Download Electronics background

Electronics background.
Electronics is based on semi-conductors i.e. materials that are half way between being
good conductors and good insulators. Silicon and germanium are on the dividing line
between metals and non-metals on the periodic table and are good examples of semiconductors. The current that flows through a semiconductor depends on temperature. At
low temperatures electrons are held firmly in the covalent bonds between atoms in a
piece of the semiconductor. As temperature rises electrons gain kinetic energy. Some
electrons will gain enough energy to break free and then it is possible to move them to
cause a current to flow.
Semi-conductors are used in electronics because it is possible to alter their ability to
conduct by adding trace amounts of an impurity to the crystal structure. This is referred to
as “doping”.
Silicon is doped in 2 ways, using boron or using phosphorous. (Aluminium can be used,
rather than boron, but students may think that adding a conductor to the silicon, of itself,
will convert it to a conductor.)
Boron and phosphorous atoms are about the same size as silicon atoms and so fit into the
crystal. Boron atoms have one electron less than silicon whereas phosphorous atoms have
an extra electron.
Silicon doped with phosphorous has one electron, per phosphorous atom, on an outer
shell that is not involved in a covalent bond. If a potential difference (voltage) is set up
across a piece of doped silicon it is relatively easy to move this electron. This doped
silicon is referred to as n-type as the charge carriers are negatively charged electrons.
Silicon doped with boron is short of one electron, per aluminium atom, to complete the
set of eight electrons in the outer shell. This creates a “hole” in which there should be an
electron. If a potential difference (voltage) is set up across a piece of doped silicon it is
possible to get electrons to move into this hole. This means that current can flow. Silicon
doped in this way is referred to as p-type.
The diode
Diode
Diode circuit symbol
In a diode the 2 types of doped silicon are used in combination. Diodes consist of a piece
of silicon one side of which is doped with boron and the other side doped with
phosphorous. (A transistor has 3 layers, either p-n-p or n-p-n with a connection to each
layer.)
At the junction of the p-type and n-type some of the electrons will migrate from the ntype into the holes in the p-type. This creates a region at the junction without any charge
carriers i.e. an insulating layer. As electrons have left the n-type it becomes positively
charged near the junction while the p-type becomes negatively charged at its side of the
junction. This creates a “junction voltage” (of about 0.6 V) where the 2 layers meet. This
means that current will not flow when there is a voltage of less than 0.6 V across the
diode.
In a diode that is connected so that the p-type material is connected to the positive
terminal of a battery a current can flow as electrons are repelled from the n-type material
to occupy the holes and continue around the circuit. A diode connected in this way is said
to be “forward-biased”.
In a diode that is connected so that the n-type material is connected to the positive
terminal of a battery a current cannot flow as electrons are attracted from the n-type
material away from the holes. This creates an area at the junction of the p-type and n-type
sections that do not have charge carriers i.e. an insulating layer. A diode connected in this
way is said to be “reverse-biased”.
A diode acts like a valve in the blood stream. It allows a current to flow in one direction
only. It can be used to rectify alternating current by blocking current that is flowing in the
“wrong” direction.
Light Emitting Diode (LED)
Different coloured LEDs
LED symbol
A light emitting diode is a diode that converts electrical energy to light, rather than to
heat. They are used as indicator switches as they require very little energy. Most
electrical appliances, TVs, electric kettles, music systems etc. have, at least, one LED to
show when it is switched on.
A Light-Dependent Resistor (LDR) is a device whose resistance to the flow of current
is decreased when light falls on it.
cadmium sulfide
track
A photon (packet of light energy) may provide enough energy to release an electron from
an atomic or molecular orbital in the cadmium sulfide. It is then easier to get this electron
to move in a particular direction i.e. the resistance is reduced.
An LDR can be used to turn on circuits when light shines on it. This can be used in a
burglar alarm.
An LDR may also be used to automatically turn on and off streetlights.