Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)

The Metal Oxide Semiconductor Field Effect Transistor or MOSFET is one of the most widely used electronic devices, particularly in digital circuits. Since it is constructed with the gate terminal insulated from the channel, it may referred to as an Insulated Gate Field Effect Transistor (IGFET). However, since most of the devices are made using silicon for the semiconductor, SiO2 for the insulator, and metal for the gate electrode, the term Metal Oxide Semiconductor Field Effect Transistor or simply MOSFET.

Fig.  N Channel Mosfet

Like, a JFET, a MOSFET is also a three terminal (source, gate and drain) device and drain current in it is also controlled by gate bias. The operation of MOSFET is similar to that of JFET and, therefore, all the equations apply equally well to the MOSFET and JFET in amplifier connections. However, MOSFET has lower capacitance and input impedance much more than that of a JFET owing to small leakage current. In case of a MOSFET the positive voltage may be applied to the gate and still the gate current remains zero.

MOSFETs are of two types namely (i) enhancement type MOSFET or E-MOSFET and (ii)depletion enhancement MOSFET or DE-MOSFET. In the depletion-mode construction a channel is physically constructed and current between drain and source is due to voltage applied across the drain-source terminals. The enhancement MOSFET structure has no channel formed during its construction. Voltage is applied to the gate, in this case, to develop a channel of charge carriers so that a current results when a voltage is applied across the drain-source terminals.

The enhancement type MOSFET (E-MOSFET) is widely used in both discrete and integrated circuits. In discrete circuits, the main use is in power switching, which means turning largest currents on and off. Although their use is declined, depletion-mode MOSFETs (DE-MOSFETs) are still found in high-frequency front-end communication circuits as RF amplifiers.

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DEPLETION-ENHANCEMENT MOSFETs (DE-MOSFETs)

Figure 1.1 shows the construction of an N-channel DE-MOSFET. It consists of a lightly doped P-type substrate into which two blocks of heavily doped N-type material are diffused forming the source and drain. An N-channel is formed by diffusion between the source and drain. This type of impurity for the channel is the same as for the source and drain. Now a thin layer of SiO₂ dielectric is grown over the entire surface and holes are cut through the SiO2 (silicon dioxide) layer to make contact with the N-type blocks (Source and Drain). Metal is deposited through the holes to provide drain and source terminals, and on the surface area between drain and source, a metal plate is deposited. This layer constitutes the gate.

A P-channel DE-MOSFET is constructed like an N-channel DE-MOSFET, starting with N-type substrate and diffusing P-type drain and source blocks and connecting them internally by a P-dopped channel region.

OPERATION

It is obvious from Fig. 1.1 that there is no P-N junction between gate and channel. Here the diffused channel N, the insulating dielectric SiO layer and the metal layer of the gate forms a parallel plate capacitor. DE-MOSFET can be operated with either a positive or a negative gate. When gate is positive with respect to the source it operates in the enhancement or E-mode and when the gate is negative w.r.t. the source, as illustrated in Fig. 1.3, it operates in depletion-mode.

When the drain is made + ve with respect to the source, a drain current will flow, even with zero gate potential and the MOSFET is said to be operating in E-mode. In this mode of operation gate attracts the - ve charge carriers from the P-substrate to the N-channel and thus reduces the channel resistance and increases the drain current. The more positve the gate is made, the more drain current flows.

On the other hand when the gate is made - ve with respect to the substrate, the gate repells some of the - ve charge carriers out of the N-channel. This creates a depletion region in the channel, as illustrated in Fig. 1.3, and, therefore, increases the channel resistance and reduces the drain current. The more - ve the gate, the less the drain current. In this mode of operation the device is referred to as a depletion-mode MOSFET.

CHARACTERISTICS

Typical drain characteristics, for various levels of gate-source voltage, of an N-channel MOSFET are shown in Fig. 1.4. The upper curves are for positive V_GS and the lower curves are for negative V_GS. The bottom drain curve is for V_GS = V_GS(OFF).

For a specified drain-source voltage V_DS, V_GS(OFF) is the gate-source voltage at which drain current reduces to a certain specified negligibly small value, as shown in Fig. 1.4. This voltage corresponds to the pinch-off voltage Vp of JFET. For V_GS between V_GS(OFF) and zero, the device operates in depletion-mode while for V_GS exceeding zero the device operates in enhancement mode. These drain curves again display an ohmic region. MOSFET has two major applications: a constant current source  and a voltage variable resistor.

The transfer (or transconductance) characteristics for an N-channel DE-MOSFET is shown in Fig. 1.5. I_DSS is the drain current with a shorted gate. Since the curve  extends to the right of the origin, I_DSS is no longer the maximum possible drain current. Mathematically, the curve is still part of a parabola and the same square-law relation exists as with a JFET. In fact, the depletion-mode MOSFET has a drain current given by the transconductance equation:

I_D = I_DSS ( 1 - V_GS / V_GS(OFF))²

Furthermore, it has the same equivalent circuits as a JFET. Because of this,the analysis of a depletion-mode MOSFET circuit is almost identical to that of a JFET circuit. The only difference is the analysis for a positive gate, but even here the same basic  formulas are used to determine the drain current I_D, gate-source voltage V_GS etc.

The foregoing discussion is applicable in principle also to the P-channel DE-MOSFET. For such a device the sign of all currents and voltages in the characteristics must be reversed.