Drexel University
ECE-E302, Electronic Devices
Lab V Semiconductor Optics
SEMICONDUCTOR OPTICS
Objective
To determine the speed and sensitivity of a CdS photoconductor and a Si phototransistor.
Theory
1. A semiconductor material can be made to change its conductivity with an optical stimulus
present. When a light source of proper wavelength is incident on the semiconductor, excess free
carriers are generated as follows:
δn = δn Gop
and δp = δp Gop
δn - excess free electrons
δn - average life time of electrons
Gop - optical generation rate
Then, the change in conductivity is given as:
 
 q ( n  n   p  p )
 q G op
 n  n

  p  p , for n or p type
(1)
Characteristic measurements of a photoconductor are its sensitivity and speed.
Sensitivity is a measure of the ratio of its photoconductivity to the intensity of the light
generating the effect. A simple way of evaluating this property is a ratio of the current generated
in a photoconductive circuit (Ipc) to the current used to produce the light (IL) in the light source:
Sensitivity (S) = Ipc / IL
When the light source is turned-off the excess carriers decay according to the relation:
n(t)  (n)0 e
 t / n
(2)
This decay can also be seen in the current flow in the device as:
t / n
I( t )  I o e 
Where I 0
is the current at t  0
(3)
2. If light impinges on a semiconductor P-N junction parallel to junction plane, an additional
1-5
Drexel University
ECE-E302, Electronic Devices
Lab V Semiconductor Optics
current term to the normal diode equation appears as follows:
I  Io
e qV
where
I op
/ kT

 1
qAG

op
I op
L p
(4)
 L n

and Lp and Ln are the diffusion lengths for holes and electrons, respectively.
When the diode is reverse biased, the current is I = -Io - τ Gop, where Io and τ are
constants. The properties of the P-N junction as a photodetector are defined similarly to the
photoconductor.
For a phototransistor, the photocurrent that would be produced by a photodiode is
multiplied by the transistor action by a factor of 10 to 100.
Typical current-voltage, and relative brightness curves for the LED are given below.
Procedure
1. Sensitivity
a) CdS Photoconductor
• Connect the DC power supply to the upper part of the optics board in series with LED and
ampermeter as shown in Fig. 1a. The source current Is is measured on the DC mA meter.
• Connect the CdS photoconductor to the +5 volt supply as shown in Fig. 2a.
• Cover the LED and photoconductor to obscure outside light.
• Vary the current (Is) of the LED from 1 mA to 25 mA in step 5 mA. Read Vc on the digital
voltmeter. Since Vc = R Id, the photocurrent (Id) can be determined. If you get no current,
check to see if diode is in backwards. Note: The peak inverse voltage (PIV) of the LED is
only 3.0 volts and Imax = 40 mA.
• Plot Id vs. Is in your report.
• Change the optics board and repeat the above steps.
b) Si Phototransistor
• Connect same LED setup as above.
• Connect the Si phototransistor to the +5 volt supply as shown in Fig. 2b.
2-5
Drexel University
ECE-E302, Electronic Devices
Lab V Semiconductor Optics
• Cover the LED and phototransistor to obscure outside light.
• Vary the current Is of the LED from 1 to 30 mA in step 5 mA. Record Vc on the digital
voltmeter. Since Vc = R Id, the photocurrent Id can be determined. Note: PIV of the LED is
only 3.0 volts and Imax = 40 mA.
• Plot Id vs. Is in your report.
• Change the optics board and repeat the above steps.
2. Speed
In this section of the experiment we will determine the speed of the two photodectors by
measuring the parameter δ in each case. This is accomplished by switching the LED on and off at
a slow enough rate so that the response of the photodetector decays completely before the light is
turned on again.
The LED is effectively switched by driving it with the transistor circuit shown in Fig. 1b.
The transistor is used so as not to load (distort) the waveform generator. Note: the polarities
shown apply to the NPN transistor only.
• Connect the circuit shown in Fig. 1b. (lower half of optics board)
• Set the DC offset of the waveform generator VBB = 1.5 V and the AC amplitude Vs = 3.0 V
peak to peak (square wave).
• Observe the current through the mA meter. The current should switch from near zero to some
maximum.
a) CdS Photoconductor (Use the board with GREEN LED)
• Connect the CdS photoconductor to a +5 volt source, as shown in Fig. 2a.
• Cover the LED and photoconductor to obscure outside light.
• Connect a DC coupled scope across the 1 kΩ load resistor in order to look at the detector
current as a function of time Id(t).
• Set the frequency of the generator and the sweep of the scope so you can see one transient
response across the screen. i.e. f ≈ 10 Hz, timebase ≈ 10ms/div
• Press Single to freeze display.
3-5
Drexel University
ECE-E302, Electronic Devices
Lab V Semiconductor Optics
• Find the lifetime τ from the display.
• Produce a hard copy of your scope trace. Set the printer switchbox to the name at the top of
your instrument rack, i.e. Station A-2. Press the scope’s “Print/Utility” button, then “Print
Screen”.
b) Si Phototransistor (Use the board with RED LED )
Due to the much smaller sensitivity of Si Phototransistor, this part is not required.
• Connect the Si Phototransistor to a +5 volt source, as shown in Fig. 2b.
• Cover the LED and phototransistor to obscure outside light.
• Connect a scope across the 1 kΩ load resistor in order to look at the detector current as a
function of time Id(t).
• Set the frequency of the generator and the sweep of the scope so you can see one transient
response across the screen. i.e. f ≈ 2 kHz, timebase ≈ 50 µs/div.
• Press DISPLAY and AVERAGE to smooth the response.
• Press Single to freeze display.
• Find the lifetime τ from the display.
• Produce a hard copy of your scope trace.
Report
1. Plot and compare the sensitivity curves of the two photodetectors (separate plots for each) for
each LED. Which photodetector is more sensitive? Explain.
2. Which photodetector is faster? Explain. If you can’t find the speed of Si phototransistor, just
give the lifetime of CdS.
4-5
Drexel University
ECE-E302, Electronic Devices
Lab V Semiconductor Optics
Figure 1. Schematics for generating a continuous output (a) and a pulsed output (b) from the
light emitting diode (LED).
Figure 2. Setup for the photodetectors
Note: These figure are schematic representations of the circuit being used. The only connections
you have to make are the power supply and ampermeter for the sensitivity measurements, and the
power supply, waveform generator, ammeter, and scope for the speed measurements.
5-5