MOSFET

In this article, we will explore the structure and operating principle of the MOSFET. This is a semiconductor device that is widely used in medium-power circuits. 

The MOSFET operates on the field effect principle to control current flow. It features very high input impedance, making it suitable for amplifying weak signals. MOSFETs are widely used in monitor power supplies, computer power supplies, and switching applications. 

MOSFET Symbol

A MOSFET has four terminals: Drain (D), Source (S), Gate (G), and Body (B). The body is usually connected to the source, making it effectively a three-terminal device similar to a BJT:

• Gate (G) ≈ Base (BJT)

• Drain (D) ≈ Collector (BJT)

• Source (S) ≈ Emitter (BJT)

Classification of MOSFETs and Their Operating Principles 

MOSFETs are classified into two main types based on their operating modes:

• Enhancement Mode MOSFET (E-MOSFET)

  • When no voltage is applied to the gate, the device is non-conductive.

  • Once the gate-to-source voltage reaches a sufficient level, the device becomes conductive and its current-driving capability is greatly enhanced.

• Depletion Mode MOSFET (D-MOSFET or DE-MOSFET)

  • When no gate voltage is applied, the device is already conductive.

  • Applying either a positive or negative voltage to the gate reduces the device’s conductivity (depletion effect).

MOSFETs are further categorized based on the channel type: N-channel or P-channel:

The resistance of the channel depends on the number of free electrons within it, and the number of free electrons is determined by the potential at the gate terminal. Since the density of free electrons forms the conductive channel, the current through the channel increases as the gate voltage increases. 

A thin layer of silicon dioxide (SiO₂) lies on top of the substrate. Although aluminum oxide (Al₂O₃) can also be used, SiO₂ is more commonly employed. This thin layer functions as a dielectric insulator.

On top of the SiO₂ layer is an aluminum plate. Together, the aluminum plate, the dielectric layer, and the semiconductor substrate form a capacitor within the device.

The two p-type semiconductor regions are connected to form the source and drain terminals. The electrode connected to the aluminum plate is the gate terminal. The source and the body (substrate) of the MOSFET are tied to ground, allowing free electrons to be supplied or removed as needed during the operation of the MOSFET. 

When a negative voltage is applied to the gate, a static negative potential appears on the aluminum plate of the capacitor. Due to the capacitive effect, positive charges accumulate beneath the dielectric layer. The free electrons in the n-type substrate are repelled by the negative plate, leaving behind a layer of positive ions.

If the negative gate voltage is further increased to reach the threshold voltage (Vth), the electrostatic force begins to break covalent bonds in the crystal beneath the SiO₂ layer. This process generates electron–hole pairs. The holes are attracted toward the gate, while the electrons are repelled by the negative gate charge. As the hole density increases, a hole-rich region is formed between the source and drain. Due to the concentration of holes, this region becomes conductive, allowing current to flow through the device.

When a negative voltage is applied to the drain, this voltage reduces the potential difference between the gate and the drain, which narrows the conductive channel near the drain region, as illustrated in the figure below. At the same time, the current flows from the source to the drain, as indicated by the arrow shown in the diagram. 

The channel formed in the MOSFET provides a resistance between the source and the drain. The resistance of this channel depends on its cross-sectional area, which in turn is determined by the negative voltage applied to the gate. Thus, we can control the current flowing from the source to the drain by adjusting the gate voltage. Since the hole density forming the channel — and therefore the current through the channel — increases as the negative gate voltage increases, this device is called a p-channel enhancement-mode MOSFET. 

When a negative voltage is applied to the gate, due to the capacitive effect, free electrons are repelled and pushed deeper into the n-type region beneath the SiO₂ dielectric layer. As a result, layers of positively charged ions appear just under the SiO₂. In this way, the number of mobile charge carriers in the channel is reduced (depletion), and the overall conductivity of the channel decreases. In this condition, the drain current decreases even if the same drain voltage is applied.

This means that we can control the drain current by adjusting the depletion of charge carriers in the channel, which is why the device is called a depletion-mode MOSFET.

In this case, the drain is at a positive potential, the gate is at a negative potential, and the source is grounded. Therefore, the voltage difference between the drain and the gate is greater than that between the source and the gate. As a result, the depletion region becomes wider toward the drain side than toward the source side.

