This article explains proper handling precautions and damage prevention techniques for MOSFETs, as well as basic methods for evaluating their condition using standard testing equipment.
Proper handling and testing of MOSFETs are crucial for ensuring reliable performance and avoiding costly failures in electronic circuits. Due to their sensitivity to electrostatic discharge and voltage spikes, even minor mishandling can lead to irreversible damage. Therefore, following recommended precautions and using appropriate troubleshooting tools helps maintain device integrity and system stability in both development and operational environments.
Safe Handling Practices for MOSFETs
MOSFETs must receive special care in handling when they are used in an electronic circuit. The silicon dioxide layer that insulates the gate from the source, drain, and substrate is very thin. A static charge or voltage less than 100 V between the gate and channel will cause permanent damage to the insulation material. This generally causes the oxide layer to rupture. As a result, gate leakage current occurs between the gate and channel. This generally causes permanent damage to the device.
The voltage supplied to an operating MOSFET generally comes from a DC power source or supply. The voltage used in most circuit applications must not exceed the breakdown rating of the device. In general, this can be avoided by checking the manufacturer’s maximum ratings so that the device is not damaged by excessively high operating voltage values. The correct source voltage values must not be exceeded. The device should not be connected to an energized or operating circuit. As a rule, these precautions apply to nearly all solid-state devices.
Static-charge damage to a MOSFET presents a number of problems that do not occur in other solid-state devices. Friction caused by rubbing the device in a plastic bag or on a piece of fabric during shipping can be damaging. Static buildup on the body of a person handling the MOSFET can also cause it to be damaged. Soldering a device into a circuit with an ungrounded soldering device can cause some form of damage. All of this requires some understanding of the special handling procedure that must be followed. Static damage is typically the most troubling problem in the use of MOSFETs.
Most manufacturers ship MOSFETs with their leads shorted together by means of a shorting ring or wire or pressed into a conducting foam material. The shorting method should not be removed from the device until the device is installed in its respective circuit. This precaution is extremely important. It is extremely important, however, that the shorting device be completely removed from the device to avoid damage to the circuit. The following precautions should be observed when handling MOSFETs:
- Turn off the power source of the circuit before inserting a MOSFET into it. Voltage transients developed by the circuit will permanently damage the device.
- Use special grounded-tip soldering devices for circuit connections. Do not use soldering guns. The tip of a soldering device can be grounded with a clip lead to reduce this problem.
- Neutralize electrostatic body voltage by using a grounding wristband or by connecting a clip lead to your metal watchband.
- Let the shorting ring or conductive foam remain on a MOSFET until all circuit connections are complete and the device is installed in the circuit. Do not forget to remove the shorting material from the device.
Most new MOSFETs have Zener diode protection built into the device to avoid electrostatic problems. Figure 1 shows how these diodes are connected to the MOSFET internally.
Figure 1. Zener diode protection is built into some MOSFETs to avoid electrostatic problems. (a) Zener diode gate protection of a MOSFET. (b) Transient voltage.
Protection is achieved when the diodes become conductive. This occurs when the voltage exceeds 15 V. The Zener diodes are connected in reverse order so that conduction will occur in either polarity of applied voltage. This type of built-in protection reduces most of the in-circuit and out-of-circuit transient problems associated with MOSFET installation. However, it is still recommended that the general precautions for handling MOSFETs be followed to avoid unnecessary damage.
Review Questions
- A voltage as low as ______ between the gate and channel can permanently damage a MOSFET.
- Static charge damage in MOSFETs typically causes a rupture in the ______ layer.
- One of the key precautions when installing a MOSFET is to never insert it into a circuit that is already ______.
- A grounded ______-tip soldering device should be used to minimize electrostatic damage during installation.
- Manufacturers ship MOSFETs with their leads shorted using a ______ or conductive foam to prevent static damage.
- The shorting device must be removed only after the MOSFET is ______ in the circuit.
- A ______ or metal watchband can be used to neutralize electrostatic body voltage.
- Zener diodes built into some MOSFETs become conductive when the gate voltage exceeds ______ volts.
- Zener diodes are connected in ______ order to allow protection from both polarities of voltage.
- Despite built-in Zener protection, following proper ______ procedures is still recommended.
Answer Key
- 100 V
- oxide
- energized
- grounded
- shorting ring
- installed
- grounding wristband
- 15
- reverse
- handling
Key Takeaways
Proper handling and protection of MOSFETs are essential for ensuring the long-term reliability and performance of electronic systems, especially in applications involving sensitive or high-speed circuitry such as power supplies, amplifiers, and switching regulators. Due to their susceptibility to static discharge and voltage transients, neglecting recommended precautions can result in immediate or latent device failure, leading to costly repairs or system downtime. Implementing careful handling procedures and utilizing built-in or external protective features like Zener diodes ensures MOSFET integrity during manufacturing, installation, and operation.