when a hemt is built from algan/gan, higher power density and breakdown voltage can be achieved. nitrides also have different crystal structure with lower symmetry, namely the wurtzite one, which has built-in electrical polarisation. since this polarization differs between the gan "channel" layer and algan "barrier" layer, a sheet of uncompensated charge in the order of 0.01-0.03 c/mformula_1 is formed. due to the crystal orientation typically used for epitaxial growth ("gallium-faced") and the device geometry favorable for fabrication (gate on top), this charge sheet is positive, causing the 2d electron gas to be formed even if there is no doping. such a transistor is normally on, and will turn off only if the gate is negatively biased - thus this kind of hemt is known as "depletion hemt", or dhemt. by sufficient doping of the barrier with acceptors (e.g. mg), the built-in charge can be compensated to restore the more customary ehemt operation, however high-density p-doping of nitrides is technologically challenging due to dopant diffusion into the channel.