Electronics I - Laboratory 13
Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
Switch
I. Objectives
1. Understand the operation of a MOSFET.
2. Design and build a MOSFET switch
II. Pre-Lab Requirements
1. The MOSFET
In a MOSFET, current flows from the drain terminal to the source terminal through a
semiconductor channel. The resistance of the channel, and therefore its ability to
conduct current, is controlled by a voltage applied to a third terminal denoted as the
gate.
MOSFETs can be either an n-channel type or a p-channel type. In a n-channel MOSFET
a positive voltage is applied to the drain terminal for operation while in a p-channel
MOSFET a negative voltage is applied to the drain terminal for operation.
An n-channel and p-channel type MOSFET may be one of two modes; enhancement
mode or depletion mode. The enhancement mode MOSFET is normally “off” (in cutoff
and conducting no current) when no voltage is applied to the gate and is “on” (in
saturation and conducting current) when a voltage greater than the gate-to-source
threshold is applied to the gate. The depletion mode MOSFET is normally “on” (in
saturation and conducting current) when no voltage is applied to the gate and is “off” (in
cutoff and not conducting current) when a voltage more negative than the gate-to-source
threshold is applied to the gate.
Figure 1. MOSFET Symbols
In this laboratory exercise we will use an n-channel, enhancement mode MOSFET.
Again, when the gate-to-source voltage (VGS) is placed at zero, the n-channel MOSFET
operates in the cut-off region of its output characteristic curves and when VGS is placed
at a value greater than the gate-source threshold voltage the n-Channel MOSFET
operates in the saturation region. When a MOSFET is operated in this manner (either
cut-off or saturation), it is called a MOSFET switch.
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As with all semiconductor components, the operating parameters are listed in its data
sheet. The parameters of particular importance for employing a MOSFET as a switch are
shown in Table 1.
Parameter
Drain-Source Voltage
Abbreviation
VDS
Gate-Source Voltage
VGS
Continuous Drain Current
ID
Maximum Power
Dissipation
PD
Gate-Source Threshold
Voltage
VGS(th)
Drain –Source On-State
Resistance
RDS(on)
Gate-Source Leakage
IGSS
Significance
Maximum voltage that should be used as
VDD with the MOSFET
Maximum voltage that should be applied
(with respect to the source) to the gate.
Maximum current that should pass
through the MOSFET from drain to
source.
The maximum power the MOSFET
dissipates when operating. It is equal to
the drain current (ID) multiplied by the
drain-to-source voltage.
The gate voltage (with respect to the
source) at which the MOSFET (drain to
source) conducts.
The resistance across the MOSFET (drain
to source) when conducting. It is the
value of VDS/ID.
The current that flows across the gate-tosource barrier when the MOSFET is
conducting. In actuality, this is the current
required to switch the device on i.e. its
input current.
Table 1. Important MOSFET Parameters
Figure 2. Basic MOSFET Switch
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Figure 2 shows a basic n-MOSFET, enhancement mode switch that you will build in the
lab exercise. When VGS = 0 the MOSFET is in cut-off, ID equals essentially 0, therefore
no current flows through the load, and VDS = VDD. When VGS > VGS(th) the MOSFET
enters saturation, current (ID) flows through the load with ID = VDD/(RLoad + RDSon),
and VDS = ID* RDSon. The gate-to-source resistor (RGS) is used in this application
because it provides a path to ground for the gate capacitance to discharge through (to
make VGS = 0) if that path is not provided by the input when it goes to the 0V state.
Occasionally a resistor called a gate resistor (RG) is placed in series with the gate and the
input. Generally the gate resistor value is small (5Ω to 100Ω) and is used to limit the
instantaneous current (often called in-rush current) that is drawn when the MOSFET is
turned on. If “in-rush current” is large, a higher resistance value resistor may be needed.
Gate resistors may also limit ringing in certain instances. A gate resistor has the adverse
effect of increasing the switching time for a MOSFET switch.
III. Laboratory Requirements
1. Required Parts and Equipment
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
2 - DC power supplies
1 - Bench DMM
1 - Fluke hand-held DMMs
1 - Proto-Board (PB-103)
1 – IRFZ34 Enhancement Mode N-MOSFET
1 – 1MΩ resistor
1 – 0.2MΩ resistor
1 – 12 Volt incandescent light
1 – SPST normally off push button switch
Wires and leads for circuit connections.
2. Required Information
A. Transistor Data Sheets
The following is a link to the specification sheet for the IRFZ34:
http://www.vishay.com/docs/91290/91290.pdf
B. IRFZ34 Pinout
Figure 3 shows the pin orientation for the IRFZ34
Figure 3. IRFZ34 Pin Orientation
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3. Laboratory Procedure - Construction and operation of a MOSFET switch
A. Using your IRFZ34, construct a MOSFET switch as shown in Figure 2. For the load
use a 12V incandescent light bulb and provide a VDD of 12V. Determine an
appropriate VGS from the specification sheet and provide this voltage from a power
supply (VGG). Employ a push button switch to connect and disconnect VGG from
the MOSFET gate.
Measure and record the parameters shown in Table 2 for you switch.
Demonstrate the operation of your MOSFET switch for your laboratory instructor.
B. Answer the following questions:
1. How much power is being used by the MOSFET when it is conducting?
2. How much power is being used to illuminate the light bulb?
3. How much power is being used to control the MOSFET? Use the
specification sheet value for the gate-source leakage current.
4. When the push button is first depresses to apply the gate voltage to the
MOSFET switch, what happens to the ID? What is the approximate value of
this “in-rush” current?
5. Place a 0.2MΩ gate resistor into the circuit. What is the approximate value
of this “in-rush” current now?
6. Operate the MOSFET switch with the gate-to-source resistor removed.
What happens?
Parameter
Measured Value
Simulation Value
VDD
VGS(th)
VGG
VGS
VDS-on
Vlight bulb (on)
ID
RDS(on)
Table 2. Parameter Table for IRFZ34 MOSFET Switch
4. Data Reduction and Lab Report
Submit the following in your report.
A. PSpice simulation of your MOSFET switch
B. A populated Table 2.
C. Answers to questions 3B1-6
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