The four-layered structure ( M - I (leaky)- n - p + ) is found to exhibit a current-controlled negative resistance region in its I-V characteristics. In this paper, a quantitative physical model of the device in the punch-through mode is presented. The negative resistance behaviour is due to a positive feedback mechanism between the tunnel MIS and the n - p + junction parts of the device. The effect of the device parameters on its I-V characteristics is studied.

Theory of switching in p-n-insulator (tunnel)-metal devices: Part I: Punchthrough mode

The theory of switching is presented for a structure consisting of a p + - n junction and a metal electrode separated from the N -section of the p + - n junction by a semi-insulating (leaky) layer. When a negative bias is applied to the electrode, the section of the n -layer under the electrode goes into deep depletion. In this mode, the current through the device is limited by generation in the deeply depleted region. This is the high-impedance or OFF state of the device. At a sufficiently high voltage, the switching voltage, V s , the p + - n junction is turned on by either avalanching in the n -layer or by the deep-depletion region extending through to the p + - n region (punch-through). When the junction turns on, the n -section goes from deep-depletion towards inversion. Thus, the voltage across the device decreases with a concomitant increase in the current through the device. This is the switching mode. The switching voltage may be tailored by varying the doping and/or width of the n -section. Following switching, the device comes into the steady-state when the current through the insulating layer is equal to the current flowing across the p + - n junction. The I-V characteristic of this highly conducting (ON state) mode is determined principally by the I-V characteristic of the semi-insulating film. By suitable choice of material this portion of the characteristic can approach zero dynamic impedance, i.e. a near-vertical characteristic, characterized by a low holding voltage. Capacitance and switching characteristics of the device are also discussed.

Theory of switching phenomena in metal/semi-insulator/n-p+ silicon devices

The direct current-voltage (I–V) characteristics of three terminal inversion controlled switches are described. These devices are layered sequences of metal/conducting “insulator”/semiconductor junction with electrical terminals at the metal and both sides of the junction. If a bias is applied between the metal and the far side of the junction, in the sense which tends to deplete the surface and forward bias the junction, the device shows bistable impedance states similar to the current-voltage characteristics of a silicon-controlled rectifier. The intermediate terminal which contacts the semiconductor region between the insulator and the junction, can be used, in proper circuit and biasing arrangements, to switch the device both into and out of its low impedance state without varying the voltage supplied to the outer terminals of the device and a series-connected resistor. The I–V characteristics of these three terminal devices support the inversion-controlled conduction model of device behavior which permits high conductivity of the device only when inversion of the semi-conductor surface occurs. The high and low impedance states and the pulses required to induce transistions between states are contained entirely within the “active” bias configuration for these devices, which is defined by analogy with the active bias region of conventional bipolar transistors.

Steady-state characteristics of two terminal inversion-controlled switches

The switching properties of silicon structures comprising a p + - n junction and a metal electrode separated from the n -section of the p + - n junction by a semi-insulating (leaky) layer are presented. Two basic types of structure were studied: devices with relatively light doped n -sections, and those with relatively heavily doped sections. The switching voltage of the first group was found to be proportional to the product of the doping density, N d and the square of the width of the n -section, and to be only very weakly temperature-dependent. The capacitance-voltage relationship of the device in the high-impedance mode was found to be of the form C −1 ∞ V 1 2 , and these measurements established that switching occurred just as the depletion region of the n -section under the gate electrode reached through to the p + - n junction. It was thus established that these devices were operating in the punch-through mode. In the second group of devices, the doping density of the n -section was increased by diffusing an n -well into the section. The switching properties were found to be quite different from the punch-through devices. The switching voltage was found to be independent of the width of the n -section and proportional to N d − 3 4 . Capacitance measurements also showed that the depletion region in the n -section under the oxide at switching, varied with the doping concentration, and was substantially less than the width of the n -layer. It was thus concluded that switching in these devices was of the avalanche-mode type.

Experimental studies of switching in metal semi-insulating n-p+ silicon devices

A new switching phenomenon in metal-oxide semiconductor tunnel junction has been discovered. With a sufficiently large negative bias applied to the electrode, incident visible light of intensity greater than about 1 microW/sq cm causes the reverse-biased junction to switch from a low-current to a high-current state. It is believed that hot-electron-induced impact ionization provides the positive feedback necessary for switching, and causes the junction to remain in its high-current state after the optical excitation is removed. The junction may be switched back to the low-current state electrically. The basic junction characteristics have been measured, and a simple model for the switching phenomenon has been developed.

Optically induced bistable states in metal/tunnel‐oxide/semiconductor (MTOS) junctions

Silicon p-n-I-M devices with thin insulating layers ( thicknesses ⩽ 30 A ), named MTIS devices, have been developed. The two terminal device shows an S-shaped negative resistance characteristics similar to a Schockley diode (or p-n-p-n diode). Typically the threshold and sustaining voltages are 10 ∼ 15 and 1.3 ∼ 2 volts, respectively. The former however can be controlled by optical illumination. Turn-on time including delay is less than 2 nsec and turn-off time − 1 nsec or less. A thyristor-like device with its third terminal connected to the n-layer shows switching operation controllable by this terminal. A monolithic linear array of p-n-I-M diodes with 30 μm spacing operates as a shift register through coupling of adjacent diodes. Life of the two terminal devices recorded at present is over 1.5 × 104 hr. These devices can be applied to low power and high-speed electrical switching and also to optical switching and integrated logic circuits.

Hiromi Shimizu

Papers

Silicon p−n insulator-metal (p-n-I-M) devices