Steady state output i-v characteristics of a MOSFET
The MOSFET, like the BJT is a three terminal device where the voltage on the gate terminal controls the flow of current between the output terminals, Source and Drain. The source terminal is common between the input and the output of a MOSFET. The output characteristics of a MOSFET is then a plot of drain current (iD) as a function of the Drain – Source voltage (vDS) with gate source voltage (vGS) as a parameter. Fig 6.5 (a) shows such a characteristics.
With gate-source voltage (VGS) below the threshold voltage (vGS (th)) the MOSFET operates in the cut-off mode. No drain current flows in this mode and the applied drain– source voltage (vDS) is supported by the body-collector p-n junction. Therefore, the maximum applied voltage should be below the avalanche break down voltage of this junction (VDSS) to avoid destruction of the device. When VGS is increased beyond vGS(th) drain current starts flowing. For small values of vDS (vDS < (vGS – vGS(th)) iD is almost proportional to vDS. Consequently this mode of operation is called “ohmic mode” of operation. In power electronic applications a MOSFET is operated either in the cut off or in the ohmic mode. The slope of the vDS – iD characteristics in this mode is called the ON state resistance of the MOSFET (rDS (ON)). Several physical resistances as shown in Fig 6.5 (b) contribute to rDS (ON). Note that rDS (ON) reduces with increase in vGS. This is mainly due to reduction of the channel resistance at higher value of vGS. Hence, it is desirable in power electronic applications, to use as large a gate-source voltage as possible subject to the dielectric break down limit of the gate-oxide layer. At still higher value of vDS (vDS > (vGS – vGS (th)) the iD – vDS characteristics deviates from the linear relationship of the ohmic region and for a given vGS, iD tends to saturate with increase in vDS. The exact mechanism behind this is rather complex. It will suffice to state that, at higher drain current the voltage drop across the channel resistance tends to decrease the channel width at the drain drift layer end. In addition, at large value of the electric field, produced by the large Drain – Source voltage, the drift velocity of free electrons in the channel tends to saturate as shown in Fig 6.5 (c). As a result the drain current becomes independent of VDS and determined solely by the gate – source voltage vGS. This is the active mode of operation of a MOSFET. Simple, first order theory predicts that in the active region the drain current is given approximately by
Equation (6.3) is shown by a dotted line in Fig 6.5 (a). The relationship of Equation (6.1) applies reasonably well to logic level MOSFETs. However, for power MOSFETs the transfer characteristics (iD vs vGS) is more linear as shown in Fig 6.5 (d).
At this point the similarity of the output characteristics of a MOSFET with that of a BJT should be apparent. Both of them have three distinct modes of operation, namely, (i)cut off, (ii) active and (iii) ohmic (saturation for BJT) modes. However, there are some important differences as well.
• Unlike BJT a power MOSFET does not undergo second break down.
• The primary break down voltage of a MOSFET remains same in the cut off and in the active modes. This should be contrasted with three different break down voltages (VSUS, VCEO & VCBO) of a BJT.
• The ON state resistance of a MOSFET in the ohmic region has positive temperature coefficient which allows paralleling of MOSFET without any special arrangement for current sharing. On the other hand, vCE (sat) of a BJT has negative temperature coefficient making parallel connection of BJTs more complicated.
As in the case of a BJT the operating limits of a MOSFET are compactly represented in a Safe Operating Area (SOA) diagram as shown in Fig 6.6. As in the case of the FBSOA of a BJT the SOA of a MOSFET is plotted on a log-log graph. On the top, the SOA is restricted by the absolute maximum permissible value of the drain current (IDM) which should not be exceeded even under pulsed operating condition. To the left, operating restriction arise due to the non zero value of rDS(ON) corresponding to vGS = vGS(Max). To the right, the first operating restriction is due to the limit on the maximum permissible junction temperature rise which depends on the power dissipation inside the MOSFET. This limit is different for DC (continuous) and pulsed operation of different pulse widths. As in the case of a BJT the pulsed safe operating areas are useful for shaping the switching trajectory of a MOSFET. A MOSFET does not undergo “second break down” and no corresponding operating limit appears on the SOA. The final operation limit to the extreme right of the SOA arises due to the maximum permissible drain source voltage (VDSS) which is decided by the avalanche break down voltage of the drain -body p-n junction. This is an instantaneous limit. There is no distinction between the forward biased and the reverse biased SOAs for the MOSFET. They are identical.
Due to the presence of the anti parallel “body diode”, a MOSFET can not block any reverse voltage. The body diode, however, can carry an RMS current equal to IDM. It also has a substantial surge current carrying capacity. When reverse biased it can block a voltage equal to VDSS.
For safe operation of a MOSFET, the maximum limit on the gate source voltage (VGS (Max)) must be observed. Exceeding this voltage limit will cause dielectric break down of the thin gate oxide layer and permanent failure of the device. It should be noted that even static charge inadvertently put on the gate oxide by careless handling may destroy it. The device user should ground himself before handling any MOSFET to avoid any static charge related problem.
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Version 2 EE IIT, Kharagpur