The difference between an N-channel MOSFET and a P-channel MOSFET is that, in an N-channel device, the MOSFET switch remains open until a gate voltage is applied; at that point, the switch closes. Conversely, a P-channel MOSFET switch remains closed until a gate voltage is applied.

Similarly, the difference between an enhancement-mode MOSFET and a depletion-mode MOSFET is that the gate voltage of an enhancement-mode device is always positive, whereas the gate voltage of a depletion-mode device can be either negative or positive.

Characteristics of MOSFETs 

DE-MOSFET – Depletion-Mode MOSFET

• When VGS = 0 V:
  • In this case, the conductive channel behaves like a resistor. As the drain-to-source voltage (VDS) increases, the drain current (ID) rises until it reaches a limiting value, IDss (the saturation current).
  • The value of VDS at IDss is also called the pinch-off voltage, similar to that in a JFET.

• When VGS < 0 V:
  • Here, the gate has a negative potential, which pushes the n-channel carriers into the p-type substrate, narrowing the channel cross-section. As a result, the channel resistance increases and the drain current (ID) decreases.
  • As the negative gate voltage increases further, ID continues to decrease until it nearly reaches zero. The corresponding gate voltage is called the cutoff voltage (VP).

• When VGS > 0 V:
  • If the gate is biased with a positive potential, minority electrons from the p-type region are attracted into the n-channel, widening its cross-section. This reduces the channel resistance, causing ID to increase beyond the saturation current IDss.
  • However, this condition produces a very large current that can easily damage the MOSFET, so it is rarely used in practice.

• Structure and Symbol of an N-Channel Depletion MOSFET:

• The output characteristic (ID vs. VDS) and the transfer characteristic (ID vs. VGS) of an n-channel depletion-mode MOSFET:

• Structure and Symbol of a P-Channel Depletion MOSFET:

• The output characteristic (ID vs. VDS) and the transfer characteristic (ID vs. VGS) of a p-channel depletion-mode MOSFET:

When VGS>VGS(th)​, the drain current ID and the gate-to-source voltage VGS​ are related by the following equation: 

The p-channel enhancement-mode MOSFET operates in a manner similar to the n-channel enhancement-mode MOSFET.

• Structure and Characteristics of a P-Channel Enhancement-Mode MOSFET:

Operating Modes of a MOSFET

The operation of a MOSFET can be divided into three different regions:

• Cut-off Region (below threshold):
  The device is always in the OFF state and no current flows through it. In this mode, the MOSFET functions as a simple switch, used only when needed.
  
  

• Saturation Region:
  In this region, the drain terminal maintains a stable voltage even as the drain-to-source voltage increases. This condition occurs when the drain-to-source voltage exceeds the allowable limit set by the gate bias. In this case, the MOSFET behaves like a closed switch, with the drain current entering saturation.
  
  

• Linear (Ohmic) Region:
  In this mode, the drain current increases proportionally with the drain-to-source voltage. MOSFETs in this region typically perform amplification functions.
  
  

Semiconductor devices such as MOSFETs or BJTs essentially operate like switches in two states: ON and OFF.

• In the ON state, they must carry current within a specified limit.

• In the OFF state, the blocking voltage has no theoretical limit.

• When operating in the ON state, the voltage drop should approach zero.

• In the OFF state, the resistance must be infinite.

• These devices have no inherent limitation on switching speed.

Key Parameters and Related Concepts

Important Parameters When Using a MOSFET:

• VDS max: Maximum drain-to-source voltage the MOSFET can withstand.

• VGS: Gate-to-source voltage required to turn the MOSFET ON or OFF.

• ID max: Maximum drain current the MOSFET can handle.

• P max: Maximum power dissipation of the MOSFET during operation.

• F cutoff max: Maximum cutoff frequency of the MOSFET.

Related Concepts

What is a “Sò” MOSFET?

• “Sò” is a term used in audio amplifiers for a type of semiconductor device combined with ICs. In amplifiers, a “sò” can integrate millions of transistors in a small area and plays a decisive role in amplifier output power, enabling high-quality signal amplification.
  
  

• A “Sò MOSFET” (Metal-Oxide Semiconductor Field-Effect Transistor) is a field-effect transistor with a structure and operation different from conventional transistors, used as an amplifier or as an electronic switch.
  
  

What is a Power MOSFET?

• A Power MOSFET, or high-power MOSFET, is a derivative designed with semiconductor structures that can be voltage-controlled, requiring only a very small gate current to handle large power levels.
  
