MOSFETs are tri-terminal, unipolar, voltage-controlled, high input impedance devices which form an integral part of vast variety of 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. However inspite of their structural difference, all of them are seen to work on a common basic principle which is explained in detail in the article “MOSFET and its Working“. This further implies that all of them exhibit almost similar characteristic curves, but for differing voltage values.
In general, any MOSFET is seen to exhibit three operating regions viz.,
- 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.
- Ohmic or Linear Region
Ohmic or linear region is a region where in the current IDS increases with an increase in the value of VDS. When MOSFETs are made to operate in this region, they can be used as amplifiers.
- Saturation Region
In saturation region, the MOSFETs have their IDS constant inspite of an increase in VDS and occurs once VDS exceeds the value of pinch-off voltage VP. Under this condition, the device will act like a closed switch through which a saturated value of IDS 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 IDS versus gate-to-source voltage VGS) of n-channel Enhancement-type MOSFETs. From this, it is evident that the current through the device will be zero until the VGS exceeds the value of threshold voltage VT. This is because under this state, the device will be void of channel which will be connecting the drain and the source terminals. Under this condition, even an increase in VDS will result in no current flow as indicated by the corresponding output characteristics (IDS versus VDS) shown by Figure 1b. As a result this state represents nothing but the cut-off region of MOSFET’s operation.
Next, once VGS crosses VT, the current through the device increases with an increase in IDS initially (Ohmic region) and then saturates to a value as determined by the VGS (saturation region of operation) i.e. as VGS increases, even the saturation current flowing through the device also increases. This is evident by Figure 1b where IDSS2 is greater than IDSS1 as VGS2 > VGS1, IDSS3 is greater than IDSS2 as VGS3 > VGS2, so on and so forth. Further, Figure 1b also shows the locus of pinch-off voltage (black discontinuous curve), from which VP is seen to increase with an increase in VGS.
p-channel Enhancement-type MOSFET
Figure 2a shows the transfer characteristics of p-type enhancement MOSFETs from which it is evident that IDS remains zero (cutoff state) untill VGS becomes equal to -VT. 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 IDS is seen to increase in reverse direction (meaning an increase in ISD, signifying an increase in the device current which will flow from source to drain) with the decrease in the value of VDS. 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 VSD).
However as VDS becomes equal to –VP, the device enters into saturation during which a saturated amount of current (IDSS) flows through the device, as decided by the value of VGS. Further it is to be noted that the value of saturation current flowing through the device is seen to increase as the VGS becomes more and more negative i.e. saturation current for VGS3 is greater than that for VGS2 and that in the case of VGS4 is much greater than both of them as VGS3 is more negative than VGS2 while VGS4 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 VGS becomes more and more negative, even the negativity of VP 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 VGS is 0V. This indicates that these devices conduct even when the gate terminal is left unbiased, which is further emphasized by the VGS0 curve of Figure 3b. Under this condition, the current through the MOSFET is seen to increase with an increase in the value of VDS (Ohmic region) untill VDS becomes equal to pinch-off voltage VP. After this, IDS will get saturated to a particular level IDSS (saturation region of operation) which increases with an increase in VGS i.e. IDSS3 > IDSS2 > IDSS1, as VGS3 > VGS2 > VGS1. Further, the locus of the pinch-off voltage also shows that VP increases with an increase in VGS.
However it is to be noted that, if one needs to operate these devices in cut-off state, then it is required to make VGS negative and once it becomes equal to -VT, the conduction through the device stops (IDS = 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 VGS. This is because they are characterized by the presence of a channel in their default state due to which they have non-zero IDS for VGS = 0V, as indicated by the VGS0 curve of Figure 4b. Although the value of such a current increases with an increase in VDS initially (ohmic region of operation), it is seen to saturate once the VDS exceeds VP (saturation region of operation). The value of this saturation current is determined by the VGS, and is seen to increase in negative direction as VGS becomes more and more negative. For example, the saturation current for VGS3 is greater than that for VGS2 which is however greater when compared to that for VGS1. This is because VGS2 is more negative when compared to VGS1, and VGS3 is much more negative when compared to either of them. Next, one can also note from the locus of pinch-off point that even VP starts to become more and more negative as the negativity associated with the VGS increases.
Lastly, it is evident from Figure 4a that inorder to switch these devices OFF, one needs to increase VGS such that it becomes equal to or greater than that of the threshold voltage VT. 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
|Kind of MOSFET||Region of Operation|
|n-channel Enhancement-type||VGS < VT||VGS > VT and VDS < VP||VGS > VT and VDS > VP|
|p-channel Enhancement-type||VGS > -VT||VGS < -VT and VDS > -VP||VGS < -VT and VDS < -VP|
|n-channel Depletion-type||VGS < -VT||VGS > -VT and VDS < VP||VGS > -VT and VDS > VP|
|p-channel Depletion-type||VGS > VT||VGS < VT and VDS > -VP||VGS < VT and VDS < -VP|