**4. Challenges in the future**

structure, but can be utilized primarily in reverse breakdown mode of operation for a voltage

Zener diodes are used uni-directional electrical overstress (EOS) protection device. Zener diodes are typically used as a voltage clamping EOS protection device, and typically used in the breakdown state. Schottky and Zener diodes can both be integrated into a given

An EOS protection device used for high voltages is the varistor. A varistor is also known as a voltage dependent resistor (VDR). The varistor element behaves like a diode, forming a non-

Another EOS protection device is the metal oxide varistor (MOV) device; this is the most common varistor composition [1, 21]. Zinc oxide, combined with other metal oxides are integrated between two metal electrodes. Metal oxide varistors can also include bismuth, cobalt, and manganese. The operation of the MOV device is based on conduction through ZnO grains; current flows "diodelike" through the grain structures creating a low current flow at low voltages. At higher voltages, the current flow is dominated by a combination of thermionic emissions and tunneling. This diode-like behavior forms the diode-like characteristic provides the high resistance/low voltage state, and the low resistance/high voltage state. An advantage of the MOV structure is it has a high trigger voltage, making it suitable for EOS protection in power electronics (e.g. 120–700 V applications) [1, 21]. The disadvantage of these elements is that it has high capacitance, high on-resistance, high trigger voltage, and variability of the device response (e.g. on-resistance and clamping voltage) in the MOV device characteristics. Key device parameters of varistor are the energy rating,

Gas discharge tubes (GDT) devices can be used to avoid electrical overstress (EOS) in systems [1, 23]. Gas discharge tubes (GDT) are bidirectional, allowing for protection for both positive and negative EOS events. GDT elements are suitable from surge protection. GDT devices have high trigger voltages (unless used as a first stage followed by other low voltage second-

Gas-filled tubes (GDT) utilize electrical discharge in gases. An applied voltage initiates the device by ionizing the electrical gas, followed by electrical glow discharge, and an electrical arc. With creation of an electrical arc, the GDT device becomes a low resistance shunt for EOS protection. These gas-filled tubes can contain hydrogen, deuterium, and noble gases (e.g. helium, neon, argon, krypton, and xenon). GDT devices can vary their electrical characteris-

GDT devices undergo three states: (1) electrical breakdown, (2) glow discharge, and (3) electrical arc [1, 23]. The electrical breakdown is a high voltage low current state prior to triggering of the GDT device. A glow discharge region forms a second state which incorporates a low current high voltage state. Lastly, after full ionization of the gas, a low voltage high current

GDT devices have high trigger voltages suitable for LDMOS power electronic applications to HV LDMOS (e.g. 120 V), and UHV LDMOS applications (e.g. 600–700 V) [1, 23]. These devices are used in a number of high voltage switch devices, such as ignitrons, krytons, and thyratrons.

operating voltage, response time, maximum current and breakdown voltages.

tics by choices of the gas type, pressure, electrode design, and spacings.

limiting EOS solution.

18 System of System Failures

ary EOS solutions) [1].

state occurs with a low "on-resistance."

linear current-voltage (I-V characteristic).

application.

Future challenges exist in improve reliability and safety in components and systems due to electrostatic discharge (ESD) and electrical overstress (EOS). Challenges include the following:

