Metal Oxide Semiconductor FET (MOSFET)

The Metal oxide semiconductor FET (MOSFET) also known as the insulated-gate FET (IGFET) is similar to JFET but exhibits even larger resistive input impedance due to the thin layer of silicon dioxide that is used to insulate the gate from the semiconductor channel. This insulating layer forms a capacitive coupling between the gate and the body of the transistor. The consequent lack of an internal DC connection to the gate makes the device more versatile than the JFET, but it also means that the insulating material of the capacitor can be easily damaged by the internal discharge of static charge developed during normal handling.

The main shortcoming of JFET is that its gate must be reverse biased for proper operation of the device that is, it can only have negative gate operation for n-channel and positive gate operation for p-channel; this means that we can only decrease the width of the channel (i.e. decrease the conductivity of the channel) from its zero-bias size. This type of operation is called depletion-mode operation. Thus, a JFET can only be operated in the depletion-mode.  But for a MOSFET, it can be operated to enhance (increase) the width of the channel (with consequent increase in conductivity of the channel) i.e. it can have the enhancement-mode operation.

The MOSFET is widely employed in large-scale integrated circuits (ICs) where its high input impedance can result in very low power consumption per component. Many of these circuits feature bipolar transistor connections to the external terminals, thereby making the devices less susceptible to damage. MOSFET also has low cost of production as compared to JFET.

Types of MOSFETs

There are two basic types of MOSFETs:

1. Depletion-type MOSFET or D-MOSFET. The D-MOSFET can be operated in both the depletion-mode and the enhancement mode. For this reason, a D-MOSFET is sometimes called depletion/enhancement MOSFET.
2. Enhancement-type MOSFET or E-MOSFET. The E-MOSFET can be operated only in enhancement-mode.

Constructional Details for D-MOSFET

The figure below shows the constructional details of n-channel D-MOSFET.

Normally in practice, the substrate is connected to the source (S) internally so that a MOSFET has three terminals source (S), gate (G), and drain (D).

Since the gate is insulated from the channel, we can apply either negative or positive voltage to the gate. Therefore, D-MOSFET can be operated in both depletion-mode and enhance-mode. On the other hand, JFET can be operated only in depletion-mode.

Construction Details of E-MOSFET

Figure 5 above shows the construction details of n-channel E-MOSFET. Its gate construction is similar to that of D-MOSFET. The E-MOSFET has no channel between source and drain unlike the D-MOSFET. The substrate extends completely to the silicon dioxide (SiO₂) layer so that no channel exists. The E-MOSFET requires a proper gate voltage to form a channel (induced channel). E-MOSFET can only be operated in enhancement mode.

Operation of D-MOSFET

Figure 6 below shows the circuit of n-channel D-MOSFET.

The gate forms a small capacitor. One plate of this capacitor is the gate and the other plate is the channel with metal oxide layer as the dielectric.

When the gate voltage is changed, the electric field of the capacitor changes which in turn changes the resistance of the n-channel and since the gate is insulated from the channel; we can apply either negative or positive voltage to the gate. The negative-gate operation is called depletion mode whereas positive-gate operation is known as enhancement mode.

Depletion Mode

Figure 6 above shows depletion mode operation of n-channel D-MOSFET. Since the gate is negative, it implies that, electrons are on the gate as shown in figure 7 above. These electrons repel the free electrons in the n-channel, leaving a layer of positive ions in a part of the channel as shown in figure 7 above. That is, we have depleted (i.e. emptied) the n-channel of some of its free electrons. Hence, a smaller number of free electrons are made available for current conduction through the n-channel. This is the same as if the resistance of the channel is increased. The greater the negative voltage on the gate, the lesser is the current from the source to drain.

Therefore by changing the negative voltage on the gate, we can vary the resistance of the n-channel and hence the current from source to drain. Take note of the fact that with negative voltage to the gate, the action of D-MOSFET is similar to JFET. Because the action with negative gate depends upon depleting i.e. emptying the channel of free electrons, the negative-gate operation is called depletion mode.

Enhancement-mode

Figure 8 below shows enhancement mode operation of n-channel D-MOSFET.

Once again, the gate acts like a capacitor. Since the gate is positive, it induces negative charges in the n-channel as shown below:

These negative charges are the free electrons drawn into the channel. Since these free electrons are added to those already in the channel, the total number of free electrons is increased. Thus a positive gate voltage enhances or increases the conductivity of the channel.

Therefore by changing the positive voltage on the gate, we can change the conductivity of the channel. The key difference between D-MOSFET and JFET is that we can apply positive gate voltage to D-MOSFET and still have essentially zero current. Because the action with a positive gate depends upon enhancing the conductivity of the channel, the positive gate operation is referred to as enhancement mode.

Related: Junction Field Effect Transistors

D-MOSFET Transfer Characteristics

 The figure below shows the transfer characteristic curve (or transconductance curve) for n-channel D-MOSFET.

The behaviour of this device can be explained as follows:

• The point on the curve where V_GS = 0, I_D = I_DSS. This is expected as I_DSS is the value of I_D when gate and source terminals are shorted i.e. V_GS = 0.
• As V_GS goes negative, I_D decreases below the value of I_DSS till I_D reaches zero when V_GS = V_GS(off) just as with JFET.
• When V_GS is positive, I_D increases above the value of I_DSS. The maximum allowable value of I_D is usually given on the datasheet of D-MOSFET.

The transconductance equation of D-MOSFET is given by:

You can also read: Bipolar Junction Transistors (BJT)