The charge profile of an AlGaN/GaN HEMT is usually modelled with six sheet charges as shown in the figure, polarization charge pair of AlGaN and GaN, and ionized surface donor states and 2DEG.
According to this model, electric field inside the both AlGaN and GaN will be constant. Since electric field is a negative gradient of potential (i.e. V = -\int E \dot dx) and conduction band is -e*V where e is an electric charge, I would expect the band diagram to be a straight line with some slope.
This is indeed the case for the band diagram corresponding to the AlGaN region, but in the GaN region, we see a band bending near the surface and flat conduction band further away from the interface. How do I understand the band diagram of AlGaN/GaN HEMT in terms of Poisson Equation?
Hello Seohong,
you are right, the curvuature (resp. band bending) of the spatial potential determines the electric field E with E = -e*grad V(x).
The broken line determines the position of the chemical potential µ in the thermal equilibrium (outer voltage = zero). The position of µ is determined by the different electron affinities near the hetero interface. This determines the trend of the bandminima. Due to this we have a potential function V(x) which generates an electric field that drives electrons to the heterointerface. Now, all electrons are present in the potential "bag" near the interface. This region is called the 2-dimensional electron gas which is used as channel for high conductive electrons (the electron see must be contacted and can than used to control electric currents(HEMT)).
The electric field is exactly so as you described, but the electron movement in the HEMT is lateral. The high fields near the interface are only used for the "containment" of electrons in the bag.
With Regard
Rüdiger Mitdank
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