Assertion–Reason (IMPATT Physics): Assertion (A): An IMPATT diode is an avalanche diode. Reason (R): Avalanche breakdown occurs when a p–n junction is reverse biased.

Introduction:
IMPATT (Impact Ionization Avalanche Transit-Time) diodes operate by generating carriers through avalanche multiplication and then using transit-time delay to achieve negative resistance at microwave frequencies. Recognizing the avalanche origin is central to identifying the device class.

Given Data / Assumptions:

• Reverse-biased p–n or p–i–n junction structure.
• High electric field sufficient for impact ionization.
• Microwave operation where transit time is comparable to an RF period.

Concept / Approach:

Under strong reverse fields, energetic carriers ionize lattice atoms, creating electron-hole pairs and initiating avalanche multiplication. The resultant phase delay between RF voltage and current, due both to avalanche buildup and carrier transit through the drift region, produces an effective negative resistance that sustains oscillation or provides gain.

Step-by-Step Solution:

Verification / Alternative check:

IMPATT oscillators and amplifiers are textbook avalanche devices; their noise properties and breakdown voltages reflect avalanche physics.

Why Other Options Are Wrong:

• R not explaining A: incorrect; R directly states the physical trigger for avalanche behavior that defines IMPATTs.
• Claims that A or R are wrong contradict device fundamentals.

Common Pitfalls:

Confusing Zener tunneling breakdown (dominant at lower fields and higher doping) with avalanche breakdown; IMPATT relies on avalanche and transit time.

Final Answer:

Both A and R are correct and R is correct explanation of A