MOSFET, Metal Oxide Semiconductor Field Effect its operation and Types of MOSFET
Table of Contents
Metal Oxide Semiconductor Field Effect
Metal Oxide Semiconductor FET (MOSFET) is also known as Insulated Gate FET (IGFET). A number of characteristics of MOSFET are quite similar to JFET’s characteristics. MOSFET is a device, the input impedance of which is extremely high. Moreover, apart from the source, drain, and gate just like a FET, it consists of a conducting channel, the resistance of which is controlled through gate voltage. The basic difference between JFET and MOSFET lies in the structure and construction of the gate. In JFET, the gate-channel path uses to be a PN junction, which is usually reverse biased. MOSFET does not have any gate-channel PN junction, instead, a narrow layer of some insulating material (silicon dioxide) exists between the gate and channel. The insulator has extremely high resistance (normally 1012 Ω), due to which MOSFET gate leakage current is extremely low ((normally 1pA) compared to JFET. As no PN junction exists between gate and channel in MOSFET, therefore, there is no limitation on the polarity of gate voltage. Insulator existing between gate and channel offers maximum resistance for any bias polarity. As its gate is insulated with a channel, therefore, MOSFET is also known as insulated gate JFET (IGJFET). However, the most common and vastly used term is MOSFET.
Operation of the MOSFET
When gate voltages in a JFET are zero, a conducting channel occurs between drain and source. And this channel can get narrower through producing depletion regions by reverse biasing gate-source. This operation mode is called depletion mode. In the depletion mode, the channel can only be made from narrow to narrower through the application of gate-source voltage and it cannot be broadened to a reasonable limit. Some of the MOSFET are such that they can operate in depletion mode as well, while the majority operate in a different mode, which is called enhancement mode. In the enhancement mode, the width of a conducting channel can be enhanced by increasing the value of width VGS=0 using a suitable polarity of gate-source voltage.
Types of MOSFET
There are two types of MOSFET
It can also be briefly mentioned as E-MOSFET. These MOSFETs operate only in enhancement mode.
Depletion -Enhancement MOSFET:
It is also briefly called De-MOSFET and its mode of operation depends on the polarity of gate voltage. That’s they function only in depletion mode or enhancement mode.
Enhancement Only MOSFET or E-MOSFET)
As the name suggests, this MOSFET operates only in enhancement mode and it is not capable of depletion mode. They only operate with positive gate voltage. From a construction viewpoint, a channel does not exist between its source and drain. Therefore, when the VGS value is zero (i.e. VGS=0), it does not conduct. Therefore, it is also called a normal MOSFET. It is vastly used in digital and computer circuits.
In diagram 5.35, the construction of an N channel E- MOSFET has been described. Its construction starts with a high resistivity P-type substrate, above which two low resistivity N-type regions are diffused inside the substrate. As is clear from the diagram. After this, the entire structure, and the surface is covered with an insulating layer of silicon dioxide (or silicon dioxide layer is spread all over its surface). The oxide layer blocks holes and establishes contact with the N region (drain and source). Afterward, a metal contact area is placed above oxide, which encompasses the entire channel from source to drain. The Gate terminal is joined with this metal area. Remember, no physical contact sets up because silicon dioxide provided insulation between the gate and P substrate.
As drain and source get separated due to P-type substrate, therefore, the current flowing from source to drain is very low due to the fixing of back to back two PN junctions. However, a gate is being used for generating a conductive channel from source to drain. The metal area of the gate, silicon dioxide layer, and semiconductor channel, assume the shape of a capacitor. (In such a situation, the gate area constitutes an upper capacitor plate and plate below the P substrate, whereas silicon dioxide is die-electric). In other words, it forms a metal oxide semiconductor (MOS).
Figure 5.36 – In the N-channel enhancement MOSFET, no channel exists until the gate is made positive with respect to the substrate. The positive potential on the gate attracts negative charge carriers to form a channel between the drain and source terminals
A P channel enhancement MOSFET is also formed in a manner similar to N channel enhancement MOSFET, with the exception that entire P and N regions are mutually interchanged. It commences from an N-type substrate and P-type drain and source blocks are diffused above it.
When drain is positive with respect to source, and no potential provided to gate, in such a situation two N blocks and P type substrate constitute two back to back PN junctions. Both these junctions cannot be forward bias simultaneously, therefore, very minor drain current (reverse leakage current) transmits (pass).
When positive voltages are supplied on gate with respect to source and substrate (diagram 5.35 b), positive charge forms up on metal plate. This positive charge builds a negative charge on semiconductor plate (because supplying positive potential on gate, (minority) charge carriers present in P type substrate move and assemble closer to surface of P substrate through a positive gate attraction). As positive voltage on gate are enhanced, holes present in P type semiconductor repel (because charges of similar nature repel each other) and maximum electrons get collected below gate (remember, these electrons cannot move towards gate through crossing the silicon dioxide layer), until the area or region below the oxide layer forms into a N type semiconductor region. Thus, a N channel builds up between source and drain. Now through positive drain biasing with respect to source (as was done for N channel JFET), current can be transmitted from source to drain through this generated N channel, the quantity of which depends on resistance of channel. Thus, with application of positive gate voltage, the current passing through, the device can be enhanced. Therefore, this device is called enhanced mode MOSFET. In fact, turning gate voltage more positive (or providing more positive voltages on gate), widens produced N channel. Due to this, channel’s resistance subsides further. Consequently, current flow from source towards drain increases. Remember, enhancement effect does not generate via zero or negative gate voltage and the device will not conduct under such a situation. Therefore, E-MOSFET is normally called MOSEFT.
