure the light output power for the high-efficiency red LED for the following
currents: 2ma, 5mA, 10mA, 15mA, 20mA, and 25mA. If you have accurate
measurements from the LED lab, you may reuse them. You will be using these powers
for parts A-D.

2. The photodetectors used in this lab are in a small parts cabinet on the counter.

Part A: Photodiode in the photovoltaic mode

Experiment
1. Measure the external voltage versus input light intensity for the photodiode operating
in the photovoltaic mode where the diode is open circuited. Use the high-output red LED
as the light source. Vary the light source intensity by taking measurements at each current
reading of the LED (2mA, 5mA,…, 25mA). Plot the photodiode voltage measured versus
the input light intensity.
Notes:
• Use the planar photodiode. It has a flat surface.
• Make sure that the LED is directly pointing at the photodiode.
• Keep the LED and the photodiode in fixed positions, free of using your hands.
Tilt the LED board on its side and bend the photodiode leads to have it face the
LED. Slight movement in distance or angle can greatly affect the readings and
the precision of your results.
• Before taking measurements, line up the diode such that you get the maximum
possible reading you can when 25mA is flowing through the HE-red LED. Doing
this will help you achieve the expected results.

2. Measure the I0, of your photodiode. Recall I0, is the current through the diode when a
reverse bias is applied and no light is shining on the diode. Set up a circuit with the diode
and a large resistance (1 MΩ) in series. Apply a reverse bias and measure the voltage
across the resistor with the diode in complete darkness. Calculate the current flow I0. Use
a reverse bias of 5 volts. Note: The measured voltage across the 1 MΩ resistor should be
very small when the circuit is in darkness (<<1V). If you are obtaining voltages > 1V,
you probably connected the diode in the wrong direction.

3. On the plot done in part 1 above, show a curve for equation 3. For equation 3 select a
value for the saturation current I0, as measured in part 2. Assume Iλ = C(Popt) where C is a
constant and Popt is the light source intensity shining on the diode as determined in part 1
above. Fit one experimental data point to equation 3 to determine the constant C. Then
plot Equation 3 using this C on the same curve as the experimental data plotted in part 1
above. Note: Assume T=300 K.
Part B: Photodiode in the photoconductive mode

Experiment
1. Connect the photodiode in series with a resistor (10 kΩ). Reverse bias the circuit and
measure the current flow versus LED light intensity. Plot the photocurrent versus light
intensity. Is the relation linear as expected from equation 2? Note: Keep the LED and the
photodiode in fixed positions.

Part C: Phototransistor

Set-up
1. A phototransistor is an npn bipolar transistor in which connections are provided to the
emitter and the collector. No connection is provided to the base. The light being absorbed
serves to generate carriers (holes and electrons) in the base region which produces base
current. This base current generated by the light then controls the flow of a larger current
between the emitter and collector terminals. The phototransistor then operates with a gain
so that a small amount of light generates a large current.

2. Connect the circuit shown in figure 4.

Figure 4: Phototransistor test schematic.

Experiment
1. Measure the current through the transistor versus input light power. Use the LED as
the light source. Plot the current measured versus light intensity. Do you get an expected
result?

Notes:
• The voltage across the 10 kΩ resistor should be close to 0V in darkness. If it is
not, check your circuit setup.
 • Keep the LED and the phototransistor in fixed positions, free of using your hands.
Tilt the LED board on its side and bend the phototransistor leads to have it face
the LED.

Part D: Photoconductor

Experiment
1. Measure the resistance of the photoconductor versus light input intensity. Use the LED
as your light source. Plot the resistance versus light intensity. Do you get the expected
result?
Notes:
• Keep the LED and the photoconductor in fixed positions
• If you can, place the face of the photoconductor directly on top of the LED

Part E: Solar Cell

Set-up
1. Using the solar cell provided, you will measure the I-V characteristics at two light
illumination levels. You will use the desk lamp at each lab station as your light source.

2. Connect the terminals of the decade resistor box to the terminals of the solar cell.

3. Connect a multimeter (in voltage read mode) across the terminals of the solar cell.

4. Place solar cell directly under the lamp. Try to angle the direction of the lamp to
directly face the angle of the solar cell to get the best readings. Be sure that the solar cell
is not resting on a metal surface, otherwise the terminals will short and you will get no
readings. Make sure that your setup is stationary and be careful not to move the solar cell
during the experiment. If you change the position of the solar cell, your readings will
change greatly and you will have to repeat this experiment.

Experiment
1. Make sure the decade resistor box is at 0Ω. You should read 0V across the solar cell.
Using the knobs of the decade resistor box, adjust the voltage to the next increment of
0.2V. Record the resistance as determined by the knob positions. Do this for 0.2V to
2.0V (10 measurements)

2. Place the neutral density filter on top of the solar cell and repeat step 1.

3. Plot I versus V for the solar cell. Also calculate and plot power output versus load
resistance. What is the maximum power in watts that the solar cell generates at each
illumination level?

IV. Conclusion

Draw conclusions on the various types of photodetectors and solar cells.