Charge Storage Mechanism of Drain Induced Dynamic Threshold Voltage Shift in  ${p}$ -GaN Gate HEMTs

The drain induced dynamic threshold voltage (<inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {th}}$ </tex-math></inline-formula>) shift of a <inline-formula> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula>-GaN gate HEMT with a Schottky gate contact is investigated, and the underlying mechanisms are explained with a charge storage model. When the device experiences a high drain bias <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSQ}}$ </tex-math></inline-formula>, the gate-to-drain capacitance (<inline-formula> <tex-math notation="LaTeX">${C}_{\textrm {GD}}$ </tex-math></inline-formula>) is charged to <inline-formula> <tex-math notation="LaTeX">${Q}_{\textrm {GD}}$ </tex-math></inline-formula>(<inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSQ}}$ </tex-math></inline-formula>). As the drain voltage drops to <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSM}}$ </tex-math></inline-formula> where <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {th}}$ </tex-math></inline-formula> is measured, <inline-formula> <tex-math notation="LaTeX">${C}_{\textrm {GD}}$ </tex-math></inline-formula> is expected to be discharged to <inline-formula> <tex-math notation="LaTeX">${Q}_{\textrm {GD}}$ </tex-math></inline-formula>(<inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSM}}$ </tex-math></inline-formula>). However, the metal/<inline-formula> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula>-GaN Schottky junction could block the discharging current, resulting in storage of negative charges in the <inline-formula> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula>-GaN layer. For the device to turn on, additional gate voltage is required to counteract the stored negative charges, resulting in a positive shift of <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {th}}$ </tex-math></inline-formula>. The dynamic <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {th}}$ </tex-math></inline-formula> shift is an intrinsic and predictable characteristic of the <inline-formula> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula>-GaN gate HEMT which is linearly correlated with <inline-formula> <tex-math notation="LaTeX">$\Delta \! {Q}_{\textrm {GD}}={Q}_{\textrm {GD}}$ </tex-math></inline-formula>(<inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSQ}}$ </tex-math></inline-formula>) <inline-formula> <tex-math notation="LaTeX">$- {Q}_{\textrm {GD}}$ </tex-math></inline-formula>(<inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSM}}$ </tex-math></inline-formula>). The <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {th}}$ </tex-math></inline-formula> shift is dependent on <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSQ}}$ </tex-math></inline-formula> as well as <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {DSM}}$ </tex-math></inline-formula>, indicating that the <inline-formula> <tex-math notation="LaTeX">${V}_{\textrm {th}}$ </tex-math></inline-formula> shift is varying along the load line during a switching operation.