A simple N-channel MOSFET can be used as a diode, Switch and Active resistor. This presentation is a part of course of Analog CMOS Design, based on textbook of same title by Allen Holberg.

1. MOS Diode & Active Resistor
Sudhanshu Janwadkar

2. MOS as Diode:
When the gate of the MOSFET is connected to
the drain, it acts like a diode with characteristics
similar to a pn-junction diode.

3. Circuit Connection

4. Region of Operation
When the gate is connected to the drain of an
enhancement MOSFET, the MOSFET is always in
the saturation region.

5. Since VT is always greater than zero for an
enhancement device, then VDG = 0 satisfies the
conditions for saturation.

6. Expression for Current/Voltage

7. The comparison is basically the difference
between an exponential and a square-law
function. However, if the designer is willing to
spend W/L, the comparison becomes more
interesting as shown below
How Does the MOS Diode Compare
with a pn Diode?

9. Small Signal Model

10. Small signal resistance of MOS Diode

11. Applications of MOS diode
• The MOS diode is a component of Current
Mirror circuits
• The MOS Diode is used for level translation

12. Application(To generate a Bias Voltage)

14. MOSFET as Active Resistor
The MOSFET Switch can be viewed as a resistor,
where the transistor’s Drain and Source form
the two terminals of a floating resistor.

16. • MOSFET acts in non-saturation region. The
Resistance can be given as:

17. Review Questions:
1. With the help of suitable schematic and
necessary expressions, explain MOSFET as
switch, diode and active resistor.