---
title: "Cutoff Region in Intro to Electrical Engineering"
description: "Cutoff region is the FET state where gate-source voltage stays below threshold voltage, so drain current is nearly zero and the transistor acts off."
canonical: "https://fiveable.me/introduction-electrical-systems-engineering-devices/key-terms/cutoff-region"
type: "key-term"
subject: "Intro to Electrical Engineering"
unit: "Unit 12"
---

# Cutoff Region in Intro to Electrical Engineering

## Definition

Cutoff region is the FET operating state where the transistor is effectively off, because the gate-source voltage is below threshold voltage. In Intro to Electrical Engineering, you use it to describe when a MOSFET stops conducting between drain and source.

## What It Is

Cutoff region is the FET operating region where the channel is not formed strongly enough for current to flow from drain to source. In this Intro to Electrical Engineering course, that usually means the gate-source voltage is below the threshold voltage, so the transistor behaves like an open switch.

For an n-channel MOSFET, cutoff happens when \(V_{GS} < V_T\). The gate is not creating the electric field needed to attract enough carriers into the channel, so the drain current \(I_D\) is ideally near zero. In real circuits, there can still be tiny leakage currents, but for circuit analysis you usually treat the device as off.

This is one of the first FET ideas that matters in DC analysis and biasing. Biasing is about choosing voltages that place the transistor where you want it, and cutoff is the simplest place to put a FET when you want no conduction. If you raise the gate voltage above threshold, the device can move into the linear region or saturation region depending on the rest of the circuit.

A common mistake is mixing up cutoff with a broken transistor. Cutoff is a normal operating state, not a failure mode. Another mistake is checking only drain voltage and forgetting that the gate-to-source voltage is what decides whether the channel turns on.

You can think of cutoff as the "off" half of a digital-like switch behavior. In a logic gate, that off state helps create clean binary levels. In analog biasing problems, though, cutoff is more than just off, it tells you whether the transistor can actually set up the circuit current you calculated.

## Why It Matters

Cutoff region shows up anytime you analyze whether a FET is conducting at all. In Intro to Electrical Engineering, that means you use it to check bias points, decide if a transistor is acting like a switch, and predict whether a circuit node is being pulled low or left floating.

It also gives you the starting point for reading FET equations and operating-region graphs. If a problem gives you \(V_{GS}\), \(V_T\), and a circuit setup, cutoff is often the first region you test before moving on to saturation or linear operation. That keeps you from plugging values into the wrong current equation.

In digital circuits, cutoff is part of how logic states are built. An "off" transistor should block current and keep power use low, which is why cutoff matters in switching and low-power design. In analog biasing, by contrast, cutoff can tell you that your chosen bias voltage is too low for amplification, so the circuit will not behave the way you intended.

## Connections

### threshold voltage

Threshold voltage is the cutoff boundary you compare against in MOSFET problems. If \(V_{GS}\) stays below \(V_T\), the device is in cutoff and the channel does not form properly. Many biasing questions are really asking you to check that relationship before doing anything else.

### [saturation region](/introduction-electrical-systems-engineering-devices/key-terms/saturation-region)

Saturation region is what you get after the FET is turned on and the channel pinches off near the drain. Cutoff is the opposite starting point, where current is essentially blocked. When you solve a DC analysis problem, you usually decide between cutoff and saturation by checking the gate-source voltage first.

### [linear region](/introduction-electrical-systems-engineering-devices/key-terms/linear-region)

Linear region happens when the FET is on but still behaving like a voltage-controlled resistor. Cutoff and linear region are separated by the threshold condition, so if the gate voltage is too low, the transistor never reaches the resistive on-state. That makes the comparison useful in switch-style circuits.

### [self-bias](/introduction-electrical-systems-engineering-devices/key-terms/self-bias)

Self-bias circuits use resistor feedback to set a stable operating point without a fixed external gate voltage. Whether the transistor ends up in cutoff depends on the chosen resistor values and source voltage. That makes cutoff a common thing to check when you evaluate bias stability.

## On the AP Exam

A quiz problem will often give you a MOSFET circuit and ask you to determine the operating region before finding current or voltages. Your first move is to compute \(V_{GS}\) and compare it to the threshold voltage. If \(V_{GS} < V_T\), you identify cutoff and set drain current to nearly zero for the ideal analysis.

In a problem set or lab write-up, you may also explain why a transistor is not switching on, especially if the output looks stuck high or the circuit is not drawing current. Good answers usually mention the gate-source voltage, the threshold voltage, and the fact that cutoff is a normal off state rather than a failure. If a graph or output characteristic is given, you identify the low-current region that corresponds to cutoff.

## cutoff region vs threshold voltage

Threshold voltage is the voltage value that marks the start of conduction, while cutoff region is the operating state below that point. In other words, threshold voltage is the boundary, and cutoff is what the transistor is doing on the low-voltage side of that boundary.

## Key Takeaways

- Cutoff region is the FET off state, where drain current is ideally near zero.
- For a MOSFET, cutoff usually means the gate-source voltage is below threshold voltage.
- Cutoff is a normal operating region, not a damaged transistor or a wiring mistake by itself.
- In DC biasing, checking cutoff is often the first step before trying current equations for on-state operation.
- In digital-style circuits, cutoff helps a transistor act like an open switch with very low input current.

## FAQs

### What is cutoff region in Intro to Electrical Engineering?

Cutoff region is the FET operating state where the transistor is effectively off and drain current is ideally zero. You get there when the gate-source voltage is below the threshold voltage, so the channel does not conduct between drain and source.

### How do you know if a MOSFET is in cutoff?

Check the gate-source voltage and compare it to the threshold voltage. If \(V_{GS} < V_T\), the MOSFET is in cutoff for the ideal model. In circuit problems, that usually means you treat \(I_D\) as zero or nearly zero.

### Is cutoff the same as off?

For most Intro to Electrical Engineering problems, yes, cutoff is the off state. The useful detail is that it is a specific operating region defined by voltages, not just a vague idea of being off. That distinction matters when you choose the right equations.

### What is the difference between cutoff region and saturation region?

Cutoff is when the transistor is not conducting, while saturation is when the transistor is on and the drain current is mainly controlled by gate voltage. They sit on opposite sides of the threshold condition, so you usually identify cutoff first before checking whether the device has moved into saturation.

## Related Study Guides

- [12.2 DC analysis and biasing of FETs](/introduction-electrical-systems-engineering-devices/unit-12/dc-analysis-biasing-fets/study-guide/HAAk1plSupJ4CSNr)

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