Microwave semiconductor devices
This page covers Microwave semiconductor devices which include 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 following reasons:
• length of the leads introduce significant inductance at microwave frequency.
• high internal capacitance
• high transit time of carriers through these device
Time taken by electron or proton to travel from one node to the other is called transit time. In the case of diode for example from cathode to anode and in transistor from emitter to base or from emitter to collector or from source to drain.
Special semiconductor material other than silicon and changed geometry overcome the problem listed above.
Many semiconductor diodes are available for microwave application mainly designed to take care of signal detection and mixing purpose. There are two main types of diodes viz. point contact diode and schottky barrier diode.
Point Contact Diode
Semiconductor material with piece of fine wire as shown in figure is called as point contact diode. Here capacitance is very low as wire touches small area of the material.Current flows from cathode to anode very easily but but not in the opposite direction. In the early days of technology point contact diodes were manufactured using germanium. Now-a-days they are manufactured using P type silicon with fine tungsten wire as cathode.
Point contact diodes are mainly used in mixers and detectors. They can not withstand high power and are ideal for low signal applications.
Hot Carrier Diode/Schottky diode
It is widely used as microwave diode, also called Schottky diode. They are made with N type silicon semiconductor (as cathode) along with thin deposited metal layer(as anode). Nickel chromium, aluminium and gold are used as anode. They are used mainly in balanced modulators as well as in mixers. Used as fast switches due to high frequency response.
The main application of this microwave semiconductor device, varactor diode is frequency multiplier. The capacitance of this varactor diode device depends on
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 upto frequency range of about 10GHz and power rating upto 50Watt.
During forward bias of operation it stores charge, the same charge keeps diode on momentarily when reverse bias is applied. Then it gets off suddenly.
Gunn diode is referred as 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 upto about 50GHz. Gunn diodes are used as Local Oscillators in receivers
and also as frequency source in transmitters. These devices exhibit negative resistance just opposite to
the standard Ohm's law.READ MORE
These IMPATT/TRAPATT diodes are also used as oscillators. They are also PN junction diodes which are 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 upto 25Watt.
This microwave semiconductor device is used to produce low power oscillators. When tunnel diodes are forward biased they produce
negative resistance.READ MORE
Both Bipolar and Field Effect Transistors are available upto 40GHz. They microwave transistors are divided into small signal and RF power categories.
Monolithic Microwave Integrated Circuits are available for amplification of small signal as well as for frequency conversion. Available upto 8-10 GHz frequency range for various applications ,Read more