Microwave transistors – Assertion–Reason on interdigitated geometry and tuning networks Assertion (A): Interdigitated geometry is widely used for microwave transistors to improve high-frequency performance. Reason (R): Input and output tuning of a transistor commonly uses on-chip MOS/MIM chip capacitors for impedance matching.

Introduction:
Interdigitated (multi-finger) geometry is a hallmark of microwave transistors such as GaAs MESFETs, HEMTs, and SiGe/Si BJTs. The arrangement minimizes parasitic resistances and spreads current to achieve large effective periphery. This question checks whether you understand why interdigitated layouts are popular, and whether common tuning practices explain that choice.

Given Data / Assumptions:

• Microwave operation where parasitics dominate performance.
• Multi-finger emitter/gate/source geometries are feasible on chip.
• Matching networks can include MOS/MIM capacitors and thin-film inductors/resistors.

Concept / Approach:

Interdigitated geometry reduces access resistance (Rg, Re, Rs) and current crowding, lowers thermal density, and keeps RC time constants small. This directly improves figures like f_T and f_max. Tuning networks indeed use on-chip capacitors (MOS or MIM) to realize impedance matching and neutralization, but their use is not the reason for the interdigitated structure. They are separate design choices that address different bottlenecks: geometry addresses intrinsic/parasitic device performance; tuning components shape the external match to 50 Ω or other impedances.

Step-by-Step Solution:

Verification / Alternative check:

Process design kits and published layouts show multi-finger gates/bases for RF transistors regardless of whether external matching is on-chip, off-chip, MOS, or MIM—confirming independence of the two facts.

Why Other Options Are Wrong:

Option A: claims R explains A, which is incorrect causality. Option C/D/E conflict with known practice (interdigitated is used; on-chip capacitors are widely used).

Common Pitfalls:

Confusing layout-driven parasitic reduction with network-level matching; assuming the presence of MOS/MIM capacitors dictates device geometry.

Final Answer:

Both A and R are correct but R is not correct explanation of A.