Page 994 - Basic Electrical Engineering

Figure 14.32 (a) Drain characteristic of DEMOSFET; (b) transfer characteristics

A depletion MOSFET can also be made to operate in enhancement mode
by applying a positive gate voltage. When a positive voltage is applied to the

gate due to capacitor action, negative charges will get induced in the channel
thereby increasing its conductivity. As a result drain current will increase.

14.9.3 Static Characteristics of MOSFET

Figure 14.31 shows the typical drain characteristics of an n-channel
enhancement type of MOSFET. Each characteristic curve shows the variation

of drain current, I with drain voltage V for a fixed gate to source voltage,
D
DS
i.e., V . The forward transfer characteristic which is the relation between I D
GS
and V has also been shown.
GS
Figure 14.32 shows the drain characteristics for a depletion-type MOSFET,
or DEMOSFET and its transfer characteristic. In the transfer characteristic
the two modes of operation of the MOSFET for negative and positive gate

voltage, respectively have been shown.
The basic difference between JFET and a MOSFET is that a MOSFET has

10
very high input resistance of the order of 10 Ω or so. MOSFETs should be
handled very carefully because the very thin layer of SiO is susceptible to
2
high voltages and can be easily punctured.