Semiconductor Devices

Physics 355

Semiconductor Devices

• The control of semiconductor electrical and optical properties make these materials useful for electronic and photonic devices.

• The properties include, for example, electrical resistivity and optical absorption, which are related to one another by the semiconductor electronic structure.

Optical Absorption

Light absorption, with photon energycan be depicted as a valence band electron making a transition to a conduction band state.Light with photon energy is not absorbed and passes through the material.

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p-n junctions

p type n type

junction

Free electrons on the n-side and free holes on the p-side can initially wander across the junction. When a free electron meets a free hole it can 'drop into it'. So far as charge movements are concerned this means the hole and electron cancel each other and vanish.

p-n junctions

p-n junctions

As a result, the free electrons near the junction tend to “eat” each other, producing a region depleted of any moving charges. This creates what is called the depletion zone.

p-n junctions

Now, any free charge which wanders into the depletion zone finds itself in a region with no other free charges. Locally it sees a lot of positive charges (the donor atoms) on the n-type side and a lot of negative charges (the acceptor atoms) on the p-type side. These exert a force on the free charge, driving it back to its 'own side' of the junction away from the depletion zone.

p-n junctions

Usually, we represent this barrier by 'bending' the conduction and valence bands as they cross the depletion zone. Now we can imagine the electrons having to 'get uphill' to move from the n-type side to the p-type side. For simplicity we tend to not bother with drawing the actual donor and acceptor atoms which are causing this effect!

p-n junctions

p-n junctions

On the basis of the explanation given above we might expect no current to flow when the diode is reverse biased. In reality, the energies of the electrons & holes in the diode aren't all the same. A small number will have enough energy to overcome the barrier. As a result, there will be a tiny current through the diode when we apply reverse bias. However, this current is usually so small we can forget about it.

p-n junctions

Light-Sensitive Diodes

If light of the proper wavelength is incident on the depletion region of a diode while a reverse voltage is applied, the absorbed photons can produce additional electron-hole pairs. This is photoconduction and many photocells are based on this property.

As electrons fall into holes, photons are emitted with energies corresponding to the band gap.

LEDs emit light in proportion to the forward current through the diode.

LEDs and photodiodes are often used in optical communication for both receiver and transmitter.

Light Emitting Diodes (LEDs)

While all diodes release light, most don't do it very effectively. In an ordinary diode, the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially constructed to release a large number of photons outward. They are housed in a plastic bulb that concentrates the light in a particular direction. As you can see in the diagram, most of the light from the diode bounces off the sides of the bulb, traveling on through the rounded end.

Conventional LEDs are made from a variety of inorganic semiconductor materials, producing the following colors:

* aluminium gallium arsenide (AlGaAs) - red and infrared * aluminium gallium phosphide (AlGaP) - green * aluminium gallium indium phosphide (AlGaInP) - high-brightness orange-red, orange, yellow, and green * gallium arsenide phosphide (GaAsP) - red, orange-red, orange, and yellow * gallium phosphide (GaP) - red, yellow and green * gallium nitride (GaN) - green, pure green (or emerald green), and blue also white (if it has an AlGaN Quantum Barrier) * indium gallium nitride (InGaN) - near ultraviolet, bluish-green and blue * silicon carbide (SiC) as substrate — blue * silicon (Si) as substrate — blue (under development) * sapphire (Al2O3) as substrate — blue * zinc selenide (ZnSe) - blue * diamond (C) - ultraviolet

Light Emitting Diodes (LEDs)

In September 2003 a new type of blue LED was demonstrated by the company Cree, Inc. to give 240 lm/W at 20 mA. This produced a commercially packaged white light giving 65 lumens per watt at 20 mA, becoming the brightest white LED commercially available at the time, and over four times more efficient than standard incandescents.

In 2006 they demonstrated a prototype with a record white LED efficacy of 131 lm/W at 20 mA. Also Seoul Semiconductor has plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be approaching an order of magnitude improvement over standard incandescents. Nichia Corp. has developed a white light LED with efficacy of 150 lm/W at a forward current of 20 mA.

Light Emitting Diodes (LEDs)

Gunn EffectJB (Ian) Gunn discovered the Gunn-effect in February 1962. He observed random noise-like oscillations when biasing n-type GaAs samples above a certain threshold. He also found that the resistance of the samples dropped at even higher biasing conditions, indicating a region of negative differential resistance.

The semiconductor materials that exhibit the Gunn Effect, such as GaAs, InP, GaN, must be direct bandgap materials that have more than one valley in the conduction band and the effective mass and the density of states in the upper valley(s) must be higher than in the main valley.

Gunn Effect

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εth < ε < εsat ε > εsatε < εth

Gunn Effect

Gunn Effect• It is important to note that the sample had to be biased in

the NDR region to produce a Gunn-domain.• Once a domain has formed, the electric field in the rest of

the sample falls below the NDR region and will therefore inhibit the formation of a second Gunn-domain.

• As soon as the domain is absorbed by the anode contact region, the average electric field in the sample rises and domain formation can again take place.

• The successive formation and drift of Gunn-domains through the sample leads to ac current oscillations observed at the contacts. In this mode of operation, called the Gunn-mode, the frequency of the oscillations is dictated primarily by the distance the domains have to travel before being annihilated at the anode. This is roughly the length of the active region of the sample, L. The value of the dc bias will also affect the drift velocity of the domain, and consequently the frequency.

Gunn Diodes

Gunn diodes are semiconductor diodes that form a cheap and easy method of producing relatively low power radio signals at microwave frequencies. Gunn diodes are a form of semiconductor component able to operate at frequencies from a few Gigahertz up to frequencies in the THz region. As such they are used in a wide variety of units requiring low power RF signals.