Physics I Lab Junction Transistor

INTRODUCTION The transistor was discovered in 1948 at Bell Labs by John Bardeen, William Shockley, and Walter Brattain, who deservedly received the Nobel Prize in Physics in 1956. Needless to say the transistor has revolutionized the electronics industry - permitting home VCR's, camcorders, microcomputers, calculators, and compact disk sound systems - machines undreamed of at the time of the Nobel awards. A study of the transistor and simple solid-state devices and circuits is appropriate in an introductory physics class. We will build a circuit in which the transistor functions as a switch - either ON or OFF. More complicated circuits include audio amplifiers which are made more accessible by the integrated circuit operational amplifiers investigated in another laboratory.
The junction transistor is made from a single crystal of silicon and doped so as to form a "sandwich" of N-P-N layers or P-N-P material as in the picture below. Each junction in the transistor by itself behaves similarly to the junction diode studied earlier.


PROCEDURE. We will take a common NPN transistor (2N3904) (there are thousands of types) and assemble a simple current amplifier with the addition of power supplies and resistors. In a succeeding experiment, the transistor will be turned on and off by means of an infrared photo transistor. This on-off signal may then be read by a computer to start and stop timing a motion event.

1.What do you think? Predict the conductivity between base-emitter, base-collector, emitter-collector in both directions for an NPN Transistor.

2.Ohmmeter Test. The three leads of a transistor may be identified from the pin-out diagram. The pin-out diagram indicates the bottom view of the transistor. Use the ohmmeter on diode mode. This mode displays the threshold voltage at which the junction begins to conduct. Record the voltage readings (on diode mode) for each pair of transistor leads in both directions and indicate if this is reasonable for a NPN transistor. There should be 6 readings. A common defect in a transistor (one that has been overheated) is a high conductivity from emitter to collector.

3.Assemble the following circuit and double check the wires.  Click here for a picture.

The circuit is quite confusing with three leads coming from  the transistor, so it is important to be extra careful. It also helps to lay-out the circuit on the plug board so that it is similar to the schematic.  The common end of the Vcc battery is not shown in order to remove clutter.

4. What do you think? Before you make any measurements, predict the output voltage when:

a) The transistor is ON and conducting
b) The transistor is OFF and non-conducting

5.  Measure Voltage and Current In order to measure the currents, the voltages on each end of both resistors will be measured and Ohm's law (I=V/R) used to calculate the currents. Here V is the voltage difference between one end of the resistor and the other (Vin - VB, for example). Measure Vin, Vb, Vout, and Vcc for a number of input voltages beginning from zero until the transistor is saturated ( as much current as the circuit allows). Plot the collector voltage (Vc = Vout) as a function of base current IB

6 .Current Gain For each of the input voltages, calculate the IB and the IC, and plot IC as a function of IB . Calculate the current gain for this circuit from the ratio 1 where the graph is linear.

7. Question. When the transistor is ON, why does the Vout drop close to zero? Remember the voltage divider experiment early in the course.

8.Question. When the transistor is OFF, why does the Vout rise to the Vcc supply voltage?

9.Compare your answers to 5 and 6 with what you predicted in item 3.

10.LED Insert a LED in series with a 300 Ohm resistance in place of RL and see qualitatively the turn-on and turn-off of the transistor by controlling the Vin. Remember to insert the diode with the correct polarity. Save your circuit and components for next week clearly labeled with your names.

BACKGROUND. The three parts of a junction transistor are referred to as emitter (e), base (b), and collector (c). Between the base and emitter or between base and collector the transistor resembles two diodes or PN junctions. The secret in a successful transistor depends on the base being very thin - on the order of the depletion layer thickness. When using an NPN transistor, the collector is made positive with respect to the emitter as shown in the diagram. Such a device does not conduct any current because of the back-to-back PN junctions (one junction is reversed and the base is all depleted because of it's thinness). However when a small (approx. 1 micro amp) current is injected into the base through the base terminal, it has the effect of making the transistor conduct current ( about 100 times Ib) from collector to emitter. The transistor thus acts as a switch or a gate. Thus a small amount of signal current (2) into the base allows a large (100 time) to flow through the collector.

The physics of the amplification of current in the transistor is described as follows: When the base is open and the collector positive with respect to emitter, the reversed biased base-collector junction prevents any current flow. The base is depleted of conductors and a large electric field exists in the base region. A small base current (from base to emitter) injects some holes into the depleted base region. This current is easily produced because the base-emitter is forward biased. In turn, electrons from the N-type emitter cross over into the base. If this were simply a diode the electrons and holes would simply re-combine at the junction like a forward-biased diode. However, the collector is generally more positive than the base and this causes an electric field in the base region. The emitter electrons which cross the junction into the base are accelerated toward the collector before they have a chance to recombine with holes. Remember, the base is thin and depleted. The collector keeps collecting more and more electrons from the emitter. Only some of these electrons actually re-combine with the holes from the base. Thus a small base current turns-on a large collector current.  We will discuss a food analogy in class to help explain the operation.

The action of a PNP transistor is analogous except the currents and biases are reversed from the NPN. Other common types of transistors are junction field-effect-transistors (J-FETs) and metal-oxide semiconductor field effect transistors (MOS-FETs). The conductivity of FETs is controlled by changing the size of a depletion zone by means of electric fields or voltages. A junction transistor is controlled by means of a current.

The following Questions make up the homework assignment for the current week. These must be completed before beginning the transistor experiment.

1. Draw the “sandwich” schematic symbol for an NPN transistor and label and label each section as to N or P and label the emitter, base, and collector.

2. Draw the schematic symbol for the NPN transistor and label the components. The symbol should be the diagram in which the emitter is labeled as an arrow.

3. How should an Ohmmeter show conductivity between e-b, c-b, and e-c? (6 measurements). State why, using the principles of PN junctions and diodes.

4. What is meant by the depletion zone in a junction?

5. When the collector is biased positive with respect to emitter and no connection is made at the base, why is the base depleted?

6. In the lab you will be measuring currents with a voltmeter rather than an ammeter. With a resistance of 10 KOhm, the voltage across the resistor is 20 mV, how much current flows through the resistance?

7. Consider the circuit below. Complete the truth table for the two states of the switch.
switch state lamp state output voltage
ON closed    
OFF or open