N-CHANNEL DEPLETION MODE MOSFET Product Summary BV DSX R DS(ON) max I DSS min T A = +25°C 600V 7 0Ω@ V GS = 0V 7mA (i.e. Parts qualified to AEC Description and Applications This new generation uses advanced planar technology MOSFET, provide excellent high voltage and fast switching, making it ideal for small-signal and level shift applications. N-Channel Depletion-mode MOSFET has negative channel cutoff voltage, which is designated as V GS(off). A designer has to know well the magnitude of the negative cutoff voltage (or threshold voltage). A negative gate-to-source voltage (V GS) will reduce the drain current until the device’s cutoff voltage level is reached and the conduction ceases. To use depletion mode MOSFETS simply reverse the circuits where an N-channel depletion mode MOSFET will use a variation of the P-channel enhancement mode circuit. In plate 1 we have the symbols for depletion mode and enhancement mode MOSFETs - notice the dashed versus solid lines. Figure 2: Cross-section view of an n-channel Depletion Mode MOSFET The gate is a metal layer (in present-day CMOS technology, this is generally made up of polysilicon) shown by black color, which is deposited over an insulator. A layer of hatched lines shows the insulator (generally, this insulator is made up of Silicon-di-oxide).

• N Channel Depletion Type Mosfet
• N Channel Depletion Mosfet Ltspice
• Depletion Mode N Channel Mosfet

In this tutorial, we will have a brief introduction to MOSFET i.e., the Metal Oxide Semiconductor Field Effect Transistor. We will learn about different types of MOSFET (Enhancement and Depletion), its internal structure, an example circuit using MOSFET as a Switch and a few common applications.

Introduction

Transistors, the invention that changed the World. They are semiconductor devices that act as either an electrically controlled switch or a signal amplifier. Transistors come a variety of shapes, sizes and designs but essentially, all transistors fall under two major families. Mac not showing code for text message forwardingfasrtrek. They are:

• Bipolar Junction Transistors or BJT
• Field Effect Transistors or FET

To learn more about a basics of transistor and its history, read the Introduction to Transistors tutorial.

There are two main differences between BJT and FET. The first difference is that in BJT, both the majority and minority charge carriers are responsible for current conduction whereas in FETs, only the majority charge carriers are involved.

The other and very important difference is that a BJT is essentially a current controlled device meaning the current at the base of the transistor determines the amount of current flowing between collector and emitter. In case of a FET, the voltage at the Gate (a terminal in FET equivalent to Base in BJT) determines the current flow between the other two terminals.

FETs are again divided into two types:

• Junction Field Effect Transistor or JFET
• Metal Oxide Semiconductor Field Effect Transistor or MOSFET

Let us focus on MOSFET in this tutorial.

Metal Oxide Semiconductor FET

The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is one type of FET transistor. In these transistors, the gate terminal is electrically insulated from the current carrying channel so that it is also called as Insulated Gate FET (IG-FET). Due to the insulation between gate and source terminals, the input resistance of MOSFET may be very high such (usually in the order of 1014 ohms.

Like JFET, the MOSFET also acts as a voltage controlled resistor when no current flows into the gate terminal. The small voltage at the gate terminal controls the current flow through the channel between the source and drain terminals. In present days, the MOSFET transistors are mostly used in the electronic circuit applications instead of the JFET.

MOSFETs also have three terminals, namely Drain (D), Source (S) and Gate (G) and also one more (optional) terminal called substrate or Body (B). MOSFETs are also available in both types, N-channel (NMOS) and P-channel (PMOS). MOSFETs are basically classified in to two forms. They are:

• Depletion Type
• Enhancement Type

Depletion Type

The depletion type MOSFET transistor is equivalent to a “normally closed” switch. The depletion type of transistors requires gate – source voltage (V_GS) to switch OFF the device.

The symbols for depletion mode of MOSFETs in both N-channel and P-channel types are shown above. In the above symbols, we can observe that the fourth terminal (substrate) is connected to the ground, but in discrete MOSFETs it is connected to source terminal. The continuous thick line connected between the drain and source terminal represents the depletion type. The arrow symbol indicates the type of channel, such as N-channel or P-channel.

In this type of MOSFETs a thin layer of silicon is deposited below the gate terminal. The depletion mode MOSFET transistors are generally ON at zero gate-source voltage (VGS). The conductivity of the channel in depletion MOSFETs is less compared to the enhancement type of MOSFETs.

Enhancement Type

The Enhancement mode MOSFET is equivalent to “Normally Open” switch and these types of transistors require a gate-source voltage to switch ON the device. The symbols of both N-channel and P-channel enhancement mode MOSFETs are shown below.

Here, we can observe that a broken line is connected between the source and drain, which represents the enhancement mode type. In enhancement mode MOSFETs, the conductivity increases by increasing the oxide layer, which adds the carriers to the channel.

