Gunn diode

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Gunn diode

The Gunn diode or Gunn element is an electronic semiconductor component that is used to generate microwaves . It is not actually a semiconductor diode , but it has become established for this component to speak of anode and cathode in order to distinguish between positive and negative contact. Based on the function of 1963 by J. B. Gunn discovered Gunneffekt .

construction

Construction of a Gunn oscillator in a cavity resonator
View of the flange and the waveguide with the Gunn diode arranged centrally in it

A Gunn element consists only of n-doped semiconductor areas, mostly of gallium arsenide (GaAs), gallium nitride (GaN) or indium phosphide (InP). The areas are arranged one behind the other and doped to different degrees . Most of the operating voltage (around 10 V) falls over a narrow, weakly doped middle layer, which shows a negative differential resistance due to the Gunne effect : the mobility of the electrons decreases with increasing field strength. As a result of the associated instability, zones of low electron mobility and high field strength migrate through this layer.

The transit time and thus the generated frequency depends primarily on the dimensions of the crystal, but can be changed slightly by the surrounding cavity resonator . Gunn diodes can generate frequencies from 1.5 GHz to approx. 200  GHz (GaN to 3  THz ).

The efficiency of a Gunnos oscillator is higher than that of reflex klystrons and is e.g. B. about 5% in the X-band (type MG1008-15, 8 ... 12 GHz). The power in continuous operation still reaches around 400 mW in the K-band  , with increasing frequency the achievable output power of power-optimized Gunn diodes decreases and reaches z. B. at 90 GHz about 50 mW with an efficiency of up to 2%. There are also pulsed gunn elements and stacks to increase the pulse output. For example, 10 watts are achieved at 9.3 GHz (type MG1060-15, pulse duration 1 µs, duty cycle 1%).

A Gunn oscillator consists of only a few components - the Gunn element and an oscillating circuit. Strictly speaking, this is not even required, but it reduces the phase noise and the spectral width of the signal generated and makes it easier to extract the high-frequency energy. At frequencies of a few terahertz, a resonator can no longer be realized, it would be too tiny.

The sketch on the right shows schematically a mechanically tunable Gunn oscillator using waveguide technology. The distance between the short-circuit slide is chosen so that the Gunn diode is loaded with a parallel resonant circuit, i.e. about a quarter of the wavelength. The coupling hole together with the tuning screw are used to adapt to the wave resistance of the waveguide.

function

Characteristic curve of a Gunn diode with hysteresis
Oblique view of the GaAs crystal. The green disk is the zone of increased field strength that migrates to the anode

The band structure of some compound semiconductors such as gallium arsenide consists of three energy bands . This additional band is usually free of conduction electrons and therefore insulates. If sufficient energy is supplied (in the case of the Gunn diode: by increasing the electrical voltage), electrons can overcome the band gap and be lifted from the valence band into this third band, which becomes electrically conductive.

As the voltage increases, more and more electrons in the valence band reach the energy required to jump into the third band . There, however, the drift speed of the electrons is lower than in the valence band and the current falls. This creates a negative differential resistance . The conductive area of ​​the third band does not fill the entire distance (a few micrometers) between the cathode and the anode, but rather forms a thin “disk” parallel to the cathode surface, which becomes detached and moves to the anode at a certain, material-dependent speed. By the time it arrives, the voltage between the cathode and anode is reduced. As soon as the disk reaches the anode, the voltage jumps back to the previous, higher value and the next disk is formed at the cathode. A drastic comparison would be a machine gun, in which a new shot is spontaneously formed at the cathode after every anode hit.

It must be avoided that the new disks form somewhere between the cathode and anode, because successive running times would then be unequal. Only if the correct cathode material is used will all new disks be created directly at the cathode connection and all run lengths correspond to the electrode spacing, which means that the generated frequency is constant even without an external resonator. For this reason, the anode and cathode must not be interchanged. As soon as a disc has formed, the voltage between the electrodes drops suddenly, which prevents the formation of additional discs.

The relationship between conductivity and field strength can be illustrated by a comparison with a multilayer structure made of metal and insulating plates: If a disk conducts the current well, the internal field strength is low. If a disk conducts the current poorly, the internal field strength is as high as in the dielectric of a capacitor. In the GaAs crystal, the difference in conductivity is considerably smaller, but the disks can move.

In the lower picture the green disk represents the zone of reduced electron mobility and increased field strength. Since this disk is "filled" with electrons, it migrates to the anode. It is surrounded by two areas of reduced field strength in which no new discs can form.

use

Gunn diodes are relatively inexpensive and are used in many oscillators if a few milliwatts are sufficient:

Web links

Commons : Gunn diodes  - collection of images, videos and audio files

Individual evidence

  1. Viktor Gružinskis, Jian H. Zhao, P. Shiktorov, E. Starikov: Gunn Effect and THz Frequency Power Generation in n + - n - n + GaN Structures . In: Materials Science Forum . tape 297-298 , 1999, pp. 341-344 , doi : 10.4028 / www.scientific.net / MSF.297-298.341 .
  2. ZS Gribnikov, RR Bashirov, VV Mitin: Negative effective mass mechanism of negative differential drift velocity and terahertz generation . In: IEEE Journal of Selected Topics in Quantum Electronics . tape 7 , no. 4 , August 2001, p. 630-640 , doi : 10.1109 / 2944.974235 .
  3. http://www.microsemi.com/document-portal/doc_view/9677-msc-gunn-diodes-cath-hs-pdf page 2
  4. Gunnelement MG1038-16 in http://www.microsemi.com/document-portal/doc_view/9677-msc-gunn-diodes-cath-hs-pdf page 3
  5. ^ BK Ridley, TB Watkins: The Possibility of Negative Resistance Effects in Semiconductors . In: Proceedings of the Physical Society . 78, No. 2, 1961, p. 293. bibcode : 1961PPS .... 78..293R . doi : 10.1088 / 0370-1328 / 78/2/315 .