Related devices Metal-Insulator-Semiconductor Capacitor

A metal-insulator-semiconductor (MIS) capacitor is usually referred to capacitor structures other than thermal oxide on silicon substrate. These substrates imply compound semiconductors or Ge.

Unfortunately, except the heterostructure, all methods yield unsatisfactory results. Problems are due to high conductance causing leakage, and unacceptable interface trap density.

Parallel-Plate Capacitor

A fixed-value parallel-plate capacitor, integrated or discrete, is one of the most common components in electronics. In the planar technology, the insulator layer can be made conveniently from thermally grown oxide or deposited dielectrics such as silicon nitride or oxide. The plates can be metal, silicide, polycrystalline semiconductor, or the substrate semiconductor that is heavily doped.

The applications of a generic capacitor are listed below:

It is commonly used in filters and tuned circuits.

Energy storage: Energy can be stored for pulsed operations such as spark plugs and pulsed lasers.

Regulation: The basic property of a capacitor is continuity of voltage and charge. A step voltage, for example, is not possible across a capacitor. This property is used to regulate DC voltage nodes and power supplies.

As a speed-up capacitor: It is used to improve the tum-on and turn-off times of a bipolar transistor when connected in parallel to the base resistor.

As an integrator: Examples of applications are analog computing and waveform generation.

As sensors: Since capacitance is proportional directly to the dielectric constant and inversely to the distance, it is used to detect the presence of any material. Examples are level sensor, position sensor, tactile switch, and humidity sensor. Capacitance is also used in a pressure sensor to detect displacement of a diaphragm under differential pressure.

Chapter 10 metal-insulator-semiconductor switch

The MISS (metal-insulator-semiconductor switch, MIS switch) was discovered by Yamamoto and Morimoto in 1972, using silicon dioxide as the insulator. A subsequent report was made by Kroger and Wegener in 1973, using another insulator-silicon nitride. Further understanding of the device was not advanced until 1977, and most analytical studies, as well as more experimental results, were presented between 1977 and 1980. These include the works by Simmons and EI-Badry, Kroger and Wegener, Habib and Simmons, Sarrabayrouse et al. and Zolomy.

The MISS structure is basically an MIS tunnel diode in series with a p-n junction. Almost all reported structures used silicon material with silicon dioxide being the tunneling insulator. This thin oxide has to be in the range of 20-50 A, and is usually thermally grown at relatively low temperature around 700°C. In the example shown, a p-type epitaxial layer of 2-10 цт thick is grown on a p+-substrate. The conjugate structure of metal, oxide on p-layer on n+-substrate has also been reported in the literature.

The MISS is characterized by having bistable states: a high-impedance, low-current off-state and a low-impedance, high-current on-state. With negative anode-to-cathode voltage, the MIS tunnel diode is under forward bias and the p-n junction under reverse bias. The current is dominated by that of generation within the depletion region of the p-n junction. The switching criterion of the MISS depends critically on the supply of holes toward the tunneling insulator. When this hole current is small, it is semiconductor-limited. In this condition, the semiconductor surface is in deep depletion, and an inversion layer of holes at the surface is not formed. If an additional supply of hole current from other sources is available, the tunneling current is not sufficient to drain the hole current and it becomes tunneling-limited, and a hole inversion layer appears. The collapse of the surface potential (surface band bending) increases the voltage across the insulator and increases the in two respects. First, the barrier height is reduced, and second, Q is also reduced. The latter is equivalent to a higher electric field across the insulator. The current mechanism in the p-n junction changes from recombination to diffusion. The MIS tunnel diode and the p-n junction pair creates a regenerative feedback and results in negative differential resistance.

The regenerative feedback can also be viewed as a result of two current gains: a gain of electron current from hole current in an MIS tunnel diode, as originally proposed by Green and Shewchun, and a gain of hole current from electron current in thep+-n junction. To achieve the current gain in an MIS tunnel diode, the precise insulator thickness is critical, and it has to lie in the range of 20-50 A for the case of silicon dioxide. Oxides thinner than 20 A cannot confine holes at the surface to support an inversion layer and current is always semiconductor-limited. Oxides thicker than 50 A do not allow deep depletion, and current is always tunneling-limited.

In practice, the current initiated by generation is not large enough to trigger switching. The two most common, additional sources are from punch-through and avalanche. In the punch-through condition the depletion region of the MIS diode merges with that of the p-n junction. The potential barrier for holes is reduced and a large hole current is injected. Besides the aforementioned punch-through and avalanche, two other sources of hole current are also possible. One is by a third terminal contact and another by optically generated current. The three-terminal MISS is sometimes called an MIS thyristor (MIST), and variations of the structure. With either a minority-carrier injector or a majority-carrier injector, the function is the same-to increase the hole current flowing toward the insulator. While the minority-carrier injector injects holes directly, the majority-carrier injector controls the potential of the n-layer, and hole current is injected from the p+-substrate. In either structure, with a positive gate current flowing into the device, a lower switching voltage results. When the MISS is exposed to a light source, the switching voltage is reduced.