Enhancement MOSFET (E-MOSFET): Types, Construction, Working, and Applications

In this article, we will discuss a type of MOSFET (Metal-Oxide Field Effect Transistor) named Enhancement MOSFET (E-MOSFET) along with its types, construction, working, V-I characteristics, and applications. But before that, let us briefly discuss the MOSFET (Metal-Oxide Field Effect Transistor).

What is MOSFET?

MOSFET stands for Metal Oxide Field Effect Transistor. Sometimes, MOSFET has also known as Metal Oxide Semiconductor (MOS) Transistor.

A MOSFET is a type of field effect transistor (FET) which has three terminals namely, Source (S), Gate (G), and Drain (D). In a MOSFET, the drain current (I_D) is controlled by the voltage level applied at the gate terminal. Thus, the MOSFET is a voltage-controlled device.

In the case of MOSFET, the gate terminal remains insulated from the channel, hence it is also termed an Insulated Gate Field Effect Transistor (IGFET).

Depending upon the modes of operation, MOSFETs are classified into the following two types:

• Depletion Type MOSFET (D-MOSFET)
• Enhancement Type MOSFET (E-MOSFET)

A Depletion Mode MOSFET is one that can operate in both the depletion mode and enhancement mode. On the other hand, the Enhancement Mode MOSFET is one that can work only in enhancement mode.

This article will explain only E-MOSFET (Enhancement Mode MOSFET). We will discuss Depletion Type MOSFET in a separate article.

What is Enhancement Type MOSFET (E-MOSFET)?

As mentioned above, E-MOSFET (Enhancement Metal-Oxide Semiconductor Field Effect Transistor) is the type of MOSFET which can be operated only in the enhancement mode of operation and cannot be operated in depletion mode.

E-MOSFET has three terminals namely, Source (S), Gate (G), and Drain (D).

E-MOSFET is the type of MOSFET in which the voltage applied at the gate terminal (G) can only be used to enhance the drain current (I_D). In E-MOSFET, a depletion mode of operation is not possible.

Based on the nature of the conduction channel, E-MOSFETs are classified into the following two types:

• N-Channel E-MOSFET
• P-Channel E-MOSFET

Now, let us discuss each of these two types of E-MOSFET in detail.

N-Channel Enhancement Type MOSFET

The type of E-MOSFET which has a conduction channel of N-type, i.e. majority of charge carriers are electrons is termed N-channel E-MOSFET. The construction and circuit symbol of N-channel E-MOSFET is shown in figure-1.

From the construction diagram, it is clear that the conduction channel from source to drain is absent physically in E-MOSFET because the semiconductor substrate extends completely to the SiO₂ layer as shown in figure-1. Hence, in E-MOSFET, the channel between the source and drain is induced only by the application of a proper voltage at the gate terminal.

Now, let us understand the working of N-Channel Enhancement Mode MOSFET.

Working of N-Channel Enhancement MOSFET

The circuit diagram of N-channel E-MOSFET with biasing batteries V_DS and V_GS is shown in figure-2. The positive potential applied at the gate terminal repels the holes (+ve charges) away from the SiO₂ layer. As a result, holes leave the area near the SiO₂ layer and enter the deeper region into the P-type substrate.

Electrons (minority charge carriers) in the P-type substrate are attracted towards the SiO₂ layer due to the positive potential of the gate terminal. Although, these electrons cannot cross the metal-oxide layer of SiO₂. Thus, they are accumulated near the SiO₂ layer. Consequently, the area near to the SiO₂ layer becomes rich in electrons and acts like an induced N-channel. This N-channel is also called the inversion layer.

This N-channel or inversion layer helps electrons of the source-side N⁺ region to move towards the drain-side N⁺ region. When this N-channel is absent, all the electrons leaving the source N⁺ region to combine with holes of the P-type substrate and no electron can reach the drain N⁺ region. Therefore, the drain current (I_D) is reduced to zero. It also proves that the gate voltage enhances the drain current (I_D) in the Enhancement Type MOSFET.

Here, the minimum value of gate-to-source voltage (V_GS) required to induce the N-channel is referred to as Threshold Voltage (V_Th). When the voltage V_GS is less than V_Th, the drain current (I_D) is very small (usually of the order of nano-amperes) and hence considered equal to zero. For an N-channel E-MOSFET, the threshold voltage is nearly equal to 3 volts.

But, when V_GS is greater than V_Th (i.e. V_GS > V_Th), the drain current (I_D) of the N-channel E-MOSFET is given by,

`\I_D=K(V_(GS)-V_(Th) )^2`

Where, K is a constant.

