Shown in Figure P9. 22 are the circuit symbols for a...

Shown in Figure P9. 22 are the circuit symbols for a depletion-mode and an enhancement-mode MOSFET. For the enhancement-mode MOSFET: $$ I_{D S s}=7 \mathrm{~mA} \quad V_T=+5 \mathrm{~V} $$ For the depletion-mode MOSFET: $$ I_{D 5 s}=7 \mathrm{~mA} \quad V_r=-5 \mathrm{~V} $$ a. Are these $n$ - or $p$-channel devices? b. Which is the depletion-mode device? The enhancement-mode device? c. For each device, state the conditions for the operation in the active region in terms of the voltages shown for the device and the threshold or pinch-off voltages given above.

Shown in Figure P9. 22 are the circuit symbols for a depletion-mode and an enhancement-mode MOSFET.
For the enhancement-mode MOSFET:

$$
I_{D S s}=7 \mathrm{~mA} \quad V_T=+5 \mathrm{~V}
$$

For the depletion-mode MOSFET:

$$
I_{D 5 s}=7 \mathrm{~mA} \quad V_r=-5 \mathrm{~V}
$$

a. Are these $n$ - or $p$-channel devices?
b. Which is the depletion-mode device? The enhancement-mode device?
c. For each device, state the conditions for the operation in the active region in terms of the voltages shown for the device and the threshold or pinch-off voltages given above.

Key Concepts

Active Region Conditions

For both enhancement-mode and depletion-mode MOSFETs, the active (or saturation) region is reached when the drain-to-source voltage (VDS) is sufficiently high relative to the effective threshold (or pinch-off) voltage. For enhancement-mode devices, the condition is typically VGS > VT (where VT is positive) and VDS > (VGS ? VT). For depletion-mode devices, while the transistor is normally on, the active region requires that the gate-to-source voltage remains above the level that would fully deplete the channel (i.e., above the negative pinch?off voltage) and that VDS is larger than (VGS ? Vp), ensuring that the channel is pinched off at the drain end and the device operates in saturation.

MOSFET Operating Modes

MOSFETs (Metal–Oxide–Semiconductor Field-Effect Transistors) can operate in different modes depending on the device structure and biasing. The two primary modes discussed here are enhancement mode—where the device is normally off at zero gate bias and requires a gate voltage above a certain positive threshold to form a conductive channel—and depletion mode—where the device is normally on at zero gate bias and must be turned off by applying a negative gate voltage (for n?channel devices) that depletes the conduction channel.

N-channel vs P-channel Devices

MOSFETs are characterized by the type of charge carriers that form the conduction channel. In n-channel devices, electrons are the majority carriers and conduction is typically enabled by applying a positive gate voltage relative to the source. In contrast, p-channel devices rely on holes and require a negative gate voltage for conduction. In the context of the provided parameters (positive threshold for one device and negative pinch-off value for the other), the devices are identified as n-channel because a positive voltage is used to turn the enhancement-mode device on and a negative voltage is used to pinch off the depletion-mode device.

Enhancement-mode MOSFET

An enhancement-mode MOSFET is a normally off transistor that requires a gate-to-source voltage (VGS) exceeding a positive threshold voltage (VT) to induce an inversion layer, creating a conductive channel. In the active or saturation region, the device conducts when VGS > VT, and the drain-to-source voltage (VDS) is high enough (typically VDS > VGS ? VT) to ensure that the channel is pinched off near the drain, resulting in a region where the drain current is relatively constant.

Depletion-mode MOSFET

A depletion-mode MOSFET is a normally on transistor that already has a conductive channel at zero bias. It can be turned off or modulated by applying a gate voltage that depletes the channel. For n-channel depletion devices, the pinch-off voltage (often denoted by Vp or VRS) is negative. The device remains in its active region as long as the gate bias does not exceed this negative pinch-off value, and conduction will typically be maintained if VGS is more positive than the pinch?off threshold, with saturation occurring when VDS exceeds the difference between VGS and Vp.

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