Switching devices and ideal ON/OFF behavior Which device(s) exhibit nearly ideal low ON resistance and extremely high OFF resistance over typical operating ranges?

Introduction / Context:
Different switching technologies display different ON and OFF resistance characteristics. Understanding which devices approximate the ideal open circuit and short circuit assists in selecting components for precision switching, isolation, and low-leakage applications.

Given Data / Assumptions:

• Compare electromechanical relays, manual switches, and semiconductor switches.
• ‘‘Ideal’’ ON means very low contact resistance; ‘‘ideal’’ OFF means extremely high resistance with negligible leakage.
• Normal operating voltages and currents; no contact contamination or arcing damage assumed.

Concept / Approach:
Metal-to-metal mechanical contacts (relays, toggles, pushbuttons) can approximate a short when closed and an open when opened. Semiconductor devices, even when ‘‘off’’, exhibit leakage currents and finite OFF resistance, and when ‘‘on’’ they have a non-zero voltage drop (e.g., diode forward drop, MOSFET Rds(on)).

Step-by-Step Solution:
1) Evaluate relays: contacts provide milliohm-level ON resistance and megaohm to gigaohm OFF resistance → near-ideal behavior.2) Evaluate manual switches: similar mechanical contacts → near-ideal ON/OFF behavior.3) Evaluate semiconductors: off-state leakage (microamps to milliamps) and on-state voltage drops or Rds(on) → not ideal.4) Therefore, devices that best approximate ideal behavior are relays and manual switches.

Verification / Alternative check:
Datasheets show relay contact resistance typically < 100 mΩ and insulation resistance > 100 MΩ, while MOSFETs specify finite Rds(on) and leakage Idss or Igss, confirming the relative differences.

Why Other Options Are Wrong:
Electromechanical relays only / Manual switches only: Each alone ignores the other mechanical option that also behaves nearly ideally.Semiconductor devices: Significant on-state drop and off-state leakage prevent ‘‘ideal’’ ON/OFF.All of the above: Incorrect because semiconductors do not meet the near-ideal criterion.

Common Pitfalls:
Overlooking contact resistance rise due to wear/oxidation, or assuming MOSFETs are ideal due to low Rds(on); they still have leakage and body diodes that impact isolation.

Final Answer:
Electromechanical relays and manual switches