Physics › Electronics › The MOSFET as a switch
The MOSFET as a switch
The gate voltage of a MOSFET controls the current between drain and source, and no channel forms until it passes the threshold voltage. Because the gate sits on an insulating oxide layer it draws no steady current, so a logic output rated in milliamps switches a load drawing amps.
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Discrete semiconductor devices, part 1 of 2. Part 2 is Zener diodes and semiconductor sensors.
Builds on Current-voltage characteristics and Potential dividers.
IN THIS TOPIC
- Read a MOSFET's drain characteristic, and use the threshold voltage to say whether the channel is open.
- Explain how a logic-level gate voltage switches a load, and why the gate itself takes no steady current.
COMMON MISCONCEPTION
Switching a heavy load needs a heavy current into the transistor's control terminal.
A MOSFET is controlled by a voltage, not a current. Its gate sits on an insulating oxide layer, so only a negligible leakage current enters it: the driver supplies the brief charge the gate capacitance needs, and a logic pin rated at a milliamp can switch amps.
A gate that takes no current
A MOSFET has three terminals. The load current flows between the drain and the source, and the gate controls whether it flows at all. The gate sits on a thin insulating layer of oxide, out of contact with the silicon beneath, so it behaves as a very small capacitor rather than as a way in. Put a voltage on it and its field reaches through the oxide and draws carriers into a conducting channel underneath.
Nothing useful happens until the gate-source voltage passes the threshold voltage . Below it no proper channel forms, so for any drain voltage within the maker's rating the drain current is negligible, a leakage far smaller than the current the device carries when it is on. Driven past that rating any MOSFET breaks down and conducts, which is a failure rather than a mode of operation.
Past the threshold the channel opens and widens, and provided the drain voltage is large enough to keep the channel saturated, the drain current is approximately proportional to the square of the excess above threshold. Those two statements define the model used here: an idealised switch, off below threshold and square-law above it, operating within its ratings.
WORKED EXAMPLE
One volt over, two volts over, three
The MOSFET drawn above has = 2.0 V, and at = 3.0 V it passes 0.10 A. Predict the drain current at 4.0 V and at 5.0 V, taking the square-law model to hold with the channel saturated.
The excess above threshold is 1.0 V, 2.0 V and 3.0 V in turn, and on the square law the current follows its square.
Doubling the excess quadruples the current, giving 4 × 0.10 = 0.40 A. Three times the excess gives nine times the current, 9 × 0.10 = 0.90 A, the top of the drawn curve.
That doubling test is the quick check on any drain characteristic. A straight line through those points would be the wrong shape entirely.
Switching a load
That characteristic makes the MOSFET a good switch. Wire the load in series with the drain and drive the gate from a logic output. At 0 V the channel is shut, the drain-source resistance is megohms, and the load sees microvolts. At a gate voltage comfortably past the threshold, 5 V on a logic-level device whose data sheet specifies its low on-resistance there, the channel is fully enhanced, its resistance falls to a fraction of an ohm, and almost the whole supply lands on the load. Reaching the threshold alone is not enough: the threshold is where conduction begins, and the guaranteed low resistance arrives at the higher gate voltage the data sheet names.
Work the on state as a potential divider, modelling the lamp as a constant 24 Ω at its working temperature. Lamp and channel carry 12/24.05 = 0.499 A between them, the lamp keeps 11.98 V, and the MOSFET dissipates = 0.012 W against nearly 6.0 W in the lamp. A component wasting a hundredth of a watt to control six of them needs no heat sink.
The gate is insulated, so only a negligible leakage current enters it once it is charged. A logic output has only to charge a gate capacitance of a nanofarad or two, which it does in microseconds, and in between it supplies almost nothing.
A MOSFET uses a small gate voltage to control a large drain current.
Holding a rail steady has moved to the next part of this lesson.
Light into current has moved to the next part of this lesson.
Measuring a field, and noticing a magnet has moved to the next part of this lesson.
ASSESSMENT FOCUS
- Use the word channel when explaining the threshold. Below no channel exists, so the drain current is negligible at any drain voltage within the device's rating, and above it, in saturation, the current rises approximately with the square of the excess.
- For a switching calculation put the channel resistance in series with the load and divide. Quote the load's pd, and the power lost in the MOSFET as if the question asks.
- Mention the insulated gate whenever a MOSFET is compared with another switching device: no steady gate current flows, so the driver supplies only the charge the gate capacitance needs to change state.
CHECK YOURSELF
A MOSFET with a threshold voltage of 2.0 V switches a 24 Ω lamp across a 12 V supply, and its channel resistance is 0.05 Ω with the gate at 5 V. State the drain current with the gate at 1.5 V. Find the lamp's pd and the power wasted in the MOSFET with the gate at 5 V. Explain how a logic output rated at 1 mA can drive it.
Show a hint
Below the threshold nothing flows. Above it, treat the open channel as a small resistance in series with the lamp.
Show the answer
At 1.5 V the gate is below the threshold, so no channel forms and the drain current is negligible, effectively zero beside the half-amp the lamp draws once the switch is on.
With the gate at 5 V the series resistance is 24.05 Ω, so the current is 12/24.05 = 0.499 A and the lamp holds 0.499 × 24 = 11.98 V.
The MOSFET dissipates = 0.4992 × 0.05 = 0.012 W, against 6.0 W in the lamp.
The gate is insulated from the channel, so it draws no steady current. The logic output supplies only the brief charging current of the gate capacitance, well within 1 mA.
No channel below the threshold voltage, so the drain current is negligible; in saturation above it the drain current climbs roughly with the square of the excess.
A MOSFET gate is insulated, so it takes no steady current and a logic pin can switch amps.
WORKBOOK
Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.
Or read them with their mark schemes on the MOSFET as a switch questions page.
USED LATER IN
CHECK YOUR PROGRESS
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- Read a MOSFET's drain characteristic, and use the threshold voltage to say whether the channel is open.
- Explain how a logic-level gate voltage switches a load, and why the gate itself takes no steady current.
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