MOSFET Characteristics | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

MOSFETs are three-terminal, unipolar, voltage-controlled devices with high input impedance, crucial in many electronic circuits.

These devices can be classified into two types viz., depletion-type and enhancement-type, depending on whether they possess a channel in their default state or no, respectively.

Further, each of them can be either p-channel or n-channel devices as they can have their conduction current due to holes or electrons respectively.

MOSFET Operation

In general, any MOSFET is seen to exhibit three operating regions viz.,

1. Cut-Off Region
   Cut-off region is a region in which the MOSFET will be OFF as there will be no current flow through it. In this region, MOSFET behaves like an open switch and is thus used when they are required to function as electronic switches.
2. Ohmic or Linear Region
   Ohmic or linear region is a region where in the current I_DS increases with an increase in the value of V_DS. When MOSFETs are made to operate in this region, they can be used as amplifiers.
3. Saturation Region
   In saturation region, the MOSFETs have their I_DS constant inspite of an increase in V_DS and occurs once V_DS exceeds the value of pinch-off voltage V_P. Under this condition, the device will act like a closed switch through which a saturated value of I_DS flows. As a result, this operating region is chosen whenever MOSFETs are required to perform switching operations.

Having known this, let us now analyze the biasing conditions at which these regions are experienced for each kind of MOSFET.

n-channel Enhancement-type MOSFET

Figure 1a shows the transfer characteristics (drain-to-source current I_DS versus gate-to-source voltage V_GS) of n-channel Enhancement-type MOSFETs. From this, it is evident that the current through the device will be zero until the V_GS exceeds the value of threshold voltage V_T.

In this state, the device lacks a channel connecting the drain and source terminals.

Under this condition, even an increase in V_DS will result in no current flow as indicated by the corresponding output characteristics (I_DS versus V_DS) shown by Figure 1b. As a result this state represents nothing but the cut-off region of MOSFET’s operation.

Once V_GS exceeds V_T, the current increases with I_DS initially (Ohmic region) and then saturates at a value determined by V_GS (saturation region). As V_GS increases, the saturation current also increases.

This is evident by Figure 1b where I_DSS2 is greater than I_DSS1 as V_GS2 > V_GS1, I_DSS3 is greater than I_DSS2 as V_GS3 > V_GS2, so on and so forth. Further, Figure 1b also shows the locus of pinch-off voltage (black discontinuous curve), from which V_P is seen to increase with an increase in V_GS.

p-channel Enhancement-type MOSFET

Figure 2a shows the transfer characteristics of p-type enhancement MOSFETs from which it is evident that I_DS remains zero (cutoff state) untill V_GS becomes equal to -V_T.

This is because, only then the channel will be formed to connect the drain terminal of the device with its source terminal. After this, the I_DS is seen to increase in reverse direction (meaning an increase in I_SD, signifying an increase in the device current which will flow from source to drain) with the decrease in the value of V_DS.

This means that the device is functioning in its ohmic region wherein the current through the device increases with an increase in the applied voltage (which will be V_SD).

However as V_DS becomes equal to –V_P, the device enters into saturation during which a saturated amount of current (I_DSS) flows through the device, as decided by the value of V_GS.

Further it is to be noted that the value of saturation current flowing through the device is seen to increase as the V_GS becomes more and more negative i.e. saturation current for V_GS3 is greater than that for V_GS2 and that in the case of V_GS4 is much greater than both of them as V_GS3 is more negative than V_GS2 while V_GS4 is much more negative when compared to either of them (Figure 2b).

In addition, from the locus of the pinch-off voltage it is also clear that as V_GS becomes more and more negative, even the negativity of V_P also increases.

n-channel Depletion-type MOSFET

The transfer characteristics of n-channel depletion MOSFET shown by Figure 3a indicate that the device has a current flowing through it even when V_GS is 0V. This indicates that these devices conduct even when the gate terminal is left unbiased, which is further emphasized by the V_GS0 curve of Figure 3b.

Under this condition, the current through the MOSFET is seen to increase with an increase in the value of V_DS (Ohmic region) untill V_DS becomes equal to pinch-off voltage V_P. After this, I_DS will get saturated to a particular level I_DSS (saturation region of operation) which increases with an increase in V_GS i.e. I_DSS3 > I_DSS2 > I_DSS1, as V_GS3 > V_GS2 > V_GS1.

Further, the locus of the pinch-off voltage also shows that V_P increases with an increase in V_GS.

However it is to be noted that, if one needs to operate these devices in cut-off state, then it is required to make V_GS negative and once it becomes equal to -V_T, the conduction through the device stops (I_DS = 0) as it gets deprived of its n-type channel (Figure 3a).

p-channel Depletion-type MOSFET

The transfer characteristics of p-channel depletion mode MOSFETs (Figure 4a) show that these devices will be normally ON, and thus conduct even in the absence of V_GS.

This is because they are characterized by the presence of a channel in their default state due to which they have non-zero I_DS for V_GS = 0V, as indicated by the V_GS0 curve of Figure 4b. Although the value of such a current increases with an increase in V_DS initially (ohmic region of operation), it is seen to saturate once the V_DS exceeds V_P (saturation region of operation).

The value of this saturation current is determined by the V_GS, and is seen to increase in negative direction as V_GS becomes more and more negative.

For example, the saturation current for V_GS3 is greater than that for V_GS2 which is however greater when compared to that for V_GS1. This is because V_GS2 is more negative when compared to V_GS1, and V_GS3 is much more negative when compared to either of them.

Next, one can also note from the locus of pinch-off point that even V_P starts to become more and more negative as the negativity associated with the V_GS increases.

Lastly, it is evident from Figure 4a that inorder to switch these devices OFF, one needs to increase V_GS such that it becomes equal to or greater than that of the threshold voltage V_T.

This is because, when done so, these devices will be deprived of their p-type channel, which further drives the MOSFETs into their cut-off region of operation. 

The explanation provided above can be summarized in the form of a following table