A device comprising: a first