Depletion MOSFET and Digital MOSFET Circuits

Fundamental of MOSFET

1. Depletion MOSFET and Digital MOSFET Circuits
Dr. Varun Kumar
Dr. Varun Kumar(IIIT Surat) 1 / 10

2. Outlines
1
Depletion MOSFET
2
Comparison of p-with n-channel FETs
3
Circuit symbol of MOSFET
4
Digital MOSFET circuits
Dr. Varun Kumar(IIIT Surat) 2 / 10

3. Depletion type MOSFET
)An n-channel depletion-type MOSFET.
)On a lightly doped p-type substrate, two highly dopedn
+
regions are
diused.
)These twon
+
sections are called as source and drain. Both terminals
are separated by a distance 510m
Dr. Varun Kumar(IIIT Surat) 3 / 10

4. Continued{
)The eld will end on
)When negative gate voltage increases, the induced +Ve charge in the
semiconductor also increases.
)Region beneath the oxide now has n-type carriers, the conductivity
decreases, and the current ows from source to drain through the
induced channel.
)Here the drain current is
device is called an.
Dr. Varun Kumar(IIIT Surat) 4 / 10

5. Continued{
)(a) The drain characteristics
)(b) The transfer curve (forVDS= 10V) for n-channel MOSFET,
which may be used as enhancement or depletion type MOSFET.
Dr. Varun Kumar(IIIT Surat) 5 / 10

6. Comparison of p-with n-channel FETs
)p-channel enhancement FET is popular in MOS, due to its easier
production.
)Most of the contaminant in MOS fabrications are mobile ions.
)+Ve charge are trapped in oxide layer between gate and substrate.
)The hole mobility in silicon at normal eld is 500Cm
2
/V-s, whereas
electron mobility is 1300Cm
2
/V-s in
)p-channel device will have more ON resistance on an equivalent
n-channel on same condition.
)p-channel requires more than double area for maintaining the same
resistance.
Dr. Varun Kumar(IIIT Surat) 6 / 10

7. Circuit symbol
)Above one are p-channel MOSFET.
)(a) and (b) are either depletion or enhancement type MOSFET.
)(c) represents only the enhancement type MOSFET.
)In (a), the substrate is understood to be connected internally to the
source
)For n-channel MOSFET, the direction of arrow will be reverse.
Dr. Varun Kumar(IIIT Surat) 7 / 10

8. Digital MOSFET circuits
1 MOS inverter or NOT gate
)(a) MOS inverter (b) Truth table
Dr. Varun Kumar(IIIT Surat) 8 / 10

9. Continued{
2 NAND gate
)(a) MOS NAND gate (b) Truth table
Dr. Varun Kumar(IIIT Surat) 9 / 10

10. Continued{
3 NOR gate
)(a) MOS NOR gate (b) Truth table
Dr. Varun Kumar(IIIT Surat) 10 / 10