Microwave Semiconductor Devices Explained

This page covers microwave semiconductor devices, including the microwave diode, point contact diode, hot carrier diode, varactor diode, step recovery diode, Gunn diode, IMPATT, TRAPATT diode, tunnel diode, microwave transistor, and MMIC.

As we know, at microwave frequencies, conventional transistors and diodes do not function as desired due to the following reasons:

• The length of the leads introduces significant inductance at microwave frequencies.
• High internal capacitance.
• High transit time of carriers through these devices.

Transit time is the time taken by an electron or proton to travel from one node to the other. In the case of a diode, for example, it’s from the cathode to the anode, and in a transistor, it’s from the emitter to the base, from the emitter to the collector, or from the source to the drain.

Special semiconductor materials other than silicon and altered geometry overcome the problems listed above.

Microwave Diode

Many semiconductor diodes are available for microwave applications, mainly designed to take care of signal detection and mixing purposes. There are two main types of diodes: point contact diodes and Schottky barrier diodes.

Point Contact Diode

A semiconductor material with a piece of fine wire, as shown in the figure, is called a point contact diode. Here, capacitance is very low as the wire touches a small area of the material. Current flows from the cathode to the anode very easily but not in the opposite direction.

In the early days of technology, point contact diodes were manufactured using germanium. Nowadays, they are manufactured using P-type silicon with fine tungsten wire as the cathode. Point contact diodes are mainly used in mixers and detectors. They cannot withstand high power and are ideal for low signal applications.

Hot Carrier Diode/Schottky Diode

It is widely used as a microwave diode, also called a Schottky diode. They are made with N-type silicon semiconductor (as cathode) along with a thin deposited metal layer (as anode). Nickel chromium, aluminum, and gold are used as anode. They are used mainly in balanced modulators as well as in mixers and function as fast switches due to their high-frequency response.

Varactor Diode

The main application of this microwave semiconductor device, the varactor diode, is the frequency multiplier. The capacitance of this varactor diode device depends on the reverse bias applied to it. They are manufactured with gallium arsenide.

Step Recovery Diode

It is a PN Junction diode manufactured with gallium arsenide or silicon microwave semiconductor materials. They are used to develop multipliers with factors of 5 or 10. Step recovery diodes operate up to a frequency range of about 10GHz and a power rating of up to 50Watt. During forward bias operation, it stores charge; the same charge keeps the diode on momentarily when reverse bias is applied. Then, it gets off suddenly.

Gunn Diode

A Gunn diode is referred to as a Transferred Electron Device (TED). It is basically a piece of N-type GaAs or InP semiconductor which forms a resistance when voltage is applied to it. They oscillate at frequencies up to about 50GHz. Gunn diodes are used as Local Oscillators in receivers and also as a frequency source in transmitters. These devices exhibit negative resistance, just opposite to the standard Ohm’s law.

IMPATT/TRAPATT diode

These IMPATT/TRAPATT diodes are also used as oscillators. They are also PN junction diodes designed using silicon, GaAs, and InP. They operate at high reverse bias, which causes these devices to break down/avalanche. When these diodes are mounted in a cavity, they will produce oscillations. They are available up to 25Watt.

Tunnel diode

This microwave semiconductor device is used to produce low-power oscillators. When tunnel diodes are forward biased, they produce negative resistance.

Microwave Transistor

Both Bipolar and Field Effect Transistors are available up to 40GHz. These microwave transistors are divided into small signal and RF power categories.

MMIC

Monolithic Microwave Integrated Circuits are available for amplification of small signals as well as for frequency conversion. Available up to 8-10 GHz frequency range for various applications.