During this lab session you will explore the characteristics of some semiconductors: diodes, LEDs, Zener diodes and bipolar transistors, to further refine your skill at determining the components of a circuit from the analysis of the waveforms observed on a scope display.

Diodes

A diode consists of some P-type semiconducting material (excess of positive charge carriers), joined to some N-type semiconductor (excess of negative charge carriers). This arrangement, a P-N junction, will preferably conduct charge in one direction. A current flows when a positive voltage, applied to the P side (anode) of the junction relative to the N side (cathode), exceeds the junction turn-on voltage. In the reverse-biased direction, the diode does not significantly conduct and behaves as a very large resistance that approximates an open circuit.

The turn-on voltage V_on is specific to the junction material. For silicon, V_on~0.7V. For a Ga-As red LED, V_on~1.5V. For LEDs of other colours, V_on varies from ~1.2V to ~3.6V. This voltage drop remains fairly constant for a wide range of junction current loads, making the diode behave as a voltage regulator.

A Zener diode also conducts when this positive V_on is exceeded, however, the junction can also become conductive when a negative voltage, the Zener reverse breakdown voltage V_Z is exceeded. The value of V_Z can be varied during the manufacturing process. The Zener diode improves on this voltage regulating ability since V_Z can be varied and is not specific to the junction material, which is typically Silicon.

The transistor

Individual transistors are rarely used today, as integrated circuits containing millions of transistors have become commonplace. There are several types of transistor technologies; bipolar, field-effect, mosfet, and others. The bipolar transistor is a current amplifier, meaning that a small current flowing through the base-emitter junction of the device controls a much larger current flowing through the collector-emitter junction, It is still frequently used as an electronic current on/off switch to drive an LED or control the charging/discharging current of a capacitor.

The bipolar transistor consists of two P-N junctions with one P or N region in common. There are two versions: NPN and PNP. For example, an NPN transistor has the base pin connected to the common P region while the collector and emitter pins are connected to the two adjoining N regions. Since these two N regions have different charge carrier concentrations, the collector and emitter pins are not interchangeable. The NPN base-emitter junction is essentially a diode with the P side connected to the base pin and the negative side connnected to the emitter pin.

A frequently used bipolar transistor is the NPN device called 2N3904. Some useful design specifications for this device, such as the current gain $\beta=I_C/I_B$, the maximum collector and base currents, $I_C$ and $I_B$, the maximum emitter-collector voltage $V_{CE}$ and power dissipation P_D can be found here.

A nice introduction to semiconductor junctions, diodes and transistors can be found here.

Prelab exercise

Assuming that the red LED should not operate with a current exceeding 50mA, verify that for the mystery circuit LED and 2N3904 transistors, their maximum rated currents and voltages are not exceeded.