Month: June 2013

13 June

P-N Junction

Teaching Resources

The following is the transcript for a video that I will be making to explain how a P-N junction works.

A p-n junction, as the name suggests, is the boundary between two types of semiconductors: P-type and N-type.

For an intrinsic or pure semiconductor such as silicon which has 4 valence electrons, each atom is bond to 4 other neighbouring atoms.
The p-type semiconductor is one with excess holes due to the addition of dopants to intrinsic semiconductors. Elements such as boron or phosphorus from Group III of the periodic table all contain three valence electrons, causing them to function as acceptors when used to dope silicon. When an acceptor atom replaces a silicon atom in the crystal, a vacant state ( an electron “hole”) is created, which can move around the lattice and functions as a charge carrier.

The n-type semiconductor is one doped with Group V elements which have five valence electrons, allowing them to act as a donor; substitution of these atoms for silicon creates an extra free electron. Therefore, a silicon crystal doped with boron creates a p-type semiconductor whereas one doped with phosphorus results in an n-type material.

When the two types of semiconductors are put together, electrons diffuse across the boundary to combine with holes, creating a depletion region where there are no charge carriers. An electric field is also set up in the depletion region because the group III atoms are now negatively charged, having gained one more electron and the group V atoms are now positively charged, having each lost an electron. This electric field prevents further charges from diffusing across the boundary.

That is, until a potential difference is applied. The p-n junction serves now as a diode. We shall illustrate this with a single cell attached to the device. In the reverse-biased mode, the positive terminal is connected to the n-type semiconductor while the negative terminal is connected to the p-type end. This causes more electrons to move away from the depletion region in the n-type semiconductor and for more holes to be filled in the p-type semiconductor. The result is a widened depletion region and a larger opposing electric field.

In the forward-biased mode, the positive terminal of the cell is connected to the p-type semiconductor while the negative terminal is connected to the n-type end. The potential difference provided offers the electrons in the n-type semiconductor a push to overcome the small electric field formed across the depletion region and flow across to the p-type semiconductor which it then passes from hole to hole into the positive terminal.

12 June

Planning Question

21 Practical / Planning

The following is the script that I used for recording the video above for our e-learning day. I’m posting it here as I can edit it easily via wordpress’s mobile app, and because I have LaTeX enabled here.

Aim:

The aim of this planning question is to investigate how the resonant frequency of a wire vibrating in its fundamental mode depends on the tension in the wire.

The independent variable is the tension in the string which can be varied by hanging masses at one end of the wire and dangling that end over the edge of a table on a pulley. The tension is represented by the symbol.

The dependent variable is the resonant frequency of the fundamental mode in the wire. The fundamental mode consists of two nodes at both ends of the wire and one antinode in the middle.

In other words, in our experiment, we shall vary tension and measure the resonant frequency of the fundamental mode.

The length of wire between the supports is kept constant throughout the experiment and we shall use the same wire throughout so that the mass per unit length is kept constant.

Procedure:

The experiment will be set up according to this diagram. One end of the wire is first tied to a fixed object. The other end is hanging over a pulley clamped on the edge of the laboratory table and tied to a mass hanger. To control the length of the wire, place a bridge at each end. Only the length of the wire between the two bridges will be vibrating. We shall keep this length constant for the whole experiment.

Record the mass of the mass hanger m and determine its weight. The weight mg is taken to be equal to the tension T on the string.

A uniform horizontal magnetic field is generated by a pair of large electromagnetic coils on both sides of the wire. The wire is connected on both ends to a function generator. An alternating current is produced by the function generator and its frequency can be varied using the same apparatus. The function generator should have a display that enables us to read the frequency. 

The resulting magnetic force acting on the wire will be driving the oscillation of the wire at the frequency shown on the function generator. By adjusting the frequency of the alternating current until the fundamental mode of a standing wave is formed on the wire, we can record the resonant frequency fo for the corresponding tension T.

We shall repeat the experiment for different values of m by adding known amounts of mass (e.g. 50 g increments each time) onto the mass hanger. All the values for the mass m and resonant frequency for the fundamental modes fo should be recorded and tabulated.

The tension in the wire is then calculated using the equation T = mg.

Assume that the resonant frequency for the fundamental mode fo and the tension T follows the equation  fo = kTn where k and n are constants. Then lg fo = lg k + n lg T. Plotting a graph of lg fo versus lg T, we can conclude that the assumption is correct if a linear relationship is observed and we can obtain the values of n and k from the gradient and the vertical intercept of the graph respectively.

As a safety consideration, the person conducting the experiment should wear goggles as the wire at high tension might suddenly snap or come loose. Always handle power supply with care.

As a precaution to improve reliabiliy, we can place a white card behind the vibrating wire so it can be seen easily. To make sure that the weight of each of the slotted masses is as indicated, measure them on a weighing balance. FInally, ensure that the pulley is smooth by measuring the tension in the wire using a force meter to check that it is indeed equal to the weight of the slotted mass.

01 June

Boyle’s Law

08 Temperature and Ideal Gases, Demonstrations

Using a hand-operated vacuum pump, we can demonstrate the relationship between pressure and volume of a gas. According to Boyle’s law, the pressure of a gas of constant mass and temperature will be inversely proportional to its volume.

In our demonstration, we will reduce the ambient pressure within the sealed container, hence allowing the higher internal pressure of a balloon to cause it to expand. When the volume within the balloon increases, the internal pressure can be observed to decrease until it is in equilibrium with the surrounding pressure.

While the relationship between pressure and volume is not exactly obeying Boyle’s law due to additional factors such as the tension due to the elastic property of the balloon, it does demonstrate an inverse relationship.