Problems All problems up to p.28 Q 4 can be done.

The p-n Junction and Applications 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 charge Electrons 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 Diagram p-type n-type Conduction band Conduction band Valence band Valence band Depletion zone

Applications of the p-n Junction 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.

n p + 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.

2. PHOTOVOLTAIC MODE (no bias) n p + V 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!

3. PHOTOCONDUCTIVE MODE (reverse bias) Reverse bias means that the p-type is connected to the negative side of the battery + n p Here the mobile charge carriers are separated further, so the depletion layer widens. This makes a BIG resistance to current.

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.

Mode: Photoconductive 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

Mode: Photovoltaic 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: L.E.D. Bias: Forward Bias Mode: no special name

Mode: Photovoltaic 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

Mode: Photoconductive 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

Mode: Photoconductive 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

Mode: Photovoltaic 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

Mode: Photoconductive 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

Mode: Photovoltaic 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: L.E.D. Bias: Forward Bias Mode: no special name

Mode: Photoconductive 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

Mode: Photovoltaic 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