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

The drain induced dynamic threshold voltage (
${V}_{\textrm {th}}$
) shift of a
${p}$
-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
${V}_{\textrm {DSQ}}$
, the gate-to-drain capacitance (
${C}_{\textrm {GD}}$
) is charged to
${Q}_{\textrm {GD}}$
(
${V}_{\textrm {DSQ}}$
). As the drain voltage drops to
${V}_{\textrm {DSM}}$
where
${V}_{\textrm {th}}$
is measured,
${C}_{\textrm {GD}}$
is expected to be discharged to
${Q}_{\textrm {GD}}$
(
${V}_{\textrm {DSM}}$
). However, the metal/
${p}$
-GaN Schottky junction could block the discharging current, resulting in storage of negative charges in the
${p}$
-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
${V}_{\textrm {th}}$
. The dynamic
${V}_{\textrm {th}}$
shift is an intrinsic and predictable characteristic of the
${p}$
-GaN gate HEMT which is linearly correlated with
$\Delta \!{Q}_{\textrm {GD}}={Q}_{\textrm {GD}}$
(
${V}_{\textrm {DSQ}}$
)
$- {Q}_{\textrm {GD}}$
(
${V}_{\textrm {DSM}}$
). The
${V}_{\textrm {th}}$
shift is dependent on
${V}_{\textrm {DSQ}}$
as well as
${V}_{\textrm {DSM}}$
, indicating that the
${V}_{\textrm {th}}$
shift is varying along the load line during a switching operation.