Choose the correct statement(s)
i) The gate circuit impedance of MOSFET is higher than that of a BJT

Question

Choose the correct statement(s)
i) The gate circuit impedance of MOSFET is higher than that of a BJT
ii) The gate circuit impedance of MOSFET is lower than that of a BJT
iii) The MOSFET has higher switching losses than that of a BJT
iv) The MOSFET has lower switching losses than that of a BJT

Examoogle · Accepted Answer

Both i & iv Explanation: The MOSFET is a voltage-controlled device with an insulated gate, resulting in extremely high input impedance, whereas the BJT is a current-controlled device with a forward-biased junction, leading to lower input impedance. Furthermore, MOSFETs have negligible minority carrier storage effects compared to BJTs, which allows for significantly faster switching and reduced switching losses at high frequencies. Working Principle: In a MOSFET, the gate is isolated from the channel by a thin $SiO_{2}$ layer, forcing the gate current to be essentially leakage current, thus resulting in near-infinite DC impedance. In a BJT, the base-emitter junction behaves like a forward-biased diode, requiring a continuous base current $I_{B}$ to maintain the collector current $I_{C}$, resulting in significantly lower impedance. Regarding switching, BJTs suffer from 'storage time' due to minority carrier recombination in the base, whereas MOSFETs switch solely by charging/discharging the gate capacitance $C_{iss}$. Key Formulas: $Z_{in(MOSFET)} \approx \infty$ — Ideal gate impedance; $I_{C} = \beta I_{B}$ — BJT current gain relationship; $P_{sw} \approx f_{sw} \cdot E_{on/off}$ — Switching loss dependency on frequency Key Points: MOSFET is a Voltage-Controlled Device (VCD).; BJT is a Current-Controlled Device (CCD).; Switching loss in MOSFETs is primarily gate drive power and capacitive discharge.; Switching loss in BJTs is dominated by storage time and fall time due to minority carriers. Advantages: High input impedance makes driving easier.; Higher switching frequency capability.; No minority carrier storage time. Disadvantages: MOSFETs suffer from high on-state resistance ($R_{DS(on)}$) at high voltages.; BJTs provide better current density in high-power, low-frequency applications. Applications: SMPS (Switching Mode Power Supplies); Inverters and Motor Drives; High-speed digital logic Additional Info: Option i is correct as the MOS gate is capacitively coupled.. Option iv is correct as MOSFETs lack minority carrier storage, enabling faster switching and lower losses.. Option ii is incorrect as MOSFET impedance is significantly higher, not lower.. Option iii is incorrect as BJTs have higher switching losses due to their inherent storage time delay. Answer: C is correct — MOSFETs offer higher gate impedance due to the oxide layer and exhibit lower switching losses compared to BJTs because they are majority-carrier devices without minority carrier storage delay. Exam Tip: Always remember: MOSFET = Majority carrier, Voltage controlled; BJT = Minority carrier, Current controlled.

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