AlGaN/GaN HEMTs

Summary

This article is talking about AlGaN/GaN high electron mobility transistor (HEMT). It is an 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) allow 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 why 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 profile, on the Nd-Na profile, 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 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 chuck 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 peizoelectric 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 that an increase of 6% of 2-DEG density. They deduce that the convex 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 little-signal model of a transistor.

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.
But this is not the only field in which we use HEMT.
For example, to make RFID, we need transistor, which have a very selective bandwidth, due to the very perturbing environment of utilization. But RFID needs microwaye power, and need to be very performant in his bandwidth, in order to supply good informations during reading. Transfer HEMT, from a rigid substrate to a flexible substrate can allow us to do RFID tag more light, more flexible, and also more performant. In fact, the RF characteristics globally still the same, and the performance in terms of strain (flat or bent) still also the same. With this technology, we can patch RFID tag wherever we want.
Another application is for flexible display. With the same way as RFID, we are able now to put a screen for example on a special paper, which will be form with HEMT transistors on flexible substrate.
HEMTs on a flexible support is a good way to mix medium power, high frequency, and flexibility.