III-V Gunn Diode Structure *S
Gunn diode has always been the preferred solid-state device for coherent power generation at millimeter wave frequencies. Typically, Gunn diodes are composed of uniformly doped n-type III-V materials (such as GaAs, InP). The most common method for manufacturing Gunn diodes is to grow epitaxial layers on n+ substrates. The thickness of the active region ranges from a few micrometers to several hundred micrometers. The doping level of the active layer is between 1014cm-3 and 1016cm-3, which is lower than the doping level used for the top and bottom regions of the device. The thickness will vary according to the required frequency. The top n+ layer can be epitaxially deposited or doped using ion implantation. The top and bottom regions of the device are heavily doped to form n+ material. This provides the high conductivity area required for connecting devices. PAM-XIAMEN can grow III-V epitaxial structure for Gunn diode, take the InP based Gunn structure for example:
1. InP Gunn Diode Structure
| Epi Layer | Material | Thickness | Doping Concentration |
| Cap layer | n+ InGaAs | 0.1um | – |
| Contact layer | n+ InP | – | – |
| Active region | n InP | – | – |
| Contact layer | n+ InP | – | – |
| Etch stop layer | n+ InGaAs | – | – |
| Substrate | n+ InP |
2. Regarding Gunn Diodes
Gunn diode, also known as transmission electronics, is a double ended semiconductor electronic component with negative resistance used in high-frequency electronic devices. It is based on the “Gunn effect” discovered by physicist JB Gunn in 1962.
2.1 Gunn Diode Characteristics
The difference between its internal structure and other diodes is that it is only composed of N-doped semiconductor materials, while most diodes are composed of P and N-doped regions. Therefore, it conducts in both directions and cannot rectify AC power like other diodes. That’s why some sources don’t use the term diode and prefer to transmit electronic devices. The Gunn diode has three regions. Two of them are heavily nitrogen doped at each terminal, with a layer of lightly nitrogen doped material in between. When voltage is applied to the device, the elevator degree in the thin interlayer is maximized. As the voltage increases, the current passing through this layer initially increases, but eventually at higher electric field values, the conductivity of the intermediate layer changes, its resistivity increases, and the current decreases. This means that there is a negative differential resistance region on the current voltage (IV) characteristic curve of the Gunn diodes, and the current decreases with increasing applied voltage. This characteristic allows them to amplify and act as high-frequency amplifiers when biased with DC voltage, or become unstable and oscillate.
Fig. 1 IV characteristic curve of Gunn diode
2.2 Working Principle of Gunn Diode
When no external voltage is applied, the free electrons, which are the main charge carriers in N-type materials, move freely with high mobility.
When it is forward biased, the movement of electrons generates current. In ordinary conductors, as the voltage increases, the mobility of electrons increases, resulting in an increase in current. As the applied voltage increases, high mobility electrons are forced into a low mobility state due to the Gunn effect in Gunn diode.
Therefore, the conductivity decreases. Due to this unusual relationship between voltage and current, high-frequency oscillating currents are generated.
When the formed current pulse reaches the end of the active region, the next pulse begins. This determines the frequency of operation. This depends on the thickness of the active region.
3. GaAs VS.InP Gunn Diodes
The most common Gunn diodes are GaAs and InP diodes, while terahertz frequency oscillators are increasingly using gallium nitride(GaN) Gunn diodes. Therefore, competition mainly exists between GaAs Gunn diodes and InP Gunn diodes.
1) Output power and AM noise
InP Gunn diodes typically have higher power output and better efficiency at millimeter wave frequencies. Moreover, under similar performance conditions, the amplitude modulation (AM) noise of InP Gunn diodes is lower than that of GaAs Gunn diodes.
2) Gradient gap hot electron injection technology
Indium phosphide Gunn diodes cannot use graded gap hot electron injection technology, so the temperature stability of InP Gunn diodes is not as good as GaAs Gunn diodes, which can be used with this injection technology. GaAs Gunn diodes generally operate in the V-band. Injected electrons (or hot electrons) have higher energy than equilibrium electrons. The increase in electronic energy greatly increases the possibility of hot electrons directly entering the high-energy conduction band or negative resistance region. In general, diodes can only oscillate under sufficiently high turn-on bias, making their oscillation frequency dependent on temperature. This problem can be effectively solved by thermal injection methods that can bypass low-energy regions.
3) Gunn Diode Applications
At higher millimeter wave frequencies, InP Gunn diode oscillators largely replace GaAs Gunn diode oscillators, while lower frequency devices still use GaAs diodes, mainly due to cost reasons. Given the wide range of applications of Gunn diode oscillators, in addition to power, frequency, and noise, many other factors have also been considered, leading to the gradual surpassing of InP in the application of GaAs, even in high-frequency millimeter wave devices.
The substrate material of InP Gunn diode has a high intrinsic electron mobility, which enables it to achieve higher power in the millimeter wave band, and the oscillator formed by its combination with waveguide cavity has good performance in the W-band.
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