Quasi-Fermi level bending in MODFET's and its effect on FET transfer characteristics

Using Shockley's diffusion/drift model we calculate the quasi-Fermi level (imref) bending in the depleted AIGaAs barrier layer of GaAs/AIGaAs MODFET's. We show that the assumption of a constant imref from the heterointerface through the barrier layer is not justified when the gate is moderately forward biased. Once the barrier-layer conduction band edge at the gate interface fails below that at the heterointerface, the imref changes both in the vicinity of the heterointerface and at the gate metal. This has important consequences for the MODFET transfer characteristics and necessitates new considerations for the gate control mechanism As a result, sheet electron concentrations in the MODFET channel exceeding the equilibrium concentrations are obtained. Despite the gate being forward biased with voltages close to or larger than the Schottky-barrier height, the gate current is suppressed not only by the barrier at the heterointerface but also by a barrier of about the same height present at the gate metal-semiconductor interface. Experimental results on AIGaAs/GaAs MODFET's are in good quantitative agreement with the theoretical calculations.