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°
2. Transit 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 × 107 cm/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.
