GUNN DIODE

Introductiony

Microwave devices that operate by a transferred electron mechanism are often called Gunn diodes. It is named after J.B.Gunn (1963) who observed these effects in semiconductors like GaAs , InP, CdTe .

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Principley

In transferred electron mechanism the conduction electrons of some semiconductors are shifted from a region of high mobility to a state of low mobility by the influence of a strong electric field. Leading to a negative conductance behaviour in the diode.

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This is the basic structore of Gunn diode. It is called a diode with reference to the positive end (anode) and negative end (cathode).

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Effective Massy

The movement of an electron in a lattice will be in general be different from that of an electron in free space. In addition to an externally applied force , there are internal forces in crystal which will influence the motion of electron in lattice. F Total= F External + F Internal The effective mass takes into account the particle mass and the effect of the internal forces.

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The effective mass of an electron in a band with a given (E,K) relationship is m*=2/(d2E/dk2

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The mobility of the electron is given by = 1/ m*

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The effective mass of a electron in high energy valley for GaAs is 1.2m0 and hence electron mobility is less compared to the lower energy valley. Since the conductivity is directly proportional to mobility, current decreases with increase in electric field beyond a threshold value.

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The semiconductors like GaAs have closely spaced energy valley in the conduction band. When a dc voltage is applied across the material an electric field is established across it. At low electric field in material most of electron will be located in lower central valley. At higher field most of the electrons will be shifted to higher energy satellite L and X valleys.

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The energy levels in conduction band shows the following properties: In the lower valley, electrons exhibit a small effective mass and very high mobility In the satellite L valley, electrons exhibit a large effective mass and very low mobility.

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The two valleys are separated by a small energy gap, approximately 0.31 eV.

E, of

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In equilibrium at room temperature most electrons reside near the bottom of the lower valley. Because of their high mobility ,they can readily be accelerated in a strong electric field.

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Valley

Effective mass Me

Mobility u Cm2/V.s

Lower

0.068

8000

Upper

1.2

180

Negative Resistance effecty

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If a voltage is applied to the nGaAs, initially the current will increase with rise in voltage. When the diode voltage exceeds a threshold value, (3.2KV/m for GaAs), the electrons are excited from the initial lower valley to the higher valley,where they become virtually immobile If there is increase in voltage beyond the threshold value there is decreas in current leading to negative resistance effect

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The energy difference between two valleys must be several times larger than the thermal energy (KT~0.0259eV) The energy difference between the valleys must be smaller than the bandgap energy (Eg) Electron in lower valley must have a higher mobility and smaller effective mass than that of in upper valley

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Modes Of OperationTransit Time(TT) mode y Limited Space Charge Accumulation(LSA) modey

Transit time modeA sample of uniformly doped n-type GaAs of length L , is biased with field in the negative conductivity region. y Resulting in space charge instability, leading to the formation of domain. y Under this negative conductivity region, the domain builds up because of the stream of electrons drifting from cathode to anode. y Eventually the drifting domain will reach anode and gives out is energy as a pulse of current.y