GUNN DIODES

Gunn diode works on the principle of transferred electron effect, which is the phenomenon for generation of Microwave oscillations in bulk semiconductor materials.

This effect is exhibited in Gallium Arsenide (GaAs) and Indium Phosphide (I_nP).

If a relatively small dc voltage is placed across a thin slice of GaAs, then negative resistance will be observed if the voltage gradient across the slice is in excess of about 3,300v/cm. The electron velocity becomes high and hence the oscillations occur at Microwave frequencies. A cavity is the tuned circuit mostly used. GUNN Effect occurs in only n-type bulk material and a domain is formed per cycle, which arrives at the positive end of the slice to excite oscillations in the associated tuned circuit.

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IRISET 81 T3 - FUNDAMENTAL OF ELECTRONICS

Fig.18.7 ENERGY BANDS

NEGATIVE RESISTANCE: Gallium arsenide is one of a fairly small number of semiconductor materials in which an n-doped sample has an empty energy band higher in energy than the highest filled/partly filled band. The size of the forbidden gap between these two is small and this does not apply to Ge or Si. When 'a voltage is applied across a slice of GaAs which is doped so as to have excess electron (n-type), these electron flow as a current towards the positive end of the slice. The greater the potential the higher the velocity with which electrons move towards the positive end and hence greater current. The energy imparted to the electrons is so high due to the high voltage gradient that they get transferred to the higher empty energy band, where they slow down. This give rise to the name transferred electron effect by which electrons are transferred from the conduction band to a higher energy band in which they are less mobile and thus the current has reduced as a result of voltage rise.(as shown in Fig.

18.7). As the applied voltage rises past the threshold negative-resistance value, current falls and when the voltage across the slice becomes sufficient to remove electrons from the higher energy band, current will again increase with voltage. The V-I characteristic is similar to that of a tunnel diode.

The intrinsic frequency of the diode is determined by the drift velocity v and diode length L as f=v/L for GaAS, v=10⁷ Cm/s, independent of applied voltage.

The diode is normally mounted in a cavity resonator and the cavity is tuned for intrinsic frequency of the diode.

GUNN DOMAINS: It is reasonable to expect that the density of doping material is not completely uniform throughout the sample of GaAs. Hence there will be a region, near the negative end, where the impurity concentration is less than the average. In such an area there are fewer free electrons than in other areas and so there will be a greater than average potential across it. Thus as the total voltage is increased, this region will be the first to have a voltage gradient across it large enough to induce transfer of electrons

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to the higher energy band. This region becomes a negative-resistance domain and the whole domain moves towards the positive end at 10⁷ cm/s velocity.

As soon as some electrons have been transferred to the less conductive energy band, fewer free electrons are left behind and the potential gradient across it increases. When this domain reaches the positive end, a pulse is received by the associated tank circuit and starts it into oscillations. With the usual applied voltage, once a domain forms, insufficient potential is left across the rest of the slice to permit another domain to form.

When the domain in a short sample arrives at the anode, there is once again sufficient potential for formation of another domain. Thus one domain is formed per RF cycle, which gives energy to the oscillation. A good equivalent circuit at X-band for the Gunn diode may be assumed as consisting of a negative resistance of 100 ohms in parallel with 0.6pF. It may require 9 volt dc- bias, 950 mA but consumes 8.5 Watts for producing 250 mw, leading to an efficiency of only 3%. Gunn diode is employed as low and medium power oscillators in Microwave receivers and as frequency modulated transmitter in CW Doppler radars.

18.10 IMPATT DIODE: Under certain reverse-bias condition, a p-n junction exhibits a negative ac resistance, which can be used to sustain oscillations or for amplifications. The name impatt is derived from impact ionization, avalanching and transit time drift mechanisms involved. Negative resistance can be defined as that property of a device which causes the current through it to be 180^O out of phase with the voltage across it. A combination of delay involved in generating avalanche current multiplicities, together with delay due to transit time through a drift space, provides the necessary 180^O phase difference between applied voltage and the resulting current is impatt diode. An extremely high voltage gradient is applied to the IMPATT diode, of the order of 400 kV/cm, eventually resulting in a very high current. Such a high potential gradient, back-biasing the diode causes a flow of minority carriers across the junction. If it is now assumed that oscillation exist the effect of a positive swing of the RF voltage super imposed on high dc voltage is considered now. Electrons and hole velocity has now become so high that these carriers form additional holes and electrons by knocking them out of the crystal structure, by impact ionization. These additional carriers continue the process at the junction, which leads to an avalanche. If the original dc field was just at the threshold of allowing this situation to develop, this voltage will be exceeded during the whole of the RF positive cycle and the avalanche current multiplication will be taking place during this entire time.

Since avalanche is not instantaneous, the current pulse maximum occurs at the instant when the RF voltage across the diode is zero and going negative, producing a 90^O phase difference between the voltage and current. Because of the reverse bias, the current pulse flows to the cathode, at a drift velocity dependent on the presence of the

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IRISET 83 T3 - FUNDAMENTAL OF ELECTRONICS

high dc field. The thickness of the n+ region is so selected that the time taken for the current pulse to arrive at the cathode corresponds to a further 90^O phase difference.

Accordingly the' voltage and current in the IMPATT diode are 180^O out of phase and a dynamic RF negative resistance has been proved to exist.(as shown in the Fig 18.8B).

IMPATT diodes are made of either silicon or GaAs. GaAs gives lower noise, higher efficiency and higher maximum operating frequencies.

But silicon can give higher output powers than GaAs for commercial diodes. The width of the drift region is determined by d= 0.37 X T X 10^-7 where T is the periodic time of microwave signal. The equivalent circuit can be shown as below in Fig. 18.8C. The diode chip consists of a resistor RD in series with a capacitor Cj, which is the capacitance of the junctions at the breakdown voltage. RD consists of two components in series; Rs the series lead resistance and negative resistance representing impatt action.

The overall value of RD is a negative number, which depends on both bias current and signal current. For a given bias current and load, oscillator operation will stabilize at tile point RD = RL, but for amplification, RL is always made higher than RD. It is used in 7D17 radio transmitter as an amplifier.

CATHODE

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Subjective:

1. What are Semiconductor microwave devices?

2. Explain the microwave bipolar transistors with their geometries 3. Draw and explain the working of a VARACTOR DIODE.

4. Draw and explain the working of a SCHOTTKY-BARRIER DIODE.

5. Draw and explain the working of a STEP RECOVERY DIODE.

6. Draw and explain the working of a PIN DIODE.

7. Draw and explain the working of a GUNN DIODE.

8. Draw and explain the working of a IMPATT DIODE.

9. Write one application of the following:

a. VARACTOR DIODE

b. SCHOTTKY-BARRIER DIODE c. PIN DIODE

d. GUNN DIODE.