What is the role of AlGaN in GaN HEMTs?

[ZeroFunGame]

TL;DR Summary

Please see below.

While trying to understand the workings of a GaN HEMT, I was looking at a conventional cross section of the device:
https://www.researchgate.net/figure/A-schematic-of-the-conventional-AlGaN-GaN-HEMT-For-structure-A-a-10-nm-thick-EBL-with_fig1_265518659

I noticed that the source and drain were in contact with the AlGaN, which as I understand it is another semiconductor used to strain the GaN to confine a 2DEG at the interface.

A few questions come to mind:

1) Since the AlGaN is 20 nm, does this mean the 2DEG needs to travel through the 20nm of AlGaN to reach the source/drain contacts?
2) If so, does traveling through the 20nm AlGaN result in much loss? I assume electrons are not allowed to flow though the AlGaN since you'd want the 2DEG confinement at the interface, so perhapse the AlGaN un-doped. This is why I find it curious that the contact is directly on the AlGaN rather than etched to contact the AlGaN/GaN interface.
3) Seems like the device will be normally on. Are there ways to create enhancement mode GaN HEMTs based on this device architecture? Or would a new structure be needed?
4) I've see people use GaN for power applications. I assume this is using HEMTs. Is the way to increase blocking voltage to just increase the channel length of the HEMT?
5) The gate is in direct contact with the AlGaN, which looks like a Schottky barrier. Is the way to turn off the device by putting a negative voltage on the gate and somehow deplete the electrons in the 2DEG?

 

[trurle]

Summary:: Please see below.

While trying to understand the workings of a GaN HEMT, I was looking at a conventional cross section of the device:
https://www.researchgate.net/figure/A-schematic-of-the-conventional-AlGaN-GaN-HEMT-For-structure-A-a-10-nm-thick-EBL-with_fig1_265518659

I noticed that the source and drain were in contact with the AlGaN, which as I understand it is another semiconductor used to strain the GaN to confine a 2DEG at the interface.

A few questions come to mind:

1) Since the AlGaN is 20 nm, does this mean the 2DEG needs to travel through the 20nm of AlGaN to reach the source/drain contacts?
2) If so, does traveling through the 20nm AlGaN result in much loss? I assume electrons are not allowed to flow though the AlGaN since you'd want the 2DEG confinement at the interface, so perhapse the AlGaN un-doped. This is why I find it curious that the contact is directly on the AlGaN rather than etched to contact the AlGaN/GaN interface.
3) Seems like the device will be normally on. Are there ways to create enhancement mode GaN HEMTs based on this device architecture? Or would a new structure be needed?
4) I've see people use GaN for power applications. I assume this is using HEMTs. Is the way to increase blocking voltage to just increase the channel length of the HEMT?
5) The gate is in direct contact with the AlGaN, which looks like a Schottky barrier. Is the way to turn off the device by putting a negative voltage on the gate and somehow deplete the electrons in the 2DEG?

Generally in high-voltage GaN transistors breakdown starts at gate corner, where field concentration do appear. GaAlN layer is likely used mostly to reduce that local electric field, without introducing much other detrimental effects. Additional resistance (or diode drop) of GaAlN will be countered by higher doping of GaN active layer. Simple transistor will have same breakdown voltage at lower GaN doping.

Regarding shottky contact, no. Electrodes material (typically tungsten) plus surface doping is high enough for tunnel (ohmic) contact happens instead of shottky.