  

What is a MOSFET Driver?

• A MOSFET driver is a circuit that provides low-voltage control signals and sufficient current to drive power devices such as MOSFETs or IGBTs.

Advantages and Disadvantages of MOSFETs

Advantages:

• Provide higher efficiency while operating at lower voltages.

• Absence of gate current results in very high input impedance, enabling high switching speeds.

• Operate at low power without continuous current draw.

• Much higher input impedance than JFETs.

• Easier to manufacture compared to JFETs.

• Higher operating speed than JFETs.

• High scalability in device size.

• No intrinsic gate diode, allowing operation with either positive or negative gate voltages.

• Low power consumption enables higher integration density on a chip.

Disadvantages:

• The thin oxide layer makes MOSFETs vulnerable to damage from electrostatic discharge (ESD), leading to reduced lifespan.

• Overvoltage stress can cause instability.

• Poor performance at low-frequency radio applications.

Applications of MOSFETs

MOSFETs are widely used in power circuits, such as power supplies and load-control circuits.

The invention of the MOSFET also enabled their use as the storage element in memory cell arrays, a function previously performed by magnetic core memories in early computers

MOSFET Connection Methods

The most common application of MOSFETs is in electronic switches. The image below shows a diagram of a MOSFET operating in an ON/OFF switching mode. When the input voltage applied at the gate (VGS between G and S) is positive, the motor is in the ON state. Conversely, when the gate voltage is negative, the motor is OFF.

If we turn on the MOSFET by supplying a required gate voltage, it will remain ON until the gate voltage is brought back to 0V. To prevent this issue, a pull-down resistor (R1) is always used.

In applications such as motor speed control or light dimming, a fast-switching PWM signal is often used. In this case, the MOSFET gate produces a reverse current due to parasitic capacitance. The solution is to use a capacitor with limited current.

When the load is resistive, the circuit is quite simple. However, when the load is inductive or capacitive, protective components should be added to prevent MOSFET damage. The reason is that:

• For capacitive loads, if there is no initial charge, a short-circuit fault may occur, causing a high inrush current.

• For inductive loads, when the voltage is disconnected, a large reverse voltage spike will appear in the circuit.

How to Test a MOSFET

Pin configuration of MOSFET:
Unlike transistors, MOSFET pins are standardized:

• G (Gate): left pin

• D (Drain): middle pin

• S (Source): right pin

Tools required:

• A digital multimeter set to x1KΩ range.

• Test probes, component holder or insulation pad.

• Before measuring, short all three pins together (using a wire or screwdriver) to discharge any stored charge, since MOSFETs are very sensitive to static charge.

A MOSFET is considered good when:

• Measuring the resistance between G and S and between G and D shows infinite resistance (the meter needle does not move in both directions).

• After the Gate (G) is discharged, the resistance between D and S must also be infinite.

Testing MOSFETs In-Circuit

Set the multimeter to x1Ω range and measure between D and S:

• If the needle rises in one direction but not the other → the MOSFET is normal.

• If the needle shows 0Ω in both directions → the MOSFET is shorted (DS short).

Quick Test to Check MOSFET Channel Operation

Testing an N-Channel MOSFET

1. Set the multimeter to the x10K range. Place the MOSFET on an insulating surface or clamp it with a non-conductive tool.

2. Place the red probe on Source (S) and the black probe on Drain (D). Normally, the multimeter will show some value (because residual charge on the Gate keeps the channel open).

3. Keep the probes as in step 2, then:

   • Touch your finger from Gate (G) to Drain (D) → the needle will move upward (often close to 0Ω).

   • Touch your finger from Gate (G) to Source (S) → the needle will drop (sometimes almost back to infinity).

   • To see clearer needle movement, moisten your finger slightly (or touch G with your tongue tip).

If the needle changes as described, the MOSFET is alive.
If there is no change, the MOSFET is dead.

Testing a P-Channel MOSFET

The procedure is the same as for the N-channel MOSFET, but you need to reverse the probes.

Notes When Using MOSFETs

• The gate drive voltage must be clean and within the specified operating range.

• By nature, MOSFETs are well-suited for high-speed applications.
  
  

Conclusion

MOSFETs are indispensable components in power circuits and erasable memory devices. Their relatively simple control makes them widely applied in practice.