Metal-Insulator Device Background

A metal-insulator-metal (MIM) diode is formed by sandwiching two metal layers around a thin insulator. When a voltage is applied between the two metal layers, electrons are induced to quantum mechanically tunnel from one metal to the other through the thin insulator. For this to occur at low voltages (1 volt and below), the insulator must be very thin, typically less than 50 angstroms.

Using two different metals results in two different barrier heights (which are a function of metal work function and the insulator electron affinity Electrons tunnel preferentially in one direction over the other, thus forming a diode. The band diagram in Figure 1 shows a MIM diode with two different barrier heights.

The tunneling is extremely fast; on the order of a femtosecond (10^–15 seconds). Therefore, in principle, the device can rectify extremely high-frequency signals, beyond terahertz and into the optical range. In practice, the speed is limited by the

RC time constant, which is determined by the diode and circuit resistances and the diode capacitance. This capacitance is made smaller by decreasing the area of the device.

MIM diodes have been investigated since the 1960s, but these basic devices have suffered from poor performance and reliability.

Next, we contrast single insulator (MIM) diodes with Phiar’s double insulator (MIIM) technology.