Generally, this oxide layer is called as ‘Inversion layer’. The channel is formed between the drain and source in the opposite type to the substrate, such as N-channel is made with a P-type substrate and P-channel is made with an N-type substrate. The conductivity of the channel due to electrons or holes depends on N-type or P-type channel respectively.

Structure of MOSFET

Here, we can observe that the gate terminal is situated on top of thin metal oxide insulated layer and two N-type regions are used below the drain and source terminals.

In the above MOSFET structure, the channel between drain and source is an N-type, which is formed opposite to the P-type substrate. It is easy to bias the MOSFET gate terminal for the polarities of either positive (+ve) or negative (-ve).

If there is no bias at the gate terminal, then the MOSFET is generally in non-conducting state so that these MOSFETs are used to make switches and logic gates. Both the depletion and enhancement modes of MOSFETs are available in N-channel and P-channel types.

Depletion Mode

The depletion mode MOSFETs are generally known as ‘Switched ON’ devices, because these transistors are generally closed when there is no bias voltage at the gate terminal. If the gate voltage increases in positive, then the channel width increases in depletion mode.

As a result the drain current I_D through the channel increases. If the applied gate voltage more negative, then the channel width is very less and MOSFET may enter into the cutoff region. The depletion mode MOSFET is a rarely used type of transistor in the electronic circuits.

The following graph shows the Characteristic Curve of Depletion Mode MOSFET.

The V-I characteristics of the depletion mode MOSFET transistor are given above. This characteristic mainly gives the relationship between drain- source voltage (V_DS) and drain current (I_D). The small voltage at the gate controls the current flow through the channel.

The channel between drain and source acts as a good conductor with zero bias voltage at gate terminal. The channel width and drain current increases if the gate voltage is positive and these two (channel width and drain current) decreases if the gate voltage is negative.

Enhancement Mode

The Enhancement mode MOSFET is commonly used type of transistor. This type of MOSFET is equivalent to normally-open switch because it does not conduct when the gate voltage is zero. If the positive voltage (+V_GS) is applied to the N-channel gate terminal, then the channel conducts and the drain current flows through the channel.

If this bias voltage increases to more positive then channel width and drain current through the channel increases to some more. But if the bias voltage is zero or negative (-V_GS) then the transistor may switch OFF and the channel is in non-conductive state. So now we can say that the gate voltage of enhancement mode MOSFET enhances the channel.

Enhancement mode MOSFET transistors are mostly used as switches in electronic circuits because of their low ON resistance and high OFF resistance and also because of their high gate resistance. These transistors are used to make logic gates and in power switching circuits, such as CMOS gates, which have both NMOS and PMOS Transistors.

N Channel Depletion Type Mosfet

The V-I characteristics of enhancement mode MOSFET are shown above which gives the relationship between the drain current (ID) and the drain-source voltage (VDS). From the above figure we observed the behavior of an enhancement MOSFET in different regions, such as ohmic, saturation and cut-off regions.

MOSFET transistors are made with different semiconductor materials. These MOSFETs have the ability to operate in both conductive and non-conductive modes depending on the bias voltage at the input. This ability of MOSFET makes it to use in switching and amplification.

N-Channel MOSFET Amplifier

When compared to BJTs, MOSFETs have very low transconductance, which means the voltage gain will not be large. Hence, MOSFETs (for that matter, all FETs) are generally not used in amplifier circuits.

But, none the less, let us see a single-stage ‘class A’ amplifier circuit using N-Channel Enhancement MOSFET. The N-channel enhancement mode MOSFET with common source configuration is the mainly used type of amplifier circuit than others. The depletion mode MOSFET amplifiers are very similar to the JFET amplifiers.

The input resistance of the MOSFET is controlled by the gate bias resistance which is generated by the input resistors. The output signal of this amplifier circuit is inverted because when the gate voltage (V_G) is high the transistor is switched ON and when the voltage (V_G) is low then the transistor is switched OFF.

The general MOSFET amplifier with common source configuration is shown above. This is an amplifier of class A mode. Here the voltage divider network is formed by the input resistors R1 and R2 and the input resistance for the AC signal is given as Rin = RG = 1MΩ.

N Channel Depletion Mosfet Ltspice

The equations to calculate the gate voltage and drain current for the above amplifier circuit are given below. League for mac.

V_G = (R₂ / (R₁ + R₂))*VDD

I_D = V_S/ R_S

Where,

V_G = gate voltage

V_S = input source voltage

V_DD = supply voltage at drain

R_S = source resistance

R₁ & R₂ = input resistors

The different regions in which the MOSFET operates in their total operation are discussed below.

Cut-off Region: If the gate-source voltage is less than the threshold voltage then we say that the transistor is operating in the cut-off region (i.e. fully OFF). In this region drain current is zero and the transistor acts as an open circuit.

