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In my previous experiences working with specialized semiconductor devices, our primary concern was to avoid forming a Schottky barrier at the metal-semiconductor interface. While this behavior could pose challenges in some applications, it can be beneficial due to its unique properties. The Schottky diode, a specific type of diode, is designed to leverage the rectification achieved between a metal and semiconductor, making it crucial in applications requiring minimal voltage drop.

When contrasting with p-n diodes, Schottky diodes demonstrate a lower forward voltage drop at low reverse bias. Their typical applications range from serving as rectifiers in switching regulators to providing discharge protection in power electronics and enabling rectifying circuits that demand high switching speeds. If you are considering simulating circuits that incorporate Schottky diodes or any circuit with rectifying elements, it is vital to consider their highly nonlinear behavior. Here are some essential points to keep in mind while designing those circuits.

What is a Schottky Diode?

A Schottky diode, also referred to as a Schottky barrier diode or simply a barrier diode, is constructed by bringing a metal film into contact with a semiconductor layer, typically n-type. In forward bias, the metal is held at a higher potential than the semiconductor, while in reverse bias, the situation is reversed. Metals commonly employed in Schottky diodes include platinum, chromium, molybdenum, and tungsten, with certain metal silicides such as palladium silicide and platinum silicide also finding usage.

For efficient operation, it is crucial to have a metal layer on the semiconductor's other side facilitating the movement of charge carriers. In Schottky diodes, two different metals serve as electrical contacts, with the metal at the anode constructing the rectifying junction known as the Schottky barrier. The cathode side, in contrast, lacks a rectifying junction, leading to a metal-semiconductor interface that behaves like a small resistor—this is referred to as an Ohmic contact.

Diagram of Schottky diode symbol and structure

When compared to a p-n diode, a Schottky diode consists of a single Ohmic contact, as opposed to the two present in a p-n diode. This distinction contributes to the lower forward voltage drop in Schottky diodes; only a single Ohmic contact is responsible for the voltage drop, while the other contact facilitates rectification. Typically, a Schottky diode exhibits a forward voltage drop of approximately 300 mV, which is significantly lower than the ~600 mV seen in silicon diodes.

Apart from this, Schottky diodes perform similarly to standard p-n diodes under DC bias conditions. If you plan on simulating these components before physically constructing your circuit, it is crucial to recognize that their unique recovery times and doping characteristics warrant the use of SPICE models for ease, accuracy, and enhancement of your overall design process. However, in scenarios involving switched DC biases or AC signals, the behaviors of Schottky diodes vastly differ from those of standard p-n or Shockley diodes.

Schottky Diode Reverse Recovery Time

A notable feature of Schottky diodes is their reverse recovery time when transitioning between rectifying and non-rectifying states. Due to the metal contact present in the device, Schottky diodes boast significantly faster reverse recovery times compared to standard p-n diodes. All diodes have some capacitance linked to their metal contacts, yet the parasitic capacitance at the metal-semiconductor interface in Schottky diodes is considerably less than that seen at the junction in silicon diodes, resulting in much faster reverse recovery times.

The reverse recovery time for Schottky diodes can reach as low as approximately 100 picoseconds. However, larger Schottky diodes utilized in power electronics, such as switched-mode power supplies, typically encounter longer reverse recovery times, approximately around 10 nanoseconds. In contrast, a typical fast p-n diode may exhibit a reverse recovery time of at least 100 nanoseconds.

This fast reverse recovery time allows Schottky diodes to excel in switching regulators, where they can efficiently work with PWM frequencies of MHz levels. Coupled with an increased edge rate for the PWM signal, this capability ensures successful operation at higher frequencies, fully switching off the MOSFET driver in the regulator. In contrast, employing a p-n diode in such a scenario would limit the potential PWM frequency and edge rate due to its comparatively slower reverse recovery time.

Schottky Diodes for RF and Power Electronics

In situations where the transistor in your regulator reaches saturation, Schottky diodes can serve a critical role in voltage clamping by limiting the voltage fed to the base while redirecting some current to the emitter/collector (or source/drain in a MOSFET). They also find application in high-frequency clipping circuits, where a pair of Schottky diodes arranged in a back-to-back configuration prevent output voltage from exceeding a specific level, mitigating the risk of damage to downstream devices.

Utilizing Schottky diodes for voltage clamping and RF detection

Smaller Schottky diodes are essential in RF detectors and mixers that can function at frequencies up to 50 GHz. Although these smaller diodes may have limitations on the maximum voltage they can handle, their low parasitic capacitances enable rapid switching times essential for RF detection applications. Schottky diodes are beneficial across an array of uses, courtesy of their low forward voltage drop and fast reverse recovery times.

Regardless of the specific type of Schottky diode you are developing, accurate evaluation of circuit behavior becomes achievable with the appropriate PCB design and analysis software, complemented by a selection of validated component models for simulations. The tools available in PSpice Simulator for OrCAD and comprehensive analytical tools from Cadence are ideally suited for assessing rectification, switching behaviors, and various other aspects of these components within a broader system. Additionally, manufacturing preparation tools are indispensable for ensuring your components can be sourced at scale.

For insights on how Cadence can meet your needs, we encourage you to engage with our team of experts.

The Schottky diodes you are considering are custom-made by a specialized electronics manufacturer. These diodes are 25 mm in size and are fabricated on 250 µm thick p-type wafers with a resistance of 35 mΩ and a lifetime of 10 µs. The manufacturer employs advanced technology to create Schottky barriers on p-type silicon with a barrier height of 0.4 eV, designed for silicon layers containing 1x10^15 cm^-3 with a lifetime of 10 µs. They incorporate a low-doped layer atop the wafer that can be up to 5 µm thick, tailored by the design engineer to achieve the desired breakdown voltage. It is crucial that the breakdown voltage is at least 20V higher than the maximum reverse bias voltage anticipated in any circuit to ensure reliability. Note that for reverse-biased I-V simulations, a reduction in substrate thickness to 2 µm may be required for simulation convergence. Additionally, a high surface recombination rate at the Schottky metal interface (e.g., 10^10 cm/s) should be considered in the simulation parameters.

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