A Closed-Form Charge Control Model for the Threshold Voltage of Depletion- and Enhancement-Mode AlGaN/GaN Devices

A closed-form charge control model (CCM) for threshold voltage $(V_{\rm th})$ of single-heterojunction (AlGaN/GaN) devices is developed. The CCM reveals the mechanisms of how the space charges across the barrier of a heterojunction deplete or enhance 2-D electron gas. The $V_{\rm th}$ of both depletion- and enhancement-mode heterostructure field-effect transistors may be calculated through this model. Considering the strained and relaxed states, and surface states, the calculated results for $V_{\rm th}$ are in good agreement with simulation results. The generalized form for CCM may have wide applications for theoretical-based design for $V_{\rm th}$ of AlGaN/GaN devices.