S-Type Negative-Resistance Semiconductor Device

a semiconductor device whose operation is based on an S-shaped volt-ampere characteristic, which has one (AB) or more regions of negative resistance (see Figure 1). Semiconductor devices have two types of nonlinear volt-ampere characteristics. One is the 5-type. The other is the N-type, which is exhibited by the tunnel diode and the Gunn diode.

S-type devices can be made in various ways. The first S-type device was the crystal detector. S-type devices include four-layer structures, in which semiconductor layers with n-type conductivity

Figure 1

alternate with layers that have p-type conductivity. An example of a four-layer structure is the tetrode thyristor. The four-layer structure contains three p-n junctions. The working ranges of tetrode thyristor devices vary from a few amperes to tens or hundreds of amperes for currents and from tens of volts to several hundred volts, or more, for voltages. The unijunction transistor, or double-base diode, is another widely used S-type device. It has three electrodes and two circuits: an emitter circuit and an interbase circuit. When a current flows through the inter-base circuit, an S-type characteristic is produced in the emitter circuit. Under certain conditions, an S-type characteristic is also exhibited by avalanche transistors, Gunn diodes, and Zener injection diodes.

Four-layer structures are the most widely used S-type devices. They are employed in the electrical industry, in electronics, in power engineering, and in conversion technology. In conversion technology, they have supplanted the cumbersome and unreliable thyratron. Unijunction transistors are also widely used. They are basic components in relaxation oscillators and delay lines. The use of four-layer structures and unijunction transistors in microelectronics appears promising.

The resistance of a semiconductor can be substantially increased by doping the semiconductor with impurities that create energy levels deep in the energy gap. When a current flows, the initial low resistance is restored. This rise in the semiconductor’s conductivity is often accompanied by a drop in the voltage across the semiconductor as the current increases. This effect accounts for the S-shaped volt-ampere characteristic. S-type devices exist that make use of compensated Si, Ge, GaAs, and other materials. In most cases, the transition from high to low resistance is accompanied by current pinching—that is, by a reduction in the cross section of the current channel. Current pinching occurs (if the self-magnetic fields of the current are ignored) only in S-type devices. For example, an abrupt decrease in the diameter of the current-channel cross section from 400 micrometers (µm) to 80–100 (im has been observed in silicon S-type devices that are compensated with Cd. Current pinching has also been observed in, for example, compensated Ge and four-layer structures. As the current increases, the pinched channel expands, so that the current density in the channel remains constant. In fact, the channel may occupy the entire contact area, no matter how large it is. The channel can be displaced as a whole—for example, in a magnetic field—without the size of its cross section being changed. These two properties indicate that S-type devices can be used to construct highly reliable current switches.

S-type devices have at least two stable states. As a result, they can be used to make neuristors. The neuristor represents an electronic model of the nerve fiber known as an axon. In S-type devices made from compensated CaAs, luminescence is observed when the device passes from the high-resistance state to the low-resistance state. In other words, the S-type device can be used as a controlled light source.

Tetrode thyristors are also finding applications. Such devices can be controlled by an incident beam of light.