ProblemsAll problems up to p.28 Q 4 can be done.

The p-n Junction and ApplicationsThe p-n junction

n-type p-type

•Electrons in Conduction band•So electrons are mobile•Overall neutral

•Spaces in Valence band•Positive holes are mobile•Overall neutral

n p

+++++

depletion zone (layer)Some free electrons meet up with holes at the junction – thus the region at the junction is empty (depleted) of mobile chargeElectrons entering the p – type have create a negative barrier at the p – type edge which stops an influx of more electrons

Similarly, electrons leaving the n – type have created a positive barrier at the n – type edge which stops holes.

p – n Junction - Band Energy Diagramp-typen-type

Conduction bandConductio

n band

Valence bandValence

band

Depletion zone

Applications of the p-n Junction

1. p – n Junction in Forward Bias Bias means a voltage is applied and

forward means that the n side is connected to the negative terminal of the battery.

•If the pd across the junction is sufficient to force electrons to cross the depletion zone, then electrons in the n-type conduction band can pass into the conduction band of the p-type material. •The electron then loses energy (or “deexcites”) so it moves down to recombine with a hole in the valence band of the p-type material.•When an electron recombines with a hole it loses energy and a photon is emitted (also called “quanta” of radiation).•This is the basis of an L.E.D.

n p

+++++

+

2. PHOTOVOLTAIC MODE (no bias)

•Incoming photons mobilise electrons in the depletion zone so a potential difference is set up across the p-n junction.•If a circuit is connected between its sides, a current will flow and can power a load. •This is how a solar cell works!

n p

+++++

V

3. PHOTOCONDUCTIVE MODE (reverse bias)Reverse bias means that the p-type is connected to the negative side of the battery

Here the mobile charge carriers are separated further, so the depletion layer widens. This makes a BIG resistance to current.

+

n p

+++++

BUT …….If we shine light on the depletion zone to mobilise electrons, then the resistance decreases.A photodiode in photoconductive mode is the basis of a light sensor .

Problems All problems up to p30 Q.10.

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Light sensor

Bias: Reverse Bias

Mode: Photoconductive Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

L.E.D.

Bias: Forward Bias

Mode: no special name

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Solar Cell

Bias: no bias

Mode: Photovoltaic Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

L.E.D.

Bias: Forward Bias

Mode: no special name

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Solar Cell

Bias: no bias

Mode: Photovoltaic Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Light sensor

Bias: Reverse Bias

Mode: Photoconductive Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

L.E.D.

Bias: Forward Bias

Mode: no special name

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Light sensor

Bias: Reverse Bias

Mode: Photoconductive Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Solar Cell

Bias: no bias

Mode: Photovoltaic Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Light sensor

Bias: Reverse Bias

Mode: Photoconductive Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

L.E.D.

Bias: Forward Bias

Mode: no special name

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Solar Cell

Bias: no bias

Mode: Photovoltaic Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

L.E.D.

Bias: Forward Bias

Mode: no special name

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Light sensor

Bias: Reverse Bias

Mode: Photoconductive Mode

Let’s Play!What’s the bias voltage and “mode” – if relevant - for each of the following:

Solar Cell

Bias: no bias

Mode: Photovoltaic Mode