MOSFETs – Metal Oxide Semiconductor Field Effect Transistor
There are 2 types of MOSFETs – Depletion Type MOSFET and Enhancement Type MOSFET
Depletion Type MOSFET, D-Type MOSFET
Physical Diagrams
N channel
P channel
The N channel device is for use with positive supply voltages and the P channel device is for use with
negative supply voltages.
Schematic Diagrams
N channel
P channel
The MOSFET has 3 new terminals Gate (G), Drain (D) and Source (S). The Gate is isolated from the
channel by an insulating layer of Silicon Dioxide, SiO2.
Questions:
If a voltage is applied at the Gate terminal, how much Gate current flows?
What is the input resistance of the MOSFET device – from the Gate terminal?
Recall for a Bipolar transistor that the Base Current (the Input current) controlled the Collector Current
(the Output current) and that β = IC/IB. the Bipolar transistor was called a current controlled device.
Transconductance
As there is no Gate current for a MOSFET the MOSFET is not a current controlled device. The GateSource voltage VGS controls the output current – the Drain current. The MOSFET is referred to as a
voltage controlled device. The Transconductance is a MOSFET parameter that relates the input voltage
VGS to the output current – the Drain current. Transconductance is referred to as gm and is equivalent to
β for a BJT.
gm = ID/VGS
Transconductance Curve
The Transconductance curve relates the value of the Drain current ID to the Gate Source voltage VGS.
The relationship between drain current and gate-source voltage can be expressed as
ID = IDSS (1 - VGS/VGS,Off)2
The D-Type MOSFET works in 2 modes – the Depletion Mode where VGS < 0 (left half of graph) and the
Enhancement Mode where VGS > 0 (right half of graph).
There is a special value of Drain current called IDSS – this is the value of ID when VGS = 0. IDSS is a data
sheet value.
Depletion Mode - VGS < 0
When a negative VGS is applied to the transistor, the negative charge at the Gate terminal depletes the N
channel of free electrons. This narrows the channel, increases its resistance and this decreases the
Drain current. At some value of VGS called VGS,Off the channel is pinched off completely and the drain
current ID = 0. The VGS,Off value is also a data sheet
value.
Depletion Mode
Enhancement Mode
Enhancement Mode - VGS > 0
In Enhancement Mode the positive VGS applied to the Gate widens the channel, decreases its resistance
and increases the Drain current.
Example
A D-Type MOSFET has IDSS = 1 mA and VGS,Off = -6 V, find the drain current ID at VGS = -3 V, -1 V, 1 V and
3V
VGS = -3 V
ID = 1 mA (1 - (-3 V/-6 V))2 = 0.25 mA
VGS = -1 V
ID = 1 mA (1 - (-1 V/-6 V))2 = 0.69 mA
VGS = 1 V
ID = 1 mA (1 - (1 V/-6 V))2 = 1.36 mA
VGS = 3 V
ID = 1 mA (1 - (3 V/-6 V))2 = 2.25 mA
If these values of ID and VGS were plotted the result would be the Transconductance curve for the
MOSFET.
Enhancement Type MOSFET, E-Type MOSFET
Physical Diagrams
N channel
P channel
Schematic Diagrams
N channel
P channel
The E-Type MOSFET works only in Enhancement Mode where VGS > 0. When VGS = 0 the transistor is off
and the Drain current is 0.
Transconductance Curve
The Transconductance curve relates the value of the Drain current ID to the Gate Source voltage VGS. The
E MOSFET only turns on (ID > 0) when VGS > VGS,ON. VGS,ON is a data sheet value. The positive VGS attracts
electrons and creates a thin layer of free electrons in the substrate creating an N channel. If the gatesource is increased further the conductivity and width of the channel increases allowing for more drain
current to flow. So when the MOSFET is conducting, as VGS increases the Drain current also increases.
E MOSFET Switch
A MOSFET switch is a useful application of this transistor. Because of the high input resistance (low
current draw) of the MOSFET device the switch circuit can be directly connected to a computer post that
supplies a TTL level output waveform. In this circuit the switch has an inductive load so the protective
diode is required. The gate resistor RG is needed to ensure that the transistor is off when the switching
input signal is 0.
The input and output switch waveforms as seen with an oscilloscope are shown.
VIN waveform
+10
Induced “Spike’ voltage
VOUT waveform
+20
On
Off
On
Off