GaN on Si for Power, E-mode

GaN on Si for Power, E-mode

PAM-XIAMEN offers GaN on silicon HEMT wafer for Power, E-mode. Because the normally-on characteristic will Increase the complexity of circuit design and power consumption, designing an enhanced (E-Mode) HEMT that is turned off under zero grid bias will be crucial for advancing the application of GaN-on-Silicon HEMT in the power field.

1. Structures of E-MODE GaN HEMT on Silicon Substrate

No. 1 GaN on Silicon Epitaxial HEMT Structure

GaN on silicon HEMT epitaxy size 2″, 4″, 6″,8″
AlGaN/GaN HEMT structure Refer 1.2
Residual 2DEG density (Vg=0 V) <1e18/cm3
AFM RMS (nm)of 5x5um2 <0.25nm
Bow(um) <=30um
Edge exclusion  <5mm
p-GaN /
u-GaN cap layer  /
Al composition 20-30%
AlGaN barrier layer  /
GaN channel /
AlGaN buffer /
Substrate material Silicon substrate
Si wafer thickness (μm) 675um(2″), 1000um(4″), 1300um(6″), 1500um(8″)

 

No. 2 4-inch GaN / Si Epi-Wafer PAM200211-HEMT

GaN / Si HEMT Epi Structure
Layer Material Thickness
6 p-GaN, Mg-doped
5 AlN
4 AlGaN
3 AlN
2 GaN (channel layer)
1 Buffer (Al,Ga)N > = 4 um
Substrate Si (111), 1000+/-25um thick, deflection <10 um

 

Remark:

1/ At present, our standard structure AlGaN 12NM, 25% Al has the corresponding parameter carrier concentration > 6e12, mobility > 2000, RS ~ 500, much better;

2/ The current Mg doping concentration is ~ 2e19 and the activation concentration is ~ 4e17. 

2. What is E Mode GaN on Silicon HEMT?

Changing the process structure of the GaN on silicon HEMT gate to switch the threshold voltage polarity can produce an enhanced (E-type) GaN HEMT device. The purpose of the process structure change is to deplete the 2DEG under the gate with the condition of no applied voltage, so that the forward threshold voltage can be enhanced by the 2DEG to form a channel. P-doped GaN gate is a structure similar to a diode in the gate structure, and the threshold voltage is raised by the diode voltage drop.

Currently, there are three main methods for using p-type gate to realize GaN-based enhanced mode HEMT:

  • The first method of p-type gate is mainly to use the entire epitaxial p-type layer on the barrier layer, and then etch and retain the p-type layer under the gate to realize the enhancement;
  • The second is to trim the p-type epitaxial layer, then retain the p-type layer under the gate, and etch most of the p-type layer in the non-gate area, leaving 5-20 nanometers of the p-type layer. During the etching process, plasma will damage the interface and affect the stability of the GaN on silicon power device;
  • The third method of p-type gate is to do secondary epitaxy in the gate area on the barrier layer to grow the p-type gate.

3. FAQ about GaN on Si Wafer for Power, E-mode

Q1: About the surface treatments for E-mode GaN on Si HEMT wafer: would you also give us some hints to overcome this hard problem? Especially gate process is in our minds. We are not very sure about the treatments before gate metal evaporation.

A: Concerning the surface of GaN on Si HEMT wafer, organic treatments are important after the pGaN etching, to repair the etched surface.

Q2: For the GaN on Si epi structure for E-mode HEMT, we also performed some simulations and achieved similar results in simulations. I mean 13nm of AlGaN barrier will be good for the expected Vth value. You can obtain 180mA/mm but at which DS distance you observe this current density?

According to our simulations decreasing the AlGaN thickness to 13nm is good but to be safe we also need to keep the Al mole fraction at 0.18. Do you have any predictions how much this will change the IDSS?

A: Our source to drain distance is ~25 μm based on the HEMT epitaxial structure. Concerning the Idss of 13 nm Al(0.18) GaN barrier, it is difficult to say, as it is also influenced by the etching accuracy. For your reference, we have worked on a device with 12 nm Al(0.15)GaN barrier, the Idss is ~72 mA/ mm.

Q3: About GaN-on-Si based devices with the threshold voltage, we have tried different gate metals (Pd, tungsten, Ni, NiAu, Ti, etc) but it is always 0.7V. We need to increase it up to 1.2V do you also have some recommendations about this problem? Your guidance will help too much.

A: Concerning the breakdown voltage (BV): Wafers with the same/similar epi structure of GaN/Si HEMT is always processing in our own fab, and a stable BV output is obtained, with BV >650 V (Ieakage current Ids < 1 μA). A higher BV should be achieved by proper surface treatments and optimized passivation layer.

Q4: We are thinking to modify the AlGaN thickness of GaN HEMT structure to increase the Vth. What do you think? Or maybe you can suggest some other things to increase the Vth.

A: Concerning the threshold voltage Vth of E-mode GaN/SiC HEMT wafer: We believe the Vth can be enhanced by reducing the barrier thickness. We can obtain devices with Vth >1.5 V easily in our fab, with a ~13 nm Al0.2Ga0.8N barrier, while a relative high on state current can still be maintained (>180 mA/ mm). In your case, the AlGaN barrier is a bit thicker (18 nm), which may limit the Vth values to be < 1 V.

For more information, please contact us email at [email protected] and [email protected].

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