Why MIIM?

For a diode rectifier to operate efficiently, it must have a sufficiently nonlinear current-voltage characteristic (sharp turn-on), which appears as substantial curvature in the current-voltage curves. And for a rectifier to operate at high frequencies (and also to match the impedance of antennas), it must have low “on” state resistance. Unfortunately, in the simple MIM diode (Figure 1) one encounters a trade-off between nonlinearity and conductivity: A MIM diode can have one or the other but not both.

Phiar’s earliest invention was a superior diode using two insulators instead of one to produce metal-insulator-insulator-metal (MIIM) diodes (Figure 2). In the MIIM diode, low resistance and high nonlinearity may be obtained simultaneously, resulting in an efficient detector capable of operating with carrier frequencies above 10 THz.

To understand the theory of operation, it is useful to look at band diagrams.

The three diagrams in Figure 3 show energy states of the MIIM diode under zero, forward, and reverse biases.

Looking at the zero bias diagram, one sees a different interface height at M1-I1 vs. I2-M2. These different barrier heights result from choosing two different metals and are the key to the device’s operation as a diode.

In forward bias, a quantum well forms between the two insulators. When the Fermi level of the first metal rises to the quantum well’s energy level, the probability of tunneling spikes (Figure 4). In part, this is due to the shorter path for electrons across the insulators due to the quantum well and the sloped band diagrams. Together, these factors reduce the effective barrier thickness of the insulators. Because the magnitude of the electron tunneling current increases exponentially as the tunneling distance decreases, this “thickness reduction” results in a sharp current turn-on.

In reverse bias, no quantum well forms and electrons must tunnel through both barriers without the aid of an intermediate pathway. The band diagram for reverse bias shows that the distance across the barrier is significantly greater than in the forward bias state. This reduces the tunneling current to almost zero.

As one recent visitor to Phiar aptly said, “One can think about this as the electrons climbing two small hills under forward bias, versus one large mountain in reverse.”

To better understand the tunneling probabilities of electrons as a function of bias voltage, see Figure 4. Multiple quantum wells occur under forward bias. The green curve represents simulated data of electron tunneling probabilities.

One sees a very sharp increase in electron transmission at the energy level corresponding to the lowest quantum well. In fact, this simulation predicts the tunneling probability increases roughly 10 orders of magnitude when the Fermi Level reaches the bottom quantum well.

The result is the very nonlinear current-voltage curve shown in Figure 5 – much more like a semiconductor diode than the simple MIM device.

The materials and structure of the diode determine its characteristics. Carefully engineered materials systems and optimized manufacturing techniques are the heart of Phiar’s diode technology.

The diode must be stable, be fabricated from materials that are compatible with standard silicon IC fabrication facilities and processes, and must provide the desired impedance and nonlinearity.

The device whose current-voltage characteristic is shown in Figure 5 is one example of an MIIM diode that fulfills these requirements. The result is a semiconductor-like characteristic, using a technology that has an extremely high frequency response, is low cost, and can be deposited on nearly any substrate.

To learn a bit more about Phiar’s plans for its diodes, click the diodes page under “products.”