MOSFETs

Owned by Kevin Li
Last updated: 2022-06-12 by Michael Botros
8 min read

  • What is a MOSFET?

A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is Junction Field Effect Transistor which behaves like most transistors in that it amplifies and/or switches signals. A MOSFET differs from other transistors in that it has a metal oxide gate which results in extreme resistance at the gate section of the MOSFET. This means that no current flows through the gate despite having high voltage. MOSFETs are the most popular type of transistor and are found in almost all electrical devices.

  • What are the 3 terminals of a MOSFET? And since it’s technically 4, what is the 4th? Which terminal is it often connected to?

A MOSFET has 4 terminals:

  1. Source

  2. Gate

  3. Drain

  4. Substrate/Base

The Substrate is often connected to source which essentially creates 3 terminals for a MOSFET. This is done as Source and Substrate are both essentially grounded together.

  • What is the main difference between a p-channel and n-channel MOSFET on a semiconductor level?

In a MOSFET, there is a channel between the Source and Drain that can open up to allow current to flow between the two terminals. This channel can be opened and closed by applying voltage to the gate terminal of the MOSFET

In a P-Channel MOSFET, the terminals and channel of the semiconductor is p-type and has electron holes which results in a mostly positive charge and the body of the semiconductor are n-type which means an overall negative charge due to impurities in the semiconductor material. In a P-Channel MOSFET, the movement of positive holes is what carries current.

N-Channel MOSFETs are the more popular type. They are cheaper to build, easier to operate and smaller overall. The channel is mostly negatively charged and the movement of excess electrons across the channel is what carries the current. The terminals are n-type while the body of the transistor is p-type.

  • What is the difference between an enhancement mode and depletion mode MOFSET?

Enhancement Mode: The MOSFET is “normally open”. Applying voltage to the gate closes the switch (opens the channel) and allows current to flow between the Source and Drain.

Depletion Mode: The MOSFET is “normally closed”. Applying voltage to the gate opens the switch (closes the channel) and stops current from flowing between the Source and Drain.

  • What are the symbolic differences between p-channel and n-channel symbol, and enhancement mode and depletion mode symbol?

The diagram above displays the symbols for N-Channel, P-Channel, Enhancement and Depletion type MOSFETs.

What differentiates the N-Channel from the P-Channel is the arrow direction on the substrate pin. Facing left is N-Channel and facing right is P-Channel.

 

What differentiates enhancement mode vs depletion mode is if the terminal connect the Drain, Source and Substrate is broken or not. Enhancement is broken (normally open) and depletion is solid (normally closed).

 

  • Describe the semiconductor construction of an N-channel enhancement mode MOSFET?

The above diagram shows an enhancement mode N-Channel MOSFET. The Source and Drain terminals are N-Type semiconductor while the substrate is P-Type. By applying a positive voltage to the gate, the section of semiconductor nearest to the gate becomes N-Type (holes move away, electrons move closer) and this opens a channel between the Source and Drain in which current can flow.

  • How does a MOSFET work? Describe the workings of these different types

A MOSFET has a source, drain and gate. Across all modes of a MOSFET, applying voltage to the gate terminal changes the resistance between the drain and the source (changing the resistance of the channel). This can mean that changing the gate voltage can either make the channel more conductive or less conductive.

In an N-Channel MOSFET, the terminals are N-Type and the channel is N-Type. In depletion mode, there is a positive voltage between the drain and source and a negative voltage applied to the gate. This negative voltage at the gate makes the channel positive to due its field effect (like a capacitor) and closes the channel. When no voltage is applied, the channel is open and allows some current to flow through. This is why this mode is called “normally open”.

In an enhancement mode N-Channel MOSFET, the opposite is the case. There is a positive voltage applied to the gate. This positive voltage attracts electrons to the channel making it an N-Type channel in which current can flow. When no voltage is applied, the channel is closed and this is why this mode is called “normally closed”.

In a P-Channel MOSFET, the terminals and channel are P-Type. In depletion mode, applying a positive voltage to the gate attracts electrons to the channel which closes the P-Channel and stops current from flowing. When no voltage is applied to the gate, current flows and this is also a “normally closed” setup.

