# 13.2 Active circuit elements

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If the diode is reverse-biased, the $+$ terminal of the battery is connected to the n-type semiconductor. This makes it even more negatively charged. It also removes even more of the free electrons near the depletion band. At the same time, the $-$ terminal of the battery is connected to the p-type silicon. This will supply free electrons and fill in more of the holes next to the depletion band. Both processes cause the depletion band to get wider. The resistance of the diode (which was already high) increases. This is why a reverse-biased diode does not conduct.

Another explanation for the increased resistance is that the battery has made the p-type semiconductor m ore negative than it used to be, making it repel any electrons from the n-type semiconductor which attempt to cross the depletion band.

On the other hand, if the diode is forward biased, the depletion band is made narrower. The negative charge on the p-type silicon is cancelled out by the battery. The greater the voltage used, the narrower the depletion band becomes. Eventually, when the voltage is about 0,6 V (for silicon) the depletion band disappears. Once this has occurred, the diode conducts very well.

## The diode

1. What is a diode?
2. What is a diode made of?
3. What is the term which means that a diode is connected the `wrong way' and little current is flowing?
4. Why is a diode able to conduct electricity in one direction much more easily than the other?

## The light-emitting diode (led)

A light-emitting diode (LED) is a diode device that emits light when charge flows in the correct direction through it. If you apply a voltage to force current to flow in the direction the LED allows, it will light up.

## Circuit symbols

This notation of having two small arrows pointing away from the device is common to the schematic symbols of all light-emitting semiconductor devices. Conversely, if a device is light-activated (meaning that incoming light stimulates it), then the symbol will have two small arrows pointing toward it. It is interesting to note, though, that LEDs are capable of acting as light-sensing devices: they will generate a small voltage when exposed to light, much like a solar cell on a small scale. This property can be gainfully applied in a variety of light-sensing circuits.

The colour depends on the semiconducting material used to construct the LED, and can be in the near-ultraviolet, visible or infrared part of the electromagnetic spectrum.

## Interesting fact

Nick Holonyak Jr. (1928) of the University of Illinois at Urbana-Champaign developed the first practical visible-spectrum LED in 1962.

## Light emission

The wavelength of the light emitted, and therefore its colour, depends on the materials forming the p-n junction. A normal diode, typically made of silicon or germanium, emits invisible far-infrared light (so it can't be seen), but the materials used for an LED can emit light corresponding to near-infrared, visible or near-ultraviolet frequencies.

are UV rays dangerous?
no
Lianda
what's this?
Lianda
i hv a problem with this chapter because i also have problem with Newton's laws of motion
what is the wave model of atom ?
Can you state the Doppler effect in words
State Coulomb's law in words
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
Cassiey
Newton's first law of motion
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
Tumelo
yes that's true
Mphoko
Always know Newton's second law of motion. It appears to be in every question paper
Botho
Is the normal force always 0
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
Courtney
define the term rate of reaction in word
I think it a chemical process in which substance act mutually on each other.
Sphesihle
how many bones are in the human body?
Asali
it's the rate at which the reactants are able to change to products
Courtney
what is wave lengh
is the de broglie wavelength of the particle
Hope
calcium and magnesium, which one can displace aluminum from its compound?
how is aluminum ion formed?
Dickens
a substance that has the lower electronegativity which will be in this case the Ca -1.0 this is because the Ca would have a lower ionization energy -needs less energy to fill its outer most shell and therefore will cause a displacement of the Al
Courtney
an Al ion would form when it has lost electrons and will normally be a cation -positively charged (3+)
Courtney
molecules with the same molecular formula but different chain
.angel
chain isomer
Anele
what is structural isomer?
Kearabetswe
Hey if you define an isomer it will come as an organic molecule with the same number of atoms,same molecule formula ,same molecular mass but different STRUCTURAL FORMULA so what we have is only the chain ,positional and functionally isomer.Never heard of STRUCTURAL isomer
Phethego
what does the newton's law says?
there are 3 newtons laws which are newton's law of universal gravitation, law of cooling, law of motion
Imagine
Did the feather or leaf hit the ground first?
leaf
both at the same time. air friction ignored
Courtney
2 to 30 minut long distance race 20 km what's the avarage speed for the race
what is meaning of covalent bonds
bonds that exists between non metal atoms. each atom contribute electron(s) which will form a bond joining the two atoms. electrons in the bond now belongs to both atoms
Matome
how do we name haloalkanes
how do we name haloalkanes
lesedi
what are examples of haloalkanes
Mpho
haloalkanes are compounds that consist of the halogen group chlorine bromine flourine and so on like 2-bromobutan
Samkele