S.V. Beletsky – Post-graduate Student, Department of Nanoelectronics, Moscow Technological University (MIREA); Deputy Head of the Technology Department, JSC «GZ «Pulsar» (Moscow)
A.M. Konovalov – Head of Department of Scientific and Technical Developments, JSC «GZ «Pulsar» (Moscow)
A.A. Schuka – Dr.Sc.(Eng.), Professor, Moscow Technological University (MIREA)
The paper presents an overview of the problem of current collapse in in High Electron Mobility Transistors (HEMTs) based on the Al-GaN/GaN heterojunction. The reason for this parasitic effect is the large voltage between the gate and the drain, which entails the «ejection» of electrons from the channel, limiting the maximum achievable currents and microwave power densities. The «ejection» of electrons is due to their trapping in traps in the buffer and barrier layers. This effect is most pronounced for powerful field-effect transistors and is called a «current collapse».
To reduce the current collapse in the barrier layer of AlGaN, one of the main methods is passivation-depositing a thin dielectric layer on the surface of the transistor. As a possible dielectric, SixNy was chosen and the collapse effect was shown down to almost complete disappearance. As one of the reasons for the positive effect of passivant on dynamic transistor characteristics, along with the filling of broken surface bonds, the diffusion of silicon, which is a donor in the lattice (Al,Ga)N, is indicated in a thin near-surface region. The liberated silicon electrons can also fill the surface states, thus preventing the capture of carriers from the channel.
It should be noted that other passivating layers, for example, SiO2, SiON and AlN, are used in addition to SixNy to reduce the current collapse in HEMT. The main parameters (maximum drain current, maximum slope, threshold voltage, breakdown voltage and output power) of AlGaN/GaN HEMT devices, passivated SiO2, Si3N4 and SiON, and without passivation grown on a sapphire substrate.
Also, before applying the dielectric to the barrier layer, it is processed by the plasma of any gas. For example, in order to obtain a greater effect of passivation with a SiN layer, pre-treatment of the grown AlGaN/GaN structure in N2 nitrogen plasma is suggested before the application of the passivation agent. Impulse measurements of the dependence of the drain-source current on the drain-source voltage of the structures without and with preliminary treatment of N2 showed a reduction in the effect of current collapse.
As already noted, in order to combat the current collapse in AlGaN/GaN HEMT in the AlGaN barrier layer, a passive SiN film is deposited on its surface. However, this reduces the breakdown voltage of the transistor. To compensate for this effect, a so-called field electrode or field plate is introduced into the gate region, which is a gate electrode that is stretched onto the passivation layer. Thanks to this, it is possible to obtain high operating voltages. A significant improvement in the breakdown voltage is the result of a depleted layer formed under the field plate electrode, which leads to a decrease in the strength of the electric field at the edge of the gate with respect to the drain.
Another way to reduce the current collapse in the barrier layer is a thin p-type GaN layer, used instead of the conventional field metal plate that has reached its limit. Due to this layer, it was possible to realize the polarization effect in the upper and lower part of the AlGaN layer interface, in which equal negative and positive charges are generated, creating a 2DHG and 2DEG at the interface. This polarization effect almost completely suppresses both the current collapse and the gate leakage currents of the HEMT.
A necessary condition for preventing the causes of current collapse associated with the capture of electrons by «deep» traps in the buffer layer of GaN is an increase in the barrier for electrons in the channel. The use of compensating doping of the GaN layer to increase the electronic limitation leads to the appearance of additional traps. Therefore, double heterostructures (DHS) are more effective, the limitation in which is achieved due to the bending of zones under the influence of polarization fields. A possible variant of such a structure can be AlGaN/GaN/AlGaN DHS. Field-effect transistors based on such heterostructures demonstrate performance in the microwave range without the occurrence of a current collapse. However, the full realization of the DHS method is hampered by mechanical stresses due to the discrepancy between the GaN and AlN lattice parameters, as well as the associated piezosoles, which substantially distort the band diagram.
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