Question

At very high frequencies, MOSFET is preferred because of:

• A. Infinite gain
• B. Low threshold
• C. High \(\beta\)
• D. No minority carrier storage

Hint:

MOSFETs are unipolar devices \(\implies\) No minority carriers \(\implies\) Zero minority carrier storage time \(\implies\) Fast switching capability at high frequencies.

Answer 1

The Correct Option is D

Concept: Field-Effect Transistors (MOSFETs) and Bipolar Junction Transistors (BJTs) differ in their primary charge conduction mechanisms:
• BJT (Bipolar device): Relies on both majority and minority charge carriers. When switched into saturation, excess minority carriers accumulate in the base region. When turning the device off, these stored charges must recombine or be swept out first, creating a delay known as storage time (\(t_s\)). This restriction limits the maximum operating switching frequency of BJTs.
• MOSFET (Unipolar device): Relies exclusively on majority charge carrier conduction through an induced channel. Because minority carriers do not participate in channel conduction, there is no minority carrier storage charge accumulation during operation.

Step 1: Identifying the operational limitations at high frequencies.
As the operating signal frequency increases into the megahertz or gigahertz range, the total time period allocated for a single switching cycle shrinks. Any internal propagation delay, such as the minority carrier storage time found in bipolar devices, slows down the response and causes severe switching power losses.

Step 2: Evaluating the benefits of a unipolar structure.
Because a MOSFET is a majority-carrier device, it does not exhibit minority carrier storage delays. Its switching speed is limited only by the parasitic RC time constants of its internal charging capacitances (such as the gate-to-source capacitance \(C_{gs}\) and gate-to-drain capacitance \(C_{gd}\)). Since these capacitors can charge and discharge quickly given adequate drive current, MOSFETs can operate at high speeds, making them preferable for high-frequency applications. This matches Option (D).