When the gate to source voltage (V_GS) is kept constant and the drain to source voltage (V_DS) is increased, the drain current (I_D) reaches a saturation level after some time as shown in figure-3.

This saturation of I_D happens due to the pinching-off of the induced N-channel. In this case, a depletion layer is developed between the N-channel and P-type substrate whose thickness depends upon the amount of reverse bias between the PN-junction.

When voltage V_DS increases, the potential of the induced N-channel increases in a positive manner, whereas the potential of the P-substrate still remains at zero volts. As a result, the reverse bias between PN-junction increases, which in turn, increases the size of the depletion layer. This depletion layer is thinnest at the source side and thickest at the drain side. The increased size of the depletion layer reduces the effective width of the induced N-channel.

Ultimately, at a particular value of the drain to source voltage (V_DS), the N-channel will be reduced to the point of pinch-off and the drain current (I_D) saturates as shown in figure-3.

P-Channel Enhancement Type MOSFET

The type of E-MOSFET which has a conduction channel of P-type, i.e. majority charge carriers are holes is called P-channel E-MOSFET. The constructional diagram and circuit symbol of P-channel E-MOSFET is shown in figure-4.

P-Channel E-MOSFET with biasing batteries V_GS and V_DS is shown in figure-5. 

The working of P-Channel E-MOSFET is similar to that of N-Channel E-MOSFET. The only difference is that the negative potential applied at the gate terminal induces a P-channel between the source-side P⁺ region and the drain-side P⁺ region.

After discussing the theory of N-Channel and P-Channel E-MOSFET, let us now discuss the V-I characteristics of Enhancement MOSFET.

V-I Characteristics of Enhancement MOSFETs

Enhancement MOSFETs (E-MOSFETs) have the following two types of V-I characteristics:

• Output Characteristics or Drain Characteristics
• Transfer Characteristics

Drain Characteristics of E-MOSFET

The graphs or curves that show the relationship between drain current (I_D) and drain-to-source voltage (V_DS) for a given value of gate-to-source voltage (V_GS) are called drain characteristics or output characteristics of E-MOSFET.

The drain characteristics (output characteristics) of N-Channel and P-Channel E-MOSFETs are shown in figure-6.

From the drain characteristics of E-MOSFETs, we can conclude the following important points:

• For small values of V_DS, the drain current (I_D) increases linearly with the increase in V_DS. This region is called the ohmic region of operation.
• After a certain value of V_DS, the pinch-off takes place and the drain current I­_D saturates and becomes independent of the voltage V_DS. This is called a constant current region.
• If the gate-to-source voltage (V_GS) is below the threshold voltage (V_Th), the drain current (I_D) is reduced to zero.
• For N-Channel E-MOSFET, if the voltage V­_GS is increased in a positive manner, the drain current (I_D) increases. This happens because the positive increment in V_GS strengthens the induced N-channel.
• For P-Channel E-MOSFET, if the voltage V­_GS is increased in a negative manner, the drain current (I_D) increases. This happens because the negative increment in V_GS strengthens the induced P-channel.

Transfer Characteristics of E-MOSFET

The graphs or curves showing the relationship between drain current (I_D) and gate-to-source voltage (V_GS) for a constant value of V_DS are called transfer characteristics of E-MOSFET.

Figure-7 below shows the transfer characteristics of N-channel E-MOSFET and P-channel E-MOSFET.

From these transfer characteristics, we may conclude the following important points:

• For V_GS < V_Th, the drain current (I_D) is zero. This is because no significant channel is induced between the source and drain regions to support the flow of charge carriers (electrons or holes).
• For V_GS > V_Th, the drain current (I_D) increases sharply with the increase in V_GS in a parabolic sense.

Applications of Enhancement MOSFET

The following are some major applications of enhancement-type metal-oxide-semiconductor field effect transistor (E-MOSFET):

• E-MOSFETs are used in switching applications.
• E-MOSFETs are used in amplifiers.
• E-MOSFETs are used in inverter circuits.
• E-MOSFETs are used in different types of motor drivers.
• E-MOSFETs are used in digital controllers.
• E-MOSFETs are used in power electronic integrated circuits (ICs).
• E-MOSFETs are used in digital electronic systems, etc.

Hence, this is all about E-MOSFET (Enhancement Type Metal-Oxide Semiconductor Field Effect Transistors), its types, working, V-I characteristics, and applications.