In the case of a P channel device, negative voltages are provided on a gate, due to which an N-type channel develops. Through this, the flow of current is enhanced.
In diagram 5.37, symbols of PN Enhancement MOSFET (or E-MOSFET) have been shown. According to the symbols, the gate has been kept isolated from the rest of the MOSFET, which indicates that the gate is electrically insulated from the channel. Like the JFET symbol, an inward arrow sign in E-MOSFET also represents an N channel while an outward arrow sign implies a P channel. Besides, the dashed or broken vertical line, which connects the drain with the source, must also be kept in mind because it reveals a normally OFF device (in case of zero VGS value, the channel tends to operate as an open circuit)
Both substrate symbols of this device have been reflected via an arrow sign and the same has been connected with the source. These connections are usually internally fitted by the designers. However, sometimes, MOSFET is provided supply just like a four-terminal device through jutting substrate out as a lead. In such a situation, the MOSFET symbol is amended. In diagram 5.38, two circuit symbols of an N channel Enhancement MOSFET have been illustrated. One of the symbols has three terminals (this symbol is most prevalent) while the other symbol has four terminals, wherein substrate and lead have been demonstrated separately instead of being combined together.
Figure 5.38- circuit symbols for N-channel enhancement MOSFET. The gate bar is separated from the channel to show that there is no direct connection between them. The channel line is broken to indicate that no channel exists until an appropriate bias is applied
In figure 5.39, drain characteristics of N channel E-MOSFET have been displayed with the help of curves. Its pinch-off (saturation) and ohmic regions are exactly analogous to a JFET. It is evident from the diagram that as long as the VGS value does not get to +2V, the flow of the drain current does not initiate. This voltage is called a threshold voltage i.e. VGS (th). It can also vividly be seen in the diagram that the polarity of VGS and VDS are the same. As against a JFET, it is a manifested advantage of this device because just a single power supply is sufficient for operating several amplifiers and switching circuits.
When the biasing of the device is done, ID does not increase at all in case the VGS value is zero, until some positive voltages are applied. Drain current values increase via an increase in positive gate-source voltage value. We have to sustain enough positive high gate voltage for attaining an explicit quantity of drain current. These voltages create a narrow layer of free electrons juxtaposed to a metal oxide layer which spreads across the entire region from source to drain. The minimum gate-source voltage which builds up this narrow layer and also produces drain current is called a threshold voltage (VGSth). When the VGS value exceeds VGSth, as VGS is increased on a given VDS value, the channel becomes deeper and the ID value increases. As the MOSFET gate is insulated by means of a channel, therefore no leakage current occurs, which indicates a very high input resistance of this device which sometimes exceeds even 1015 ohm.
Depletion- Enhancement MOSFET or De-MOSFET
This type of MOSFET has been named De-MOSFET because it can operate both in the depletion mode and enhancement mode by means of variations in VGS polarity. When the negative gate to source voltages are provided, N channel De-MOSFET operates in depletion mode. However, it starts operating in enhancement mode as soon as the gate voltage turns positive. As this kind of channel exists between drain and source, therefore ID transmits even if the VGS value is zero. That’s the reason a De-MOSFET is also known as a (normally on) MOSFET.
The construction of a De-MOSFET is almost exactly similar to that of an E-MOSFET with the exception that a conducting channel exists between the source and drain in De-MOSFET despite the absence of a gate voltage. In other words, a lightly doped N-type channel is mounted midway between two heavily doped sources and drain blocks in a De-MOSFET. Resistance of the channel can fluctuate through an adequate gate-source bias voltage. In figure 5.40, the construction of an N channel De-MOSFET with suitable resistivity is also visible. By means of this initial N channel, current flows from the source towards the drain with the necessity of any gate voltage.
A De-MOSFET also comprises a source, gate, and drain similar to a JFET, however, its gate is insulated from the conducting channel via an insulating layer of silicon dioxide (SiO2). As a result of this insulating property, it is also known as an insulated gate field effect (IGFET) or simply IGT. Here too, drain current is controlled through gate voltage. However, the fundamental difference between a JFETB and a MOSFET is that in the case of MOSFET, we can provide both positive or negative voltages on the gate (because its gate has been insulated with a channel). Moreover, the gate, SiO2 insulator, and channel adopt the shape of a parallel plate capacitor. Contrary to JFET, De-MOSFET consists of just one P or N region, which is known as the substrate. Normally, it shorts the source intrinsically. In figure 5.41, both P and N channel De-MOSFET have been illustrated along with the symbols.
The working mechanism of this device can easily be understood with the help of the below-mentioned two modes:
- Depletion mode of N channel De-MOSFET
- Enhancement mode of N channel De-MOSFET
when the drain is made positive with respect to the source and its gate voltage is zero (i.e. VGS=0), electrons or drain current starts flowing from the source towards the drain through the conducting channel (which is located between the two) Figure 5.42. if the gate is made negative with respect to the substrate (i.e. if negative voltages are provided on the gate), some of the gates’ negative charge carriers (i.e. electrons) repel the channel’s negative charge carriers, due to which they emit from N-type channel and as a consequence, a depletion region forms within the channel. As a result, channel resistance increases. Due to an increase in resistance, drain current reduces. In other words, when negative voltages are applied to the gate, holes attract towards the N channel from the P substrate and start neutralizing free electrons, which results in the creation of a depleted or reduced N channel. Channel’s resistance increases due to a reduction in size and thus, the flow of current passing through the channel declines. Through increasing values of negative voltage on the gate (or increasing negative charge on the gate), the number of electrons drifting through the channel reduces. As a result, the channel’s conductivity also declines. Remember that channel may cut off completely in case excessive negative gate voltages VGS (OFF) are provided. Thus, with negative gate voltage, a DE-MOSFET operates just like a JFET. As the N-type channel of free charge carriers depletes due to the operation of negative gate voltage, therefore, this device is also called depletion mode MOSFET and this process of the device is known as depletion mode operation.
Figure 5.42 -The N – channel depletion enhancement MOSFET is similar to the enhancement device, except that N-type channel is included when the device is fabricated. The channel resistance may be increased or decreased by appropriate gate-source bias voltages.
Working of De-MOSFET in N-Channel Enhancement Mode
When positive voltages are supplied on a gate, input gate capacitors become capable of producing free electrons in the channel, due to which ID increases. These free electrons increase the total number of electrons by connecting with electrons already present in the channel, which enhances the conductivity of the channel. As positive gate voltages are added, the number of electrons produced also increases, and channels’ conductivity located between source and drain also keeps increasing. Thus, maximum current passes through the terminals. Therefore, the positive gate operation of a De-MOSFET is also known as enhancement mode operation.
Actually, the width of the initial N channel increases with the supply of positive gate voltage. Resultantly, channel current also increases under a given drain-source voltage (as channel resistance reduces with an increase in its area/ region)
Standard De-MOSFET symbols issued by the Institute of Electrical and Electronics Engineers (IEEE) have been illustrated in diagram 5.43, in which normal drain-source bias polarities can also be seen. Gate -source polarity has not been shown in the figure, because they might have any polarity. It should be remembered that in the case of an N-type channel device, the arrow sign seems to be inward going whereas in the case of P-type channel device, the arrow sign is outward going. It has also to be a mind that in a De-MOSFET symbol, drain to the source has been reflected vide a solid/unbroken line, which implies that the device channel is conducting normally or that De-MOSFET is normally ON the device.
Drains Curves of De-MOSFET
The characteristics of an N channel De-MOSFET have been shown in figure 5.44 with the help of some specific curves. These curves reflect the cumulative operation of the device. The curves resemble in shape the characteristic curves of the rest of the FET devices and various curves indicate different VGS values. At zero VGS value i.e. VGS=0, a pinch-off current value is obtained (which is represented by the symbol IDSS). In the situation of a positive VGS value, channel size enhances or becomes spacious, and drain current value also becomes higher than IDSS. This is the enhancement region. As a result of negative VGS values, the channel gets depleted and the value of drain current also gets lower compared to IDSS. Finally, when the VGS value becomes excessively negative, the channel current gets almost completely OFF (i.e. flow of current via channel stops). This happens when VGS= VGS (OFF), which has been denoted in the figure. Thus, a negative VGS value builds up a depletion region.
A particular set of drain curves can also be obtained by means of reversing the voltage polarity. However, due to the difficulties being observed in the manufacturing process, P channel De-MOSFET are not so well-accepted as compared to the N channel.
Comparison of JFET and MOSFET
Mode of Operation
JFET is operated by means of reverse biasing its gate to the source junction. It is called “operating in the depletion mode”. Enhancement mode MOSFET can only be operated in enhancement mode (as its name suggests). It means that its gate should be forward-biased with respect to its source. Depletion type MOSFET can be operated both in the depletion as well as enhancement modes.
JFET and MOSFET are normally considered ON devices because if no bias exists between the gate and source of such devices (i.e. VGS=0V), a reasonable quantity of drain current can still transmit through it. Whereas in a JFET, drain current is the maximum current (IDSS), which gets transmitted through the device. E-MOSFET is normally considered an OFF device because no current can actually transmit through it at VGS=0V. It has, therefore, to be kept in mind that tend to consume power even if no signals are provided to them.
The value of leakage current flowing from source to drain in a MOSFET is quite negligible as compared to JFET. Its reason is that gate is insulated from the source via a layer of silicon dioxide (SiO2)
Application of MOSFETs
MOSFETs are normally used for switching and amplification purposes. When these are used as an amplifier, their voltage gain is less than compared to a BJT amplifier. However, where ever high frequencies matter, it performs relatively better. MOSFETs are mainly used for switching purposes in computer circuits, computer memories, or microprocessors.
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