In an enhancement mode P-Channel MOSFET, applying a negative voltage to the gate repels electrons and attracts holes to the channel which creates a P-Channel for the current to flow. When no voltage is applied, the channel remains closed. This is also “normally open”.

 

  • Describe the circuit parameters of a MOSFET

The circuit parameters of a MOSFET are:

VGS: The voltage between the gate and the source (or substrate)

VDS: The voltage between the drain and the source (or substrate)

Vth: The threshold voltage when a channel is established

  • What are the different modes of operation?

The three modes of operation for an enhancement mode N-Channel MOSFET are Cutoff, Triode/linear mode and Saturation/active mode.

In Cutoff mode, no current flows from the source to the drain.

In Triode/linear mode, the channel acts like a resistor that depends on the voltage of the gate. The flow of current is controlled by adjusting the gate voltage with respect to VDS. The voltage-current relationship resembles a linear relationship to a certain extent.

In Saturation/active mode, the maximum current flowing through the channel is reached and increasing the voltage has little effect on the current through the channel at this point.

  • What are the circuit conditions for the different modes of operation?

For the cutoff mode: VGS< Vth

For Triode/linear: VGS > Vth and VDS < VGS − VGS

For Saturation/active mode: VGS > Vth and VDS ≥ (VGS – Vth)

  • What does the IV curve for a MOSFET look like? Describe it

The IV curve demonstrates the three different modes of operation in effect. The current increases when VDS increases but only up to a certain saturation point dictated by VGS. There is also a lower bound illustrated as the cutoff. There is no current if VGS is 0.

  • What are some differences between a BJT and a MOSFET?

Bipolar Junction Transistors (BJTs) use holes and electrons to carry current. They use an alternation of P-Type and N-Type sections of semiconductor. They have two junctions and can be built and PNP or NPN transistors.

  • What is the parasitic internal body diode of a MOSFET?

A MOSFET with N-Type terminals and a P-Type section in between can be viewed as an NPN transistor with no lead connecting to the P-Type base. This means that if for some reason the body gets triggered, the transistor will activate and have no control. By connecting the middle P-Type section to the source, the internal transistor has its base connected to its emitter and can no longer activate randomly. This creates a diode in the MOSFET which only allows current to flow from the source to the drain. It is parasitic as it is an unintended but necessary part of MOSFET structure. 

  • How is VGS related to Rds(on)?

VGS is inversely related to Rds(on). The voltage of the gate controls the current through the channel by affecting the resistance between the source and drain (Rds(on)). For an enhancement N-Channel MOSFET, increasing the gate voltage lowers the resistance between the source and drain which opens the channel and allows current to flow through. 

  • What type of MOSFETs are typically used for high-side switching? What about low-side switching? Why?

 

Low-side switching is when a transistor is placed between a load and ground while high-side switching is when a transistor is placed between V+ and the load. N-Type MOSFETs are better for low-side switching since N-Type MOSFETs carry more current and are better for dealing with higher loads. P-Type MOSFETs are better for high-side switching since N-Type MOSFETs need the gate voltage to be higher than the voltage at the source which requires a charge pump to the gate on high-side. 

  • What are the two main sources of power loss from a MOSFET?

The two main sources of power loss in a MOSFET are conduction losses and switching losses. Conduction losses occur because the channel between the source and drain acts as a resistor and therefore dissipates energy in the system as current passes. Switching losses occur due to parasitic capacitance stored in the MOSFET that is discharged every time the MOSFET switches on and off. 

  • What is CMOS?

Complementary Metal Oxide Semiconductors are devices and circuits that use a combination of enhancement P-Channel (PMOS) and N-Channel (NMOS) MOSFETs to create fundamental logic gates. In combining these two MOSFETs, ICs of all kinds can be built and properties of CMOS devices are high noise immunity and low power consumption. This generates little waste heat and can increase CMOS density on chips.

The image above shows an example of a NAND gate in CMOS logic. The MOSFET symbols are simplified and NMOS can be differentiated from PMOS by the “not” circle on the top MOSFETs on A and B.

  • What does a CMOS inverter and buffer look like?

The diagram below shows an inverter in CMOS logic. It takes the signal A and inverts it into signal Q.

The diagram below shows a buffer in CMOS logic. A buffer takes a potentially weak signal and strengthens/corrects it to an ideal voltage level.