Do Your Circuits Need a Schottky Diode?
In my earlier years working on unique semiconductor devices, the last thing we wanted was to form a Schottky barrier at a metal-semiconductor interface. The resulting rectifying behavior is undesirable in many applications, but you can take advantage of this rectification between a metal and semiconductor. This type of diode is called a Schottky diode, and it finds its home in a number of important applications requiring rectification with low voltage drop.
Compared to p-n diodes, a Schottky diode provides lower voltage drop across the diode at low reverse bias. Some applications of Schottky diodes include rectifiers in switching regulators, discharge protection in power electronics, and rectifying circuits requiring high switching rate. If you’re planning on simulating the behavior of circuits with Schottky diodes, or any circuit with a rectifying element, pay attention to the highly nonlinear behavior of these components. Here’s what you need to keep in mind when designing these circuits.
What is a Schottky Diode?
A Schottky diode is sometimes called a Schottky barrier diode, or simply a barrier diode. These diodes are built by placing a metal film in contact with a semiconductor layer (normally n-type). These diodes are forward biased when the metal side is held at higher potential than the semiconductor side, and vice versa for reverse bias. Typical metals used in a Schottky diode are platinum, chromium, molybdenum, or tungsten. Certain metal silicides, such as palladium silicide and platinum silicide, are also used in Schottky diodes.
Obviously, there must be a metal on the other side of the semiconductor layer to provide a path for charge carriers to move through the device. In a Schottky diode, two dissimilar metals are used for electrical contacts. The metal at the anode forms the rectifying junction in a Schottky diode, known as a Schottky barrier. At the cathode side, there is no rectifying junction, and the metal-semiconductor interface acts like a small resistor (called an Ohmic contact).
Schottky diode symbol and structure
Compared to a p-n diode, there is only a single Ohmic contact in a Schottky diode, while a p-n diode has two Ohmic contacts (one on each side of the device). This is one reason a Schottky diode has lower forward voltage drop than a p-n diode; voltage is only dropped across a single Ohmic contact, while the other contact in a Schottky diode provides rectification. The forward voltage drop across a Schottkey diode is ~300 mV, while it is ~600 mV in a silicon diode.
Aside from this characteristic, Schottky diodes exhibit the same behavior as standard p-n diodes when run with DC bias. If you’re looking to simulate these components prior to making your actual circuit, it’s important to note, especially with their unique recovery times and doping considerations, that SPICE models can make this easy, accurate, and advantageous for your overall design process. But, when the DC bias is switched, or when run with an AC signal, Schottky diodes have very different behavior than standard p-n diodes or Shockley diodes.
Schottky Diode Reverse Recovery Time
One important aspect of Schottky diode behavior is its reverse recovery time when switched between the rectifying and non-rectifying states. Thanks to the metal contact in the device, a Schottkey diode has much faster reverse recovery time than a typical p-n diode. Any diode will have some capacitances at the metal contacts. In a Schottky diode, the parasitic capacitance at the metal-semiconductor interface is lesser than that at the junction in a silicon diode, thus its reverse recovery time is much faster.
The reverse recovery time in a Schottkey diode can reach as low as ~100 ps. Larger Schottkey diodes that are used in power electronics (e.g., in switched-mode power supplies) have longer reverse recovery times, usually reaching ~10 ns. Compare this with a typical fast p-n diode, where the reverse recovery time is at least ~100 ns.
This is why a Schottkey diode finds its home in switching regulators. The fast recovery time of a Schottkey diode allows it to be used with PWM frequencies reaching MHz levels. Combine this with a faster edge rate for the PWM signal, and you have a system that can run successfully at higher frequencies that fully switches off the MOSFET driver in the regulator. If a p-n diode were used in such a system, the maximum PWM frequency and edge rate would be limited by the slow reverse recovery time of the p-n diode.
Schottky Diodes for RF and Power Electronics
If the transistor in your regulator is saturating, a Schottky diode is also useful for voltage clamping, which limits the voltage applied to the base by channeling some current to the emitter/collector (or source/drain in a MOSFET). Another possible application is in a high frequency clipping circuit, where a pair of Schottky diodes in a back-to-back configuration will limit the output voltage at the reverse saturation current. This nicely limits the amplitude of a switching signal to some maximum, preventing potential damage to a downstream device.
Voltage clamping and RF detection with a Schottky diode
Smaller Schottky diodes are also important in RF detectors and mixers, which can operate up to 50 GHz. These smaller diodes are limited in the maximum voltage they can handle, but their low parasitic capacitances provide the fast switching time needed for RF detection (see the above circuit). There are many other applications that can benefit from a Schottky diode, thanks to its low forward voltage drop and fast reverse recovery time.
No matter which type of Schottky diode you’re building, you can accurately evaluate circuit behavior when you use the right PCB design and analysis software and a set of verified component models for your simulations. The design and simulation tools in PSpice Simulator for OrCAD and the full suite of analysis tools from Cadence are ideal for evaluating rectification, switching behavior, and other aspects of these components in a larger system. The manufacturing preparation tools also help ensure your components will be sourceable at scale.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.