Mr Powell

VI Characteristics Ohms Law

12.1.2 Current/voltage characteristics

For an ohmic conductor, a semiconductor diode and a filament lamp Candidates should have experience of the use of a current sensor and a voltage sensor with a data logger to capture data from which to determine VI curves

12.1.3 Ohm’s law understood as a special case where I V

Resistor

As the voltage increases the current also increases at the same rate.

This is what is called “ohms law”

True for resistor at a constant temperature

It is directly proportional

Filament Lamp / Bulb

The resistance of a filament lamp increases as the temperature of the filament increases.

i.e. the resistance changes as the temperature of the wire changes.

The gradient of the graph represents the resistance

Semi Conducting Diode

The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

Often used in transformers to change A.C to D.C. currents or computers!

Why does this happen?

30V

Summary Ohms law states that V I when T = const

If we plot a graph of a component of v on y-axis and I on the x-axis the gradient is the resistance of the component.

Light bulb and diode are examples of non-ohmic components

Semiconductors?

Carbon, silicon and germanium (germanium, like silicon, is also a semiconductor) have a unique property in their electron structure - each has four electrons in its outer orbital.

This allows them to form nice crystals. The four electrons form perfect covalent bonds with four neighbouring atoms, creating a lattice.

In carbon, we know the crystalline form as diamond.

In silicon, the crystalline form is a silvery, metallic-looking substance. Which is a near perfect insulator.

Well on its own that is not amazing but a semi-conductor is made from a silicon lattice with an impurity which will enable it to conduct. We often refer to these types of materials as intrinsic semiconductors.

2D morphed view of lattice

Doping p or n? In N-type doping, phosphorus or arsenic is added to the silicon in small quantities. Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. N-type silicon is a good conductor. Electrons have a negative charge, hence the name N-type.

In P-type doping, boron or gallium is the doping agent. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbour, moving the hole over a space. P-type silicon is a good conductor.

p-type

n-type

2D morphed view of lattices

PN junctions?

N-type and P-type silicon are not that amazing by themselves; but when you put them together, you get some very interesting behaviour at the junction. That's what happens in a diode.

The place where they meet in the diagram forms what is called a depletion zone.

Where free electrons have filled positive “holes” to form an area where there are no free charge carriers.

P N P N P N

PN junctions - behaviour to applied pd

V< 0.7V V> 0.7V

This typically takes 0.7V to overcome and bridge the gap and make it conduct!

If we reverse the flow then the gap gets larger and does not conduct (up to 30V)

V< 0V V<< 0V

P N

P N

PN junction superimposed

Thermistor

The resistance of a thermistor decreases as the temperature increases so if we look at it from the VI perspective it is the opposite of a bulb!

How do they work?

The exact conduction mechanisms are not fully understood but metal oxide NTC thermistors behave like semiconductors, as shown in the decrease in resistance as temperature increases. The physical models of electrical conduction in the major NTC thermistor materials are generally based on this theory;

A model of conduction called "hopping" is relevant for some materials. It is a form of ionic conductivity where ions (oxygen ions) "hop" between point defect sites in the crystal structure.

The probability of point defects in the crystal lattice increases as temperature increases, hence the "hopping" is more likely to occur and so material resistivity decreases as temperature increases.

Extension

Temperature Sensors?

They are inexpensive, rugged and reliable. They respond quickly to changes and are easy to manufacture in different shapes.

An example could be made from a combination of Fe3O4 + MgCr2O4 (metallic oxides)

A NTC thermistor is one in which the resistance decreases with an increase in temperature.

The circuit shows how you can use the thermistor as a potential divider. As the temperature changes the division of voltage or energy will change. You need the 5k resistor or the voltage would be that of the cell a constant 3V.

A common use is the glass heat sensor in a car or the temperature sensor in a conventional oven.

Mr Powell

Extension Devices

As Physics

LDR

The resistance of a light-dependent resistor (LDR) decreases as light intensity increases. This is a similar process to a thermistor