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Determine the resonant frequency of an IMPATT diode having the following specifications:

Carrier drift velocity: 2 × 107 cm/s

Drift-region length: 5 μm

Maximum operating voltage: 100 V

Maximum operating current: 250 mA

Efficiency: 20%

Breakdown voltage: 90 V


1. 16.67 GHz
2. 15 GHz
3. 20 GHz
4. 25 GHz

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Correct Answer - Option 3 : 20 GHz

Concept:

1. An IMPATT diode has n+ - p - i - p + structure and is used with reverse bias. It exhibits negative resistance and operates on the principle of avalanche breakdown. IMPATT diode circuits are classified as broadly tunable circuits, low Q circuits,s, and high Q circuits. The impedance of the IMPATT diode is a few ohms. The word IMPATT stands for Impact Avalanche Transit Time diode.

2. The features of the IMPATT diode oscillator are Frequency 1 to 300 GHz, Power output (0.5 W to 5 W for single diode circuit and up to 40 W for combination of several diodes), the efficiency of about 20%. Its applications include police radar systems, low power microwave transmitters, etc.

IMPATT diode:- Impact Avalanche Transit time diode - This diode exhibits a differential negative resistance by two effects.

1. Impact Ionization effect: This cause the carrier current and ac voltage to be out of phase of 90°

2Transit time effect: Which further delays the external current relative to the ac voltage by 90°

Resonant frequency in IMPATT is given by

\(f = \frac{{{V_d}}}{{2L}}\)

Vd: carrier drift velocity

L: drift region length

The efficiency of IMPATT diode is

\(\eta = \left( {\frac{{{V_a}}}{{{V_d}}}} \right)\left( {\frac{{{I_a}}}{{{I_d}}}} \right)\)

Vd, Id → Voltage and current

Va, Ia → Ac voltage and current.

Efficiency is approx 30%

Calculation:

Given that Vd = 2 × 10cm/s, L = 5 × 10-4 cm

The frequency of resonance

\(\frac{{{V_d}}}{{2L}} = \frac{{2 \ \times \ 10^{7} }}{{2 × 5 \ \times \ 10^{-4}}}\) 

= 20 GHz

Hence option (3) is the correct answer.

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