Impatt Diodes

       IMPATT diode means Impact Ionization Avalanche Transit Time Diode.

       Its operation depends on the reverse breakdown voltage characteristics of p-n junction and phase delay of the applied RF signal.

1.Mechanism of Oscillation of IMPATT Diode 

       In a diode resonator circuit, noise voltage excites resonant component. The reverse biased d.c. field and a.c. field make the diode to swing into and out of the avalanche condition. The carriers drift to the end contacts before the diode swings out of the avalanche region. This happens as the hole drift time is very short. As a result, the a.c. field takes energy from the carriers or d.c. bias source. This process builds the microwave oscillations in the circuit.

       The frequency of oscillations is given by 

Here, v = Drift velocity of the holes.

      l = Length of the drift region.

2.Principle of Operation of IMPATT Diode

       It is basically a p-n junction diode and is reverse biased to breakdown. In this, an avalanche of electron-hole pairs are created by impact ionization in the high field depletion layer of the device. The oscillations are produced by the transit of the field carriers through the depletion layer. 

       The negative resistance of IMPATT diodes is due to the impact ionization and the transit time effects.

       The IMPATT diode and avalanche pulse are shown in Fig. 1. 

       The transit angle is given by,

 Here, Ԏ = Transit time.

           ω = 2πf,

       The avalanche resonant frequency is given by

 Here, A = Diode cross section

           I = Current.

           ε_s = Semiconductor permittivity.

            α = Ionization coefficient.

           α= ∂α/∂ε

       The IMPATT diode is characterized by an impedance consisting of both resistive and reactive parts. The resistive part R_d is given by

       Here, R_p = Resistance of inactive region.

       In terms of transit time, the frequency of the diode is given by

       Here, Ԏ = Transit time.

3. V-I Characteristics of IMPATT Diode

       The V-I characteristics are shown in Fig 2.

4. Efficiency of IMPATT Diode

       It is given by

5. Salient Features of IMPATT Diode

. It is an avalanche transit time device.

. It can be made with germanium, silicon and gallium arsenide.

. It is useful upto 100 GHz.

. It provides highest continuous power compared to other semiconductor devices.

. Its efficiency is less than 30%.

. It is useful at microwave, millimeter and submillimeter wave frequencies.

. IMPATT diodes are popularly used in digital and analog communication and radar systems.

. It can be used as an amplifier and an oscillator.

 6. Equivalent Circuit of Impact Diode

       An equivalent circuit of IMPATT diode is shown in Fig 3.

       In this circuit,

       The load impedance is given by

       In the diode mount, the reactance of the circuit is made zero by controlling the inductance. In the circuit.

       R_md = Diode negative resistance

       C_j = Junction capacitance

       L_p = Package lead inductance.

       C_p = Capacitance between package terminals.

       R_L = Loads resistance.

       Quarter wave transformer is used tomatch the low resistance diode with the load.

7. Applications of IMPATT Diode

       It is used

. as microwave oscillator..as modulated oscillator.

. as receiver local oscillator.

. as parametric amplifier pump.

. in radar reception..in communication transmission.

. as negative resistance amplifier.

8. Advantages of IMPATT Diode

. Wideband

. Pulse power is high

. Suitable for high frequency use

9. Disadvatages of IMPATT Diode

. Noise device

. Tuning range is not high

. Noise Figure is high and is about 80 DB

Shockley Diode

       Shockley diodes are very similar to tunnel diodes in that they have a negative resistance characteristic. They are actually classified as a thyristor because they operate in only two states, on or off. The Shockley diode is constructed by joining four semiconductor layers together to form a PNPN junction, Figure 1. The Shockley diode exhibits a very high resistance, like that of an open switch, until the voltage across it reaches a point called the breakover  voltage. When breakover voltage is reached, the diode suddenly exhibits a very low resistance and begins to conduct, as in Figure 2. Once the Shockley diode begins to conduct , the voltage across it will suddenly drop to about 1 volt. Once conduction begins, it will continue to conduct until the current falls below a certain level called the holding current level.

       The negative resistance characteristic permits the Shockley diode to be used as a simple relaxation oscillator, Figure 3. In this example circuit, the battery voltage is greater than the breakover voltage for the Shockley diode, but the value of the series resistor is high enough to limit the maximum circuit current to a value that is below the diode's holding current level. When switch S1 is closed, the capacitor will begin to charge through the series resistor. When the voltage across the capacitor reaches the breakover value for the Shockley diode, it will turn on and discharge the capacitor. When the current level falls below the holding current value, the diode will turn off and the capacitor will begin charging again.