V_GS < V_TH => I_DS = 0

Ohmic (Linear) Region: If the gate voltage is greater than threshold voltage and the drain-source voltage lies between VTH and (VGS – VTH) then we say that the transistor is in linear region and at this state the transistor acts as a variable resistor.

V_GS > V_TH and V_TH < V_DS < (V_GSVGS – V_TH) => MOSFET acts as a variable Resistor

Saturation Region: In this region the gate voltage is much greater than threshold voltage and the drain current is at its maximum value and the transistor is in fully ON state. In this region the transistor acts as a closed circuit.

The gate voltage at which the transistor ON and starts the current flow through the channel is called threshold voltage. This threshold voltage value range for N-channel devices is in between 0.5V to 0.7V and for P-channel devices is in between -0.5V to -0.8V.

The behavior of a MOSFET transistor in depletion and enhancement modes depending on the gate voltage is summarized as follows.

Applications

• MOSFETs are used in digital integrated circuits, such as microprocessors.
• Used in calculators.
• Used in memories and in logic CMOS gates.
• Used as analog switches.
• Used as amplifiers.
• Used in the applications of power electronics and switch mode power supplies.
• MOSFETs are used as oscillators in radio systems.
• Used in automobile sound systems and in sound reinforcement systems.

Conclusion

A complete beginner’s guide to introduction of MOSFET. You learned the structure of a MOSFET, different types of MOSFET, their circuit symbols, an example circuit using a MOSFET to control an LED and also few areas of applications.

In this tutorial, we will learn about the working of a MOSFET as a Switch. In the MOSFET tutorial, we have seen the basics of a MOSFET, its types, structure and a few applications of MOSFET as well.

One of the important applications of MOSFET in the field of Power Electronics is that it can be configured as a simple analogue switch. With the help of such analogue switches, digital systems can control the flow of signals in analogue circuits.

Before going into the details of how a MOSFET acts as a switch, let me take you through a recap of the basics of a MOSFET, its operation regions, internal structure etc. For more information on MOSFETs, read the MOSFET Tutorial.

Introduction to MOSFET

A MOSFET or Metal Oxide Semiconductor Field Effect Transistor, unlike a Bipolar Junction Transistor (BJT) is a Unipolar Device in the sense that it uses only the majority carriers in the conduction.

It is a type of field effect transistor with an insulated gate from the channel (hence, sometimes called as Insulated Gate FET or IGFET) and the voltage at the gate terminal determines the conductivity.

Speaking of terminals, a MOSFET is typically a 3-terminal device, which are Gate (G), Source (S) and Drain (D) (even though there is a 4th terminal called Substrate or Body, it is usually not used in either input or output connection).

MOSFET Symbol

MOSFET can be classified into Enhancement type MOSFET and Depletion type MOSFET. Each of these types are further divided into N-channel MOSFET and P-channel MOSFET.

The symbols for each of these types of MOSFETs are shown in the image below.

The main difference between Enhancement Mode MOSFET and Depletion Mode MOSFET is that in depletion mode, the channel is already formed i.e., it acts as a Normally Closed (NC) switch and in case of enhancement mode, the channel is not formed initially i.e., a Normally Open (NO) switch. Corporation inc.spiter games.

MOSFET Structure

The structure of a MOSFET varies based on the application i.e., MOSFETs in IC technology are fairly lateral, while the structure of Power MOSFETs is more of a vertical channel. Irrespective of the application, a MOSFET has basically three terminals namely Gate, Drain and Source.

If we consider an N-channel MOSFET, both the Source and Drain are made up of n-type, which sit in a P-type substrate.

Working of a MOSFET

Let us now try to understand how an n-Channel Enhancement Mode MOSFET works. In order to carry a drain current, there should be a channel between the drain and source regions of the MOSFET.

A channel is created when the voltage between gate and source terminals V_GS is greater than the threshold voltage V_TH.

When V_GS > V_TH, the device is said to be in triode (or constant resistance) region or saturation region depending on the voltage across drain and source terminals V_DS.

For any V_GS, if V_DS < V_GS – V_TH, then the device is in triode region (also known as constant resistance or linear region). If V_DS > V_GS – V_TH, then the device enters into saturation region.

When V_GS < V_TH, then the device is in off state. The gate current in either regions of operation is very less (almost equal to zero). Hence, MOSFET is known as Voltage Driven Device.

MOSFET Characteristics Curve

The image below shows the characteristic curve of MOSFET in three regions of operation. It depicts the Drain Current I_D versus the Drain to Source Voltage V_DS for a given Gate to Source Voltage V_GS.

MOSFET Regions of Operation

Based on the above-mentioned working of a MOSFET, it can be concluded that a MOSFET has three regions of operation. They are:

• Cut-off Region
• Linear (or Triode) Region
• Saturation Region

A MOSFET operates in cut-off region when V_GS < V_TH. In this region, the MOSFET is in OFF state as there is no channel induced between drain and source.

