Intrinsic. N-Type P-Type The Diode and PN Junction.

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Presentation transcript:

Intrinsic

N-Type

P-Type

The Diode and PN Junction

Question: What if you put the two (P-type and N-type) semiconductors together?

Draw this in your book and label: Holes, Electrons, Occupied Holes, No Electrons, Positive, Negative, Neutral, Depletion Zone

Q: What if you connect a battery?

Reverse biased PN junction

Reverse Biased PN Junction 1)Describe what happens to the holes in the P-Type material and the electrons in the N-Type material in the reverse biased PN junction 2)What happens to the depletion region? 3)Can current flow through the reverse biased PN junction?

Reverse Biased PN Junction The electrons in the N-type material and the holes from the P-type material get pulled away from the PN junction by the positive and the negative terminals of the battery respectively. This widens the non-conductive depletion region so no current will flow.

Q: What if you connect a battery in the opposite way?

Forward biased PN junction

Forward Biased PN Junction 1)Describe what happens to the holes in the P-Type material and the electrons in the N-Type material in the forward biased PN junction 2)What happens to the depletion region? 3)Can current flow through the forward biased PN junction?

Forward Biased PN Junction The electrons supplied by the negative terminal of the battery and the holes from the positive terminal move into the depletion region, which narrows. Electrons and holes are now able to move across the narrowed depletion region, which means current can flow. The PN junction is then said to be forward biased.

The Diode

The Transistor

Question: What if you put three different type semiconductors together?

*Base = P-type material, very thin

NPN Transistor a)What are two significant design aspects of the base region? b)Explain what happens at the right hand junction as the V be rises from 0V to 0.6V c)When Vbe > 0.6V a large collector current occurs. How does this happen?

There is a depletion zone (which acts as an insulating barrier) at the collector- base junction, because electrons have been attracted out of it through the collector material towards the positive terminal. As the voltage across the base-emitter junction rises, however, electrons are attracted from the emitter into the base area. This reduces the depletion zone.

As the voltage rises at the base, eventually the electrons have enough energy to flow through the depletion zone into the collector material and then to the positive terminal and right around the circuit. So a current can now flow in the circuit.

If the voltage rises further, more and more electrons can flow through the base to the collector. The base is so thin that a small change in the voltage can cause big changes in this current.

The MOSFET

N-Channel Enhancement MOSFET

P-Channel Enhancement MOSFET

Use these words to explain how a conducting channel will appear between drain and source when a positive gate-source voltage is applied. positive gate, holes, repelled, minority carriers, SiO 2 layer, electrons, attracted, conducting channel

The gate is insulated from the p-type area (which has holes) by an insulating SiO 2 (silicon dioxide -glass) layer. When a voltage is applied to the gate, we have a positive gate, and the holes are repelled away, leaving a ‘inversion layer’ near the gate, which acts as a conducting channel. When a voltage is now applied between the drain and the source, electrons (the minority carriers) are attracted from the source to the drain along the conducting channel.

N-Channel Depletion MOSFET

P-Channel Depletion MOSFET

N-Channel Depletion MOSFET

LED – Light Emitting Diode

Electrons falling into holes drop out of the conduction band, down to a lower energy level. This drop across the ‘band-gap’ releases energy. For an ordinary diode based on silicon this energy is too low to be a visible frequency. LEDs use special materials e.g. gallium nitride (GaN) instead of silicon. N-type and P-type GaN can be made using special doping chemicals. The band-gap that can be made with this n- and p- type GaN may be wide enough to allow a greater energy release, which can be seen as light, which can be produced in a range of different visible frequencies i.e. colours.

LDR – Light Dependent Resistor

The substrate is a photo-sensitive substance such as cadmium sulphide or a lightly doped semiconductor e.g. silicon. Light falling on the substrate releases electrons. These are picked up by the metal grids, which are attached to the LDR electrodes and hence the leads. The grids are interleaved and ‘zigzaggy’ to maximise contact with the substrate, so that the uptake of electrons released by the light energy is as efficient as possible. As the light level is increased, the resistance drops sharply, so the current through the LDR increases.