Analytical Model for Schottky Barrier Height and Threshold Voltage of AlGaN/GaN HEMTs With Piezoelectric Effect

Physics-based models for calculating the threshold voltage (V
_th
) of GaN-based high electron mobility transistor (HEMT) is proposed. Assuming that the 2-D electron gas in III-V heterostructures originate from donor-like surface states, an analytical expression for φ
_b
is derived based on the condition of charge neutrality across the barrier layer and applied to develop V
_th
model. Effects of introducing a GaN cap layer on distribution of surface donor states and strain relaxation due to high barrier layer Al composition are considered in the derivation of the models. Calculated V
_th
values are in excellent agreement with the values extracted from static I
_d
-V
_g
measurement data for devices with different gate lengths. Furthermore, the V
_th
models are implemented in the physics-based I-V model previously proposed by our group for AlGaN/GaN HEMTs and tested with devices of different geometries. Excellent agreement is obtained between the model and experimental data of the drain current and transconductance of the devices over a full range of bias conditions.