What is P Channel MOSFET : Working & Its Applications

MOSFET is a three-terminal, voltage controlled, high input impedance, and unipolar device which are essential components in different electronic circuits. Generally, these devices are classified into two types enhancement Mosfet & depletion Mosfet based on whether they have channels in their default condition or not correspondingly. Again, enhancement MOSFETs are classified into p channel enhancement and n channel enhancement & depletion MOSFETs are classified into p channel depletion and n channel depletion MOSFETs. So this article discusses one of the types of MOSFETs like P channel MOSFET.

What is P Channel MOSFET?

A type of MOSFET in which the channel is composed with a majority of charge carriers as holes is known as p channel MOSFET. Once this MOSFET is activated, then the majority of charge carriers like holes will move throughout the channel. This MOSFET is in contrast to N channel MOSFET because in N MOSFET the majority of charge carriers are electrons. The P Channel MOSFET symbols in enhancement mode and depletion mode are shown below.

The P- channel MOSFET includes a P- Channel region that is arranged in between the two terminals like source (S) and drain (D) & the body is n- region. Similar to N channel MOSFET, this type of MOSFET also includes three terminals like source, drain, and gate. Here, both the source & drain terminals are doped heavily with p type materials  and the type of substrate used in this MOSFET is n-type.

Working

The majority of charge carriers in P-Channel MOSFETs are holes where these charge carriers have low mobility as compared to the electron utilized within N-Channel MOSFETs. The main difference between the p channel and n channel MOSFET is, that in the p channel, a negative voltage is necessary from the Vgs (gate terminal to source) to activate MOSFET whereas, in the n channel, it needs a positive VGS voltage. So this makes P-Channel type MOSFET a perfect option for high-side switches.

Whenever we give the negative (-) voltage at the gate terminal of this MOSFET, then the charge carriers available below the oxide layer like electrons are pushed downside into the substrate. So the depletion region occupied by the holes is connected with the donor atoms. So, the negative (-) gate voltage will attract holes from the drain region & p+ source into the region of the channel.

Please refer to this link to know more about MOSFET as a Switch

Types of P Channel MOSFET

There are two types of p channel MOSFETs available P channel enhancement MOSFET & P channel depletion MOSFET.

P -channel Enhancement MOSFET

The p channel enhancement MOSFET is simply designed with a lightly doped n-substrate. Here, two p-type materials with heavily doped are separated through the channel length like ‘L’. The thin silicon dioxide layer is deposited on the substrate which is normally called the dielectric layer.

In this MOSFET, the two P-type materials form the source (S) & the drain (D) and the aluminum is used as the plating on the dielectric to form the gate (G) terminal. Here, the MOSFET’s source and the body are simply connected to the GND.

When a negative voltage is applied to the gate (G) terminal then the +ve concentration of the charges will be settled under the dielectric layer due to the capacitance effect. The electrons available at the n substrate because of the repulsive forces will get moved.

When a negative voltage is applied at the drain terminal then negative voltage within the drain region decreases the voltage difference between gate & drain decreases, thus, the conductive channel width gets decreased toward the drain region, and current supplies from source to drain.

The channel formed within the MOSFET provides resistance to the flow of current from source to drain. Here, the resistance of the channel mainly depends on the side view of the channel & again this channel’s cross-section depends on the negative voltage applied at the gate terminal. Thus the current flow from the source to the drain can be controlled through the voltage applied at the gate terminal so MOSFET is known as a voltage-controlled device. When the hole concentration forms the channel & the flow of current throughout the channel gets improved because of an increase within negative gate voltage, so this is known as P – Channel Enhancement MOSFET.

P -channel Depletion MOSFET

The p channel depletion MOSFET construction is reversed to n channel depletion MOSFET. The channel in this MOSFET is pre-build because of the available p-type impurities in it. Once the negative (-) voltage is applied at the gate terminal then the minority charge carriers like electrons in the n-type get attracted towards the p-type channel. In this condition, once a drain is reverse biased then the device begins conducting although, when the negative voltage within the drain is enhanced then it results in the depletion layer formation.

This region mainly depends on the layer concentration formed because of the holes. The depletion layer’s region width will affect the conductivity value of the channel. So, by variations of the region’s voltage values, the flow of current gets controlled. At last, the gate & the drain will stay at the negative polarity while the source remains at the ‘0’ value.