For the channel to be induced and MOSFET to operate in either linear or saturation region, V_GS > V_TH.

The Gate – Drain bias voltage V_GD will determine whether the MOSFET is in linear or saturation region. In both these regions, the MOSFET is in ON state but the difference is in linear region, the channel is continuous and the drain current is proportional to the resistance of the channel.

Coming to saturation region, as V_DS > V_GS – V_TH, the channel pinches off i.e., it broadens resulting in a constant Drain Current.

Switching in Electronics

Semiconductor switching in electronic circuit is one of the important aspects. A semiconductor device like a BJT or a MOSFET are generally operated as switches i.e., they are either in ON state or in OFF state.

Ideal Switch Characteristics

For a semiconductor device, like a MOSFET, to act as an ideal switch, it must have the following features:

• During ON state, there should not be any limit on the amount of current it can carry.
• In OFF state, there should not be any limit on the blocking voltage.
• When the device is in ON state, there should be zero voltage drop.
• OFF state resistance should be infinite.
• Operating speed of the device has no limits.

Practical Switch Characteristics

But the World isn’t ideal and it is applicable even to our semiconductor switches. In a practical situation, a semiconductor device like a MOSFET has the following characteristics.

Depletion Mode N Channel Mosfet

• During ON state, the power handling capabilities are limited i.e., limited conduction current. The blocking voltage during OFF state is also limited.
• Finite turn on and turn off times, which limit the switching speed. Maximum operating frequency is also limited.
• When the device is ON, there will be a finite on state resistance resulting in a forward voltage drop. There will also be a finite off state resistance which results in a reverse leakage current.
• A practical switch experiences power loses during on state, off state and also during the transition state (on to off or off to on).

Working of a MOSFET as a Switch

If you understood the working of the MOSFET and its regions of operation, you would have probably guessed how a MOSFET works as a switch. We will understand the operation of a MOSFET as a switch by considering a simple example circuit.

This is a simple circuit, where an N-Channel Enhancement mode MOSFET will turn ON or OFF a light. In order to operate a MOSFET as a switch, it must be operated in cut-off and linear (or triode) region.

Assume the device is initially OFF. The voltage across Gate and Source i.e., V_GS is made appropriately positive (technically speaking, V_GS > V_TH), the MOSFET enters linear region and the switch is ON. This makes the Light to turn ON.

If the input Gate voltage is 0V (or technically < V_TH), the MOSFET enters cut-off state and turns off. This in turn will make the light to turn OFF.

Example of MOSFET as a Switch

Consider a situation where you want to digitally control a 12W LED (12V @ 1A) using a Microcontroller. When you press a button connected to the microcontroller, the LED should turn ON. When you press the same button once again, the LED should turn OFF.

It is obvious that you cannot directly control the LED with the help of the microcontroller. You need a device that bridges the gap between the microcontroller and the LED.

This device should take in a control signal from the microcontroller (usually the voltage of this signal is in the working voltage range of the microcontroller, 5V for example) and supply power to the LED, which in this case is from a 12V supply.

The device which I am going to use is a MOSFET. The setup of the above-mentioned scenario is shown in the following circuit.

When a Logic 1 (assuming a 5V Microcontroller, Logic 1 is 5V and Logic 0 is 0V) is supplied to the gate of the MOSFET, it turns ON and allows drain current to flow. As a result, the LED is turned ON.

Similarly, when a Logic 0 is given to the gate of the MOSFET, it turns OFF and in turn switches OFF the LED.

Thus, you can digitally control a high-power device with the combination of Microcontroller and MOSFET.

Important Note

An important factor to consider is the power dissipation of the MOSFET. Consider a MOSFET with a Drain to Source Resistance of 0.1Ω. In the above case i.e., a 12W LED driven by a 12V supply will lead to a drain current of 1A.

Hence the power dissipated by the MOSFET is P = I² * R = 1 * 0.1 = 0.1W.

This seems to be a low value but if you drive a motor using the same MOSFET, the situation is slightly different. The starting current (also called as in-rush current) of a motor will be very high.

So, even with RDS of 0.1Ω, the power dissipated during the start-up of a motor will still be significantly high, which may lead to thermal overload. Hence, R_DS will be a key parameter to select a MOSFET for your application.

Also, when driving a motor, the back emf is an important factor that has to be considered while designing the circuit.

One of the main advantages of driving a motor with MOSFET is that an Input PWM signal can be used to smoothly control the speed of the motor.

Conclusion

A complete beginner’s tutorial on MOSFET as a Switch. You learned some important basics of MOSFET (its internal structure and regions of operations), ideal vs. practical Semiconductor switch, working of a MOSFET as a Switch, and couple of example circuits.