A charger supplies 100 W at 20 V for charging the battery of a laptop. The power devices, used in the converter inside the charger, operate at a switching frequency of 200 kHz. Which power device is best suited for this purpose?

The correct option is MOSFET.

To determine the best suited power semiconductor device for the given charger converter, we analyze the operating parameters provided in the problem:
1. Power Rating: $P=100\text{W}$ (Low power)
2. Voltage Rating: $V=20\text{V}$ (Low voltage)
3. Switching Frequency: $f=200\text{kHz}$ (High frequency)

Let's evaluate the characteristics of the given power semiconductor options under these conditions:

1. Thyristor (SCR):
Thyristors are designed for high-power, high-voltage applications (typically in the megawatt range) and operate at very low switching frequencies (usually less than 1 kHz to a few kilohertz) because they are slow-switching devices. Therefore, they are not suitable for a high-frequency (200 kHz) low-power converter.

2. Bipolar Junction Transistor (BJT):
BJTs are current-controlled devices with moderate switching speeds (typically limited to below 20-50 kHz) due to storage charge effects during turn-off. Operating a BJT at 200 kHz would result in extremely high switching losses and efficiency degradation.

3. Insulated Gate Bipolar Transistor (IGBT):
IGBTs combine the high input impedance of a MOSFET with the low conduction loss of a BJT. They are ideal for medium-to-high voltage (above 400 V) and medium-power applications, operating at medium switching frequencies (typically up to 20-50 kHz). At 200 kHz, switching losses in an IGBT become prohibitively high.

4. Power MOSFET:
Power MOSFETs are majority carrier, voltage-controlled devices capable of very fast switching speeds. They are exceptionally well-suited for low-voltage (less than 200 V), low-to-medium power (up to a few hundred watts) applications operating at high switching frequencies (ranging from 100 kHz to several megahertz). At low voltage ratings (20 V), the on-resistance ($R_{\text{DS(on)}}$) of a MOSFET is extremely small, leading to minimal conduction losses, while its fast switching capability ensures low switching losses even at 200 kHz.

Thus, because the application requires low voltage (20 V), low power (100 W), and high switching frequency (200 kHz), the MOSFET is the most suitable power device.