Static Charge: Threat To CMOS And Other High-Impedance Devices

By David Herres

In the world of semiconductors, a device said to have a high impedance really has a high-impedance input. A high-impedance device will typically have low output impedance. This is a satisfactory arrangement because any given stage will not overload the stage preceding it and will not be overloaded by the stage that follows it.

Conventional bipolar junction transistors (BJTs) did not fall into the high-impedance category. The first transistor considered to have a high impedance was the field-effect transistor (FET). When FETs came on the scene, many applications became apparent. For example, with its high impedance, FETs often served as front-end amplifiers for high-impedance voltmeters that would not load the circuits being measured.

Rather than responding to fluctuating current as would a traditional BJT, the FET responds to a changing electric field, which controls the shape and therefore the conductivity of an internal channel. This electric field arises from a fluctuating voltage applied at the input. Because the channel is small, only a minute amount of current need be drawn from the output of the preceding stage, or whatever happens to be connected to the FET input.

Beginning in 1960, FETs were replaced by metal oxide semiconductor field-effect transistors (MOSFET’s). They resemble FETs but have higher, awesome, input impedances. A variation on the MOSFET is the insulated gate bipolar transistor (IGBT). It is generally modeled as a MOFET driving a bipolar transistor. Like the MOSFET, it has a high input impedance and draws infinitesimal current from the device that feeds the input. The current-vs-voltage output qualities resemble those of bipolar transistors.

MOSFET’S are employed in pairs to make complementary metal oxide semiconductor (CMOS) devices, the  technology that dominates integrated circuit (IC) design at present. Some ICs contain billions of CMOS devices. Because they consume little power and operate at high speed, they have become ubiquitous in contemporary circuit boards. But CMOS circuits have extremely thin insulating layers, so they are hypersensitive to damage from electrostatic charge.

Specifically, discrete CMOS components or ICs in or out of a circuit board can be destroyed by a sufficiently high static charge reaching one of their terminals. This can happen if someone who’s body contains a static charge with respect to ground touches one of the IC terminals or the terminal or some other device that connects to the IC.
Your body can easily acquire such a charge merely by brushing past an insulated object. This pick-up of static charge happens more often in a heated building that tend to have dry air — moisture in the air helps static charge bleed off. A humidifier or even a hotplate with boiling water can help mitigate static problems.

Circuit boards should be held by the edges, away from their conductive parts. And it is helpful to frequently touch a verified grounded object such as a metal receptacle faceplate. A grounding bracelet, made for the purpose, prevents people who handle static-susceptible parts from acquiring a charge. The bracelet must be plugged into a reference ground. An audio phone jack works well. But note: The grounding bracelet should be removed before working on a power circuit.

Sensitive components often come with their leads inserted into conductive foam packing, so as to ensure they are shunted and remain at the same potential. It’s best to leave such packing in place until immediately before the device is to be used.