# Optoelectronic Devices

## Discrete Semiconductor Devices and Circuits

• #### Question 1

In this circuit, the electric motor is supposed to turn on whenever the cadmium sulfide photocell is darkened:

Unfortunately, though, the motor refuses to turn on no matter how little light strikes the photocell. In an attempt to troubleshoot the circuit, a technician measures voltage between the collector and emitter terminals of the transistor with the photocell covered by a piece of dark tape, and measures full battery voltage. The technician also measures voltage between the collector and base terminals of the transistor, and measures full battery voltage. At that point, the technician gives up and hands the problem to you.

Based on this information, what do you suspect is faulty in this circuit, and how might you determine the exact location of the fault? Also, identify what you know to be not faulted in the circuit, based on the information given here.

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• #### Question 2

 Don’t just sit there! Build something!!

Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

1. Carefully measure and record all component values prior to circuit construction, choosing resistor values high enough to make damage to any active components unlikely.
2. Draw the schematic diagram for the circuit to be analyzed.
3. Carefully build this circuit on a breadboard or other convenient medium.
4. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
5. Mathematically analyze the circuit, solving for all voltage and current values.
6. Carefully measure all voltages and currents, to verify the accuracy of your analysis.
7. If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction against the diagram, then carefully re-calculate the values and re-measure.

When students are first learning about semiconductor devices, and are most likely to damage them by making improper connections in their circuits, I recommend they experiment with large, high-wattage components (1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.), and using dry-cell battery power sources rather than a benchtop power supply. This decreases the likelihood of component damage.

As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter “loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between 1 kΩ and 100 kΩ.

One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

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• #### Question 3

The characteristically colored glow from a gas-discharge electric light is the result of energy emitted by electrons in the gas atoms as they fall from high-level “excited” states back to their natural (“ground”) states. As a general rule of electron behavior, they must absorb energy from an external source to leap into a higher level, and they release that energy upon returning to their original level.

Given the existence of this phenomenon, what do you suspect might be occurring inside a PN junction as it conducts an electric current?

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• #### Question 4

What determines the color of an LED?

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• #### Question 5

What is the typical forward voltage drop for a light-emitting diode? What is the typical forward current for an LED?

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• #### Question 6

Explain the operating principle of a photovoltaic cell, otherwise known as a “solar cell.” What happens within these devices to convert sunlight directly into electricity?

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• #### Question 7

What happens when external light strikes the PN junction of an LED? Design and operate an experiment to verify your hypothesis.

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• #### Question 8

Photodiodes may be operated in either “photovoltaic” mode or in “photoconductive” mode. Describe the difference between these two modes, drawing schematic diagrams showing how a photodiode would be used in each mode.

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• #### Question 9

Describe the function of a phototransistor. What typical applications do phototransistors have in circuits? What is the schematic symbol for a phototransistor, and how is it properly biased?

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• #### Question 10

Photodiodes, phototransistors, and light-emitting diodes all play important roles in optical communications networks, where digital (on/off) signals are communicated over long distances as pulses of light energy rather than voltage or current.

Why are LEDs greatly preferred over other electro-optical devices such as incandescent lamps or gas-discharge bulbs? What can an LED do that a small electric lamp cannot?

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