How Do You Use P-Channel Mosfet?

The complementary MOSFET switch circuit for controlling the motor is shown below. This switch circuit uses two MOSFETs like the P channel and the N channel to control the motor in both directions. In this circuit, these two MOSFETs are simply connected to generate a bi-directional switch using a dual supply through the motor connected in between the common drain & GND reference.

Once the input voltage is LOW then P-channel MOSFET connected in the circuit will be turned ON & N channel MOSFET will be turned off because its gate to source junction is biased negatively as a result the motor in the circuit turns in one direction. Here, the motor is operated by using the +VDD supply rail.
Similarly when the input is HIGH, then the N-channel MOSFET switches ON & P-channel device switches OFF because its gate to source junction is biased positively. Now the motor turns in the reverse direction because the terminal voltage of the motor has been reversed when it is supplied through the -VDD supply rail.

After that, for the forwarding direction of the motor, the P-channel type MOSFET is utilized to switch the +ve supply to the motor whereas, for the reverse direction, the N-channel MOSFET is utilized to switch the -ve supply to the motor.

• Here, when both the MOSFETs are OFF the motor will stop working.
• When MOSFET1 is ON, MOSFET2 is OFF then the motor runs in the forwarding direction.
• When MOSFET1 is OFF, MOSFET2 is ON then the motor runs in the reverse direction.

How Do You Test P Channel MOSFET?

The testing of p channel MOSFET can be done by using a digital multimeter by using the following steps.

• First, you need to set the multimeter to the diode range
• Place the MOSFET on any wooden table by facing its printed side towards you.
• By using the probe of a digital multimeter, short the drain and gate terminals of the MOSFET, this will firstly allow the device’s internal capacitance to discharge, so it is very necessary for the MOSFET testing process.
• Now place the multimeter’s red color probe on the source terminal and the black probe on the drain terminal.
• You will get an open circuit reading on the multimeter display.
• After that, without changing the RED color probe from the MOSFET’s source terminal, take away the black color probe from the drain terminal & place it on the gate terminal of the MOSFET for a few seconds & place it back on the MOSFET’s drain terminal.
• At this time, the multimeter will show a low value or continuity value on the display of the multimeter.
• That’s it, this will verify your MOSFET is okay & without any troubles. Any other type of reading will specify a defective MOSFET.

P Channel MOSFET Failure Modes

The MOSFET failure occurs frequently for seemingly inexplicable reasons even with the good design, the finest components & a new motor. Generally, MOSFETs are very robust – however, they can fail very quickly due to exceeded ratings. Here we are going to explain some of the main failure modes of MOSFET & how to avoid them.

It is very difficult to find out the failures that occurred within the MOSFET because we are not aware of what exactly happened to cause failures. Here we have listed some failure modes that occurred in MOSFET like the following.

• Whenever high current supplies throughout MOSFET then it will heat up. Poor heat sinking can also damage the MOSFET from extreme temperatures.
• Faulty battery.
• Avalanche failure.
• dV/dt failure .
• Blocked or jammed motor.
• Fast acceleration or deceleration.
• Excess power dissipation.
• Excess Current
• Load with Short-circuit
• Foreign objects.

Characteristics

The P Channel MOSFET characteristics are discussed below.

• These MOSFETs are voltage-controlled devices.
• These devices have high input impedance values.
• In P-channel, the conductivity of the channel is due to the negative polarity at the gate terminal.
  As compared to the n channel, the p channel Mosfet characteristics are similar but the only difference is the polarities because the values of substrates are not the same here.

Advantages

The advantages of P Channel MOSFET include the following.

• This MOSFET design is very simple so it is applicable where space is restricted like low-voltage drives & non-isolated POLs applications.
• This is the simplified gate driving method within the high side switch place & frequently reduces the overall cost
• The efficiency provided by the MOSFETs is higher when operating at low voltages.
• As compared to JFETs, MOSFETs have high input impedance.
• They have high drain resistance because of less channel resistance.
• These are very simple to manufacture.
• It supports high-speed operation as compared to JFETs.

The disadvantages of P Channel MOSFET include the following.

• The MOSFET’s thin oxide layer will make it vulnerable to damage when induced by electrostatic charges.
• These are not stable when high voltages are used.

Thus, this is an overview of p channel MOSFET – working, types, and its applications. Here is a question for you, what is n channel MOSFET?