Basic Electronics

# MOSFET as a Switch Explained with Circuit Diagrams

## MOSFET as a Switch:

MOSFET as a Switch- MOSFETs are most widely applied in computer circuits as a switching device. Because, when its gate voltage value exceeds the threshold voltage, this device switches from its cut-off region to the saturation region. Voltages, which turn on an enhancement mode MOSFET, an inversion layer joints source to drain on these voltages. Therefore, this device is extensively used in the computer industry due to its on-off operation. Millions of such kinds of devices are applied in computer circuits, the basic task of which is to operate them like typical digital switches. Remember when there are zero voltages on the device gate, it is in OFF condition (or it is in non-conducting mode). When voltage values on the gate exceed zero, they start to conduct i.e. they turn ON.

In figure 5.45, an E-MOSFET circuit has been shown in the direction of a passive load (i.e. RD). this circuit is the simplest form of the digital switch. It is also called the E-MOSFET inverter circuit.

In this circuit, input voltages Vin are either low or they are high. When Vin is low, MOSFET is cut-off and Vout is equal to the supply voltage VDD. When Vin is high, MOSFET is saturated and Vout drops and thus becomes very low (i.e. zero). In other words, when the value of Vin equals VDD (i.e. Vin = VDD), then MOSFET turns ON and in this situation, we get an absolute zero voltage on output (this level is called digital low state). When Vin value is zero volts, i.e. Vin = 0V, then MOSFET remains OFF. Because, under such a situation no channel exists between drain and source for the flow of current, therefore the value of drain current is zero. As a result, the output voltage becomes equal to VDD (this level is called the digital high state). Remember that this circuit converts digital high state to digital low state and digital low state to digital high state. That’s the reason such a circuit is called an inverter circuit (i.e. as output voltages are opposite to the input voltages in this circuit, it is therefore called an inverter circuit). As load resistance of this circuit (RD) is not an active device like a semiconductor or load resistor, therefore such a specific type of inverter is known as a passive switch. For working with this circuit efficiently, it is necessary that drain saturation current ID (sat) must always be lower as compared to ID (on), whereas input voltage Vgs (on) must equal or be higher. This matter can be described in a simplified form as well in that when ohmic region RDS (on) is lower as compared to passive drain resistance RD, i.e. RDS (on)<<RD, then this circuit operates properly.

Remember that when input voltages of the circuit are low, its output voltages are high and when input voltages are high, output voltages are low. For analyzing such types of switch circuits, extreme caution should be exercised in terms of accuracy. Therefore, keeping in view the input and output voltages of the circuit, they are only identified only as low or high.

In figure 5.46, an active switch inverter circuit has been shown. As another MOSFET has been fixed in this circuit in place of load resistance (RD) which operates as an active device, therefore this device acts as an active switch inverter. The need for manufacturing this type of circuit was felt because the passive load resistor (which has been discussed above) is physically quite bulky in size as compared to a MOSFET, due to which the size of integrated circuits (wherein a passive resistor is being used) increased too much. Therefore, in order to reduce the size of integrated circuits considerably and for manufacturing small-sized personal computers (which are being commonly used these days) a MOSFET is fitted in the circuit instead of a passive load resistor. Furthermore, MOSFET is such a top-quality device, which operates as high impedance via applying a gate-source short. Thus, as a result of this reason, an active switch has become more popular as compared to a passive switch in digital integrated circuits.  Remember that when a MOSFET is operated within its ohmic region, it functions as a resistor.

According to the diagram, lower MOSFET acts as a voltage control switch while upper MOSFET acts as an active load fitted on the high resistance series, the value of which depends on the “ON” or “OFF” status of MOSFET. As the gate of the above-mentioned MOSFET has been connected with the drain (as has been shown in the figure), therefore it assumes the shape of a device with two terminals, the active resistance of which to the following

RD=VDS (active)/ ID (active)

For operating the circuit properly, the RD of the upper circuit should be quite large (about 10 times) as compared to the lower MOSFET RD (on). In such a situation, when the circuit is provided with high or low input voltage, it acts as a high-value resistance (RD) fitted on a switch series. As a result, inverted high or low voltages are accrued.

In the figure 5.47, a circuit equivalent to an inverter (whether fitted on an active series or passive series) has been shown

## MOSFET Characteristics:

In figure 5.48 (a) a set of drain characteristics of an NMOS has been shown, in which VGS and VDS have been retained mutually equal. From the characteristic curves, one can comprehend that when an E-MOSFET is fitted as a resistor on some other MOSFET series, it acts as a non-linear resistor (i.e. the device always remains in the active region). The value of upper MOSFET’s RD can be determined via this non-linear curve. At whatever point this curve is checked, at every point, the values of VGS and VDS always remain equal, due to which the non-linear character of this resistor can be understood.

Figure 5.48 A)- A plot of VGS=VID on a set of NMOS drain characteristics, showing the non-linear nature of the enhancement MOSFET connected as a resistor

In figure 5.49 (b), a specific set of NMOS drain characteristics of a Q1 transistor have been highlighted while in figure (a), an NMOS inverter circuit has been illustrated, in which an NMOSFET (Q2) resistor has been applied as a load resistor, while Q1 acts as a voltage control switch on this circuit, which opens and closes through Vin. When Q2 is used as a load resistor, it then acts as a non-linear load resistor. Therefore, in figure (b) a non-linear load line of a resistor has been revealed above the characteristic curves.

Figure 5.49 – The NMOS inverter switches between point A (low) and point B (High) on the non-linear load line representing Q2.

When Q1 is off, the value of IDI is about zero, and the VDS1 (inverter output voltage or Q1 drain force voltage) value is extremely high. In diagram (b), such a situation has been shown vide point B. when Q1 is conducting (i.e. when it is ON) the value of VDSI (these voltages have been reflected by Von in the diagram) is very low and Q1 remains in its voltage-controlled resistance region at that time. This condition has been shown by point A in diagram (b)

Thus, output voltages switch between low and high (or between Von and VDD-Vt) as is manifested in the figure. As an output of any other similar circuit is normally input of that inverter, therefore it is assumed about an inverter’s input that it also varies between two same/uniform levels. When an input is Von, the value of which is very low as compared to V1, Q1 does not conduct due to which high output (i.e. VDD-VT) is obtained.  This has been shown in the figure by point B.

When an input is high (i.e. VGSI=VDD-VI) then Q1 conducts, as a result of which low output produces (as has been shown by point A in the figure). Remember, prospective ratios between Q1 and Q2 have been designed in such a way that Q2 resistance is higher (about 10 times) than Q1 resistance, thus best-switching characteristics can be obtained.

## MOSFET Amplifiers

As three basic amplifier circuits (i.e. common source JFET amplifier, common drain amplifier, and common gate amplifier) can be made along with JFETs (described in detail in the previous pages). Similarly, using MOSFETs instead of JFETs, MOSFET amplifiers can also easily be manufactured. The basic difference between a JFET amplifier and a MOSFET amplifier is the type of bias used in them. However, remember that a De-MOSFET is normally supplied with a zero bias i.e. VGS=0, whereas an E-MOSFET is normally supplied biasing on a higher VGS as compared to a threshold value. Moreover, the equations of Aʋ, Zin, and Zout, which are obtained via all the three series of JFET amplifiers, exactly the same equations are also acceptable for MOSFET amplifiers.

In figure 5.50, a self-bias common source amplifier consisting of a D-MOSFET has been shown. Analyzing the DC of such an amplifier is relatively easy compared to a JFET amplifier because VGS =0 equals ID=IDSS (remember that with the exception of D-MOSFET, zero bias is not used in any other JFET or E-MOSFET. Once the value of ID is known, the value of VD can easily be ascertained with the help of the following formula.

VD= VDD-IDRD

Whereas voltage gain value and input resistance value of such an amplifier are as follows:

Aʋ= gm Rd

Rin= RG║[VGS/IDSS]

Figure 5.51- E-MOSFET common source amplifier

In figure 5.51, a voltage divider bias common source amplifier comprising an E-MOSFET has been shown. The objective of using voltage divider biased E-MOSFET in this circuit is to ascertain the value of VGS which is greater than the threshold value. Different values of such an amplifier circuit are given below:

VGS= [R2/R1+R2] VDD

ID=K(VGS-VGS (th))2

VDS=VDD-ID RD

Aʋ=gm Rd

Rin= R1║R2║[VGS/IDSS] Or R1║R2║Rin (gate)

The gain of MOSFET amplifiers is less as compared to the JFET amplifiers, however, due to a low noise attribute, their usage is quite preferred. Just like JFET, trans-conductance gm can be altered via variations in gate-source voltage, therefore, these devices are applied in automatic gain control circuits (AGC)

## Handling Precautions of MOSFETs

We know that a thin or narrow layer of silicon dioxide (SiO2) exists between the gate and the channel of a MOSFET, which behaves like an insulator for gate current, in case of a positive or negative gate voltage. If due for any reason or accidentally, gate-source voltage increases, this thin or narrow layer is destroyed or it becomes totally useless (remember only 100 volts are enough for rendering a MOSFET ineffective, therefore,100 volts can easily be obtained through static charges or defective insulated soldering iron isolated from AC power line). Hence, greater caution is imminent in the use of MOSFET

All MOSFETs have the pending danger of getting damaged or destroyed as a result of electrostatic charges. As the gate of a MOSFET is insulted with the channel, therefore its input resistance is very high. As its gate channel junction resembles a high resistance capacitor, therefore just a few electrons are required for generating a high voltage parallel to it until the MOSFET is lifted by holding it from its lead (doing so leads to the accumulation of an excessive static charge on it) even then it could get damaged or become ineffective.

In order to avoid the increase in static charges occurring as a result of holding a MOSFET in their hands, metal rings are placed on their heads before sending them to the market. And when these are desired to be placed/ installed on the circuit, the rings are removed first. Sometimes, a conducting foam is placed between MOSFETs’ leads instead of using shorting rings. MOSFET can also be protected against static charge or stray current by fitting back to back Zener diodes between it source and gate.

As ratings of these Zener diodes is less than VGS (i.e. Zener diodes are designed in such a manner that they breakdown only on 50V), therefore they start their working prior to the destruction of the narrow silicon dioxide layer in MOSFET due to the presence of a stray charge, thus this layer is protected. However, one of the defects of this method is that the input resistance of MOSFET gets slightly reduced as a result of the Zener diode, and the gate leakage current of MOSFET increases. In order to overcome this loophole, the gate of MOSFET is shorted through the source or drain, so long as it is fixed in the circuit. Further the tip of the soldering iron going to be used should be grounded properly.

Apart from electro-static discharge or supply of discrete excessive voltages, there is also a danger of MOSFET being damaged in some other way. If a MOSFET is plugged or unplugged on a circuit at a time when the circuit is energized or its power is on its gate, the source voltage can enhance due to the temporary inductive kickback, due to which the device can get useless or futile.

In short, the following aspects must necessarily be taken care of at the time of application of a MOSFET.

1). They should always be stored in the metal rings or conductive foam

2). All instruments and metal benches that are proposed to be used, should properly be earthed or grounded

3). The wrist of the person who assembles them or holds them, should be connected to the earth through some piece of wire and a high-value series resistor.

4). They should never be detached from the circuit when powered on, nor should these be inserted into the circuit in that condition.

5). When DC power supply is off, they should not be supplied signals

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