Depletion mode MOSFETs are commonly used as resistors because of their inherent characteristics and behavior. Let's explore the reasons why depletion mode MOSFETs are suitable for resistor applications:

1. Depletion mode operation:
Depletion mode MOSFETs are designed to operate in the depletion mode, which means that they are normally conducting when no voltage is applied to the gate terminal. In this mode, the channel between the source and drain is already established, allowing current to flow even without any external biasing. This behavior makes depletion mode MOSFETs ideal for resistor applications.

2. Variable resistance:
By applying a voltage to the gate terminal of a depletion mode MOSFET, the channel between the source and drain can be modulated, effectively changing the resistance of the device. Increasing the gate voltage reduces the channel width, resulting in a higher resistance. Conversely, decreasing the gate voltage increases the channel width, resulting in a lower resistance. This variable resistance characteristic allows depletion mode MOSFETs to be used as adjustable resistors.

3. Precise resistance control:
Depletion mode MOSFETs offer precise control over the resistance value due to their linear relationship between gate voltage and channel resistance. By accurately adjusting the gate voltage, the resistance can be finely tuned to achieve the desired value. This level of control makes depletion mode MOSFETs suitable for applications that require precise resistance values.

4. Wide resistance range:
Depletion mode MOSFETs can provide a wide range of resistance values. By selecting MOSFETs with different channel lengths, widths, and threshold voltages, a broad spectrum of resistance values can be achieved. This versatility allows designers to choose the most appropriate depletion mode MOSFET based on the specific resistance requirements of their circuit.

5. Low power dissipation:
Depletion mode MOSFETs exhibit low power dissipation because they operate in the depletion mode, where no gate current is required. This characteristic makes them energy-efficient and suitable for applications where power consumption needs to be minimized.

In conclusion, depletion mode MOSFETs are commonly used as resistors due to their depletion mode operation, variable resistance, precise resistance control, wide resistance range, and low power dissipation. These characteristics make them an excellent choice for applications that require adjustable resistors with high precision and efficiency.