9/24/2004 EE 42 fall 2004 lecture 11 1

Lecture #11 Metals, insulators and Semiconductors, Diodes

Reading: Malvino chapter 2 (semiconductors)

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Energy states

• In any material, there are different roles that an electron can play.

• Electrons can be tightly bond around the core of atoms

• Electrons can participate in bonding between atoms

• Electrons can be at higher energies, and free to move

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Insulators

• In an insulator, all of the electrons are either tightly wrapped around the neuclei, or they are locked into bonds between atoms.

• Since no electrons or ions can move, the material is an insulator.

• Examples:– Silicon dioxide– Plastic– glass

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Metals

• In a metal, a very large number of electrons are free to move.

• This gives metals there ductility and strength, because the sea of electrons binds the material in a distributed way

• It makes them good conductors of heat

• It also makes them very good conductors of electricity.

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Semiconductors

• In a semiconductor, there is a gap in the energies that an electron can take, between those of the bonding (valence electrons) and those that can move (conduction electrons)

• In an intrinsic semiconductor, all of the valence (bonding) states are full of electrons, but no electrons are left to go into the conduction band.

• So an intrinsic semiconductor is an insulator, but if a few extra electrons are added, they can move freely.

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Semiconductor (undoped)

} Energy gap

All empty

All full of electrons

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Semiconductor (N type)

} Energy gap

A few extra electrons

All full of electrons

Doped with Arsenic or Phosphorus atoms, each with one more proton than a silicon atom, each becomes a fixed positive ion, and neutrality means a few extra electrons will be available

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Semiconductor (p type)

} Energy gap

All empty

A few empty states

Doped with Boron, Aluminum or Gallium atoms, each with one fewer proton than a silicon atom, each becomes a fixed positive ion, and neutrality means a few extra electrons will be available

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Column 3 of the periodic table

If we look at column 3 of the periodic table, we can see that these elements are very similar to silicon, except that they have one fewer protons—one less quantum of positive charge.

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Energy bands

• In general, we won’t worry about all of the core electrons, and we will just care about electrons which are in the conduction band, or missing electrons in the valence band.

• Electrons in the conduction band are called …wait for this… electrons

• Missing electrons from the valence band are called holes

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Conduction and valence bands

Energy gap, about1 electron volt

Electrons

Holes

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Recombination

Energy gap, about1 electron volt

Electrons

Holes

If an electron comes near a hole, it can fall into the open state. This is called recombination.The extra energy can heat up the crystal, or it can be released as light (as in a light emitting diode)

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Recombination

Energy gap, about1 electron volt

Electrons

Holes

If some energy is available, on of the many electrons in the valence band can jump up into the conduction band, leaving a hole behind.This process is called generation.Of course, this always makes a pair, an electron and a hole.In a solar cell, light causes this jump, and that is how sunlight is converted into electrical energy (we just need to get the electron and hole out before they recombine!)

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Electrons and Holes

• If you were to have both electrons and holes in the crystal, the electrons could fill up the holes until one or the other was depleted.

• Silicon crystals with quite a few extra electrons running around are called N-type, and they only have a few holes in the valence band.

• Silicon crystals with many holes running around are called P-type, and they only have a few electrons in the conduction band.

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Applications

• We can take silicon wafers and put patterns of doping on their surface, and control its conductivity.

• We can further control the conductivity by applying electric fields

• We can make use of the difference between P and N type carriers moving through the crystal.

• This ability to control the conduction of silicon is the basis for the function of transistors, the foundation of the entire electronics industry

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Diodes

• If we have a P doped material next to an N doped material, the result is a diode.

• Remember:• the P type material has extra fixed negative charge

– And mobile positive charge (holes)• The N type material has extra fixed postive charge

– And mobile negative charge (electrons)

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Diode

• If we apply a voltage to push the electrons and holes toward each other, they can recombine at the junction and a current will flow (this is called forward bias)

• If we apply a voltage which pulls the electrons and holes away from each other, then the will pull back from the junction, and very little current will flow. (called reverse bias)

• So a diode acts like a one way valve for current and the symbol is an arrow in the direction that the current is allowed to flow

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• The diode can be used for many purposes, for example to charge a power supply capacitor from an AC source

~

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IV curve for a diode

• The IV curve for a ideal diode is to have zero current in the reverse direction, and no resistance when forward biased

Voltage →

Current

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Unbiased diode

• If we were to make an N type material and join it to a P type material, we would have extra electrons right next to holes.

• Near the joint, the extra electrons and extra holes recombine, leaving a region which has no carriers called the depletion zone)

• The loss of the mobile charge leaves the fixed charge from the dopants in place, and that creates an electric field which keeps more electrons and holes from wandering into the deletion layer.

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Depletion zone

- -

-

--

--

- -

-

--

--

+ +

+

++

+

+

+ +

+

+

+

+

-

-

-

-

-

-

++

+

++

+

The mobile electrons and holes are held back from the joint by the electric field from the the fixed charges

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Reverse leakage

• In a real diode, if a reverse bias voltage is put on it, a small reverse current flows. This current is called the leakage current

• It is caused by:– Generation in the depletion zone (at higher

temperatures, there is more leakage current, and light on a diode can cause a lot of leakage current)

– surface and edge effects.

• If enough reverse voltage is applied, the diode will break down and conduct a lot of current (not necessarily destructively)

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Real diode under forward bias

• In a real diode, as you reduce the depletion width, pushing the mobile carriers together, the current does not turn on instantly, but rises exponentially with the voltage

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Real diode IV curve