Elec

4. Semiconductor Diodes

Introduction

So far we have looked at only so-called passive electronic devices: capacitors and resistors respond to voltages applied across them by accumulating charge or passing currents, respectively. In the next two labs, we’ll take a look at two active devices which behave very differently depending upon what voltages are applied to them. The diode essentially acts as a one-way switch controlled by voltage. For one polarity of voltage, if the voltage across the diode is greater than a threshold value (often ~0.6 Volts), it conducts current with essentially no resistance. If the voltage is below that value, or has the opposite polarity, the diode acts as an open switch and conducts no current. The transistor differs from the other devices we’ve considered so far in that it has three leads or connections. The voltage applied between two of these leads controls whether current can be conducted between two others. Although the transistor also acts as a switch, it does not merely shift between fully off and fully on. Its importance lies in the fact that a relatively low-power voltage supply can control the flow of a more powerful current over a range of values. It is this switching and decision-making property of diodes and transistors, which makes all of modern day electronics, including computers and telecommunications, possible.

The study of semiconductors and the devices made from them falls under the category of solid state physics. In this experiment, we will work with one useful device: the diode. Although we will discuss the theory briefly, you will mainly be expected to be able to understand how diodes function in circuits.

Energy Bands

The electrons of an isolated atom have discrete allowed energies that we call energy levels. The Pauli Exclusion Principle states that at most two electrons can occupy any allowed energy level. For example, Figure 1 shows schematically the energy levels for a Lithium atom....