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TRANSFER ELECTRON DEVICESThe application of two-terminal semiconductor devices at microwave frequencies hasbeen increased usage during the past decades. The CW, average, and peak poweroutputs of these devices at higher microwave frequencies are much larger than thoseobtainable with the best power transistor. The common characteristic of all activetwo-terminal solid-state devices is their negative resistance. The real part of theirimpedance is negative over a range of frequencies. In a positive resistance the currentthrough the resistance and the voltage across it are in phase. The voltage dropacross a positive resistance is positive and a power of (12 R) is dissipated in the resistance.In a negative resistance, however, the current and voltage are out of phase by180. The voltage drop across a negative resistance is negative, and a power of(-[2 R) is generated by the power supply associated with the negative resistance. Inother words, positive resistances absorb power (passive devices), whereas negativeresistances generate power (active devices).Differences b/w Microwave Transistors and Transfer Electron Devices(TEDs)MIcrowave -transistorsTheMicrowave transistordevices include microwave BJT, FET, HBT, tunnel diode etc. Here electrons and holes both participate in the transport process.

It consists of junctions or gates.

It is fabricated from elemental semiconductor such as Ge and Si.

It operates with low energy "Warm" electrons.

Advantages:Low cost, low power supply, small size

TRANSFER ELECTRON DEVICESTheTEDdevices include Gunn diode, LSA diode and Inp diode. Here electron transfer from lower conduction valley to the upper satellite valley.

It is not having any junction or gate, it is a bulk type of device.

It is fabricated from compound semiconductor such as GaAS and Inp.

It operates with high energy "hot" electrons.

Advantages:Low noise, high gain, low power, reliable, lighter in weightBecause of these fundamental differences, the theory and technology of transistorscannot be applied to TEDs.The Gunn diode is a so-called transferred electron device, Since the Electrons are transferred from one valley in the conduction band to another valley.

AGunn diode, also known as atransferred electron device(TED), is a form ofdiode, a two-terminalpassivesemiconductorelectronic component, withnegative resistance, used in high-frequencyelectronics. Its largest use is inElectronic oscillatorsto generatemicrowaves, in applications such asRadar speed gunsandmicrowave relaydata link transmitters.GUNN DIODE

RUSSIAN GUNN DIODEIts internal construction is unlike other diodes in that it consists only ofN-dopedsemiconductormaterial, whereas most diodes consist of both P and N-doped regions.

2. It therefore does not conduct in only one direction and cannotrectifyalternating current like other diodes, which is why some sources do not use the termdiodebut prefer TED.3.Gunn diode is termed as diode because it has two electrodes.

4. In the Gunn diode, three regions exist: two of those are heavily N-doped on each terminal, with a thin layer of lightly n-doped material between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. If the voltage is increased, the current through the layer will first increase, but eventually, at higher field values, the conductive properties of the middle layer are altered, increasing its resistivity, and causing the current to fall. This means a Gunn diode has a region ofnegative differential resistancein itscurrent-voltage characteristiccurve, in which an increase of applied voltage, causes a decrease in current.

5. This property allows it toamplify, functioning as a radio frequency amplifier, or to become unstable and oscillate when it isbiasedwith a DC voltage.

Construction

A schematic diagram of a uniform n-type GaAs diode with ohmic contacts at theend surfaces is shown in Fig.GUNN EFFECT1.)Above some critical voltage, corresponding to an electric field of 2000-4000 volts/cm, the current in every specimen became a fluctuating function of time.

2.) This fluctuation took the form of a periodic oscillation superimposed upon the pulse current.

3.) The frequency of oscillation was determined mainly by the specimen, and not by the external circuit.

4.) The period of oscillation was usually inverselyproportional to the specimen length and closely equal to the transit time of electrons between the electrodes.

From Gunn's observation the carrier drift velocity is linearly increased fromzero to a maximum when the electric field is varied from zero to a threshold value.When the electric field is beyond the threshold value of 3000 Wcm for the n-typeGaAs, the drift velocity is decreased and the diode exhibits negative resistance.

By applying a voltage pulse of 16-V amplitude and 10-ns duration to aspecimen of n-type GaAs 2.5 X 10-3 cm in length.The oscillation frequencyWas 4.5GHz.Gunn found that the period of these oscillations was equal to the transit timeof the electrons through the specimen calculated from the threshold current.

Gunn also discovered that the threshold electric field Eth varied with the lengthand type of material. A specimen of n-type GaAs of length L = 210 x 10^-6 mand cross-sectional area 3.5 x 10^-3 cm2 with a low-field resistance of 16 Ohms. Currentinstabilities occurred at specimen voltages above 59 V, which means that thethreshold field is

Eth=V/L=2810 volts/cm

In some materials (III-V compounds such as GaAs and InP), after an electric field in the material reaches a threshold level, the mobility of electrons decrease as the electric field is increased, thereby producing negative resistance.Electrons (GaAs) can exist in a high-mass low velocity state as wellas their normal low-mass high-velocity state and they can be forced into thehigh-mass state by a steady electric field of sufficient strength. In this state they form clusters or domains which cross the field at a constant rate causing current to flow as a series of pulses. This is the Gunn effect.

GUNN DIODE AS OSCILLATORTHANK YOUYAGNESH ASHAR5TH BTELECOMMUNICATION ENGINEERINGCMRIT