AlGaN/GaN HEMTs
Sommaire
Summary
This article is talking about AlGaN/GaN high electron mobility transistor (HEMT). HEMT are heretostructure field effect transistor (HFET).
This kind of transistor allows to the electron a best mobility. In fact, a heterojunction with a highly n-doped bandgap (here AlGaN) and a undoped bandgap (here GaN) allows to electrons to go faster on the other side. The layer between this 2 sides is called 2-DEG (two-dimensional electron gas).
Authors introduces first the subject, their target, and how fabricate this kind of transistor. It's a molecular beam epitaxy. A 21-nm AlGaN barrier is grown on a silicon substrate. In a second time, this barrier is processed on a rigid Si substrate by lithography. Only after this 2 steps, the transistor is transferred on a flexible substrate.
After an explanation of DC characteristics of the transistor on a Silicium (Si) subtrate, they explain the effect of the transfer on a flexible substrate on the DC transistor characteristics. They try to explain all changes on the Ids-Vds characteristics, on C-V (Capacitance = f(Gate voltage)) profile, on the Nd-Na profile (Nd-Na : representative of doping level), in order to show which parameters are modified, and what are the consequences of this modifications.
The first observation is on the maximum drain current density. It drops down when the transistor is transferred on a flexible substrate (930 mA/mm before, 450 mA/mm after the transfer).
The second observation is the behavior of the transfer characteristics. It does not follow the ideal transfer characteristics.
This 2 observations are due to the poor thermal conductivity of the polymer which form the flexible tape. In order to confirm this hypothesis, they decided to plot the C-V profile. They show that no deviation is observed in C-V profile in terms of frequency dispersion and charges density. They confirm that the 2-DEG density is not affected and assume that no additional defects are created during the transfer.
After this study, they try to show the effect of the bending on the DC characteristics. They explain that the characteristics are not the same if the flexible tape is flat or if it is bent. They realize the bending with a semicylindrical chuck (chuck : place where you put the wafer on - wafer : silicium disc where all devices are engraved) with 15 mm raduis of curvature. With this chuck, strain is in the gate direction. The effect on the I-V characteristic is light. And due to the difference between the thermal conductivity between the 2 chucks they use for analysis, they do analysis with a low drain-source voltage. They show that the on-resistance (Ron) is decreased, and the sheet resistance (Rs - 11% decrease) also. It is due to the increase of 2-DEG density (in 2-DEG, the parameter which translate the mobility of the electron (μn) increased, and sheet resistance is directly close to this parameter)
In a thrid part, they try to characterize the piezoelectric behavior of the transistor in flat and bent configuration. All semiconductors which are in III-N column of Mendeleiev Table of Elements is piezoelectric. It means that any deformation or strain involve a tension inside the semiconductor.
They show an increase of 6% of 2-DEG density. They deduce that the convex (or concave) configuration involve an increase of 6% of 2-DEG density, so involve a tension.
Finally, they study RF characteristics of this kind of transistor. They show a difference of 8 GHz between the theoretical cutoff frequency (40 GHz) and the experimental cutoff frequency (32 GHz). This difference is due to the approximation to determine the gate-to-2-DEG capacitance and the gate-to-drain capacitance, which are used in the small-signal model of a transistor.
Main contribution
This article has the objective to show the implementation of transistors on a flexible substrate.
The main contribution of this project is the use of method for making a transistor, not on a rigid substrate but on a flexible substrate.
Authors show that it is possible to transfer a transistor, which had growing on a rigid substrate, on a flexible substrate, and have the same performances.
In addition, the piezoelectric effect adds performances when the transistor is under strain.
Applications
With an industrial asking to have more and more of large bandwidth, HEMT are very used in the industry of telecommunication, for 3G or 4G technology. With their abilities to work at high-frequency, HEMTs are also used in radio-astronomy and radars, and in general all application with low noise and maximun of power.
But this is not the only field in which we use HEMT.
Transfer HEMT, from a rigid substrate to a flexible substrate can allow us to do electronics PCB more light, more flexible, with the same RF and DC performances than a PCB with classic transistors. The flexible substrate provides to the PCB RF and DC performances when it is strained (flat or bent). In a compact package, this is a good solution to match space and complexity (a lot of wires or circuits with a small space can be possible now with flexible circuits).
Another application is for flexible display. For example, in LCD fabrication, glass is used as substrate. Now, with flexible substrate, we are able to make flexible LED (flexible OLED for flexible Organic LED) in order to make flexible LCD screen, as thin as a paper
Another example is for developping solar cells. You can developp solar cells with flexible circuits. This solar cells are light, and very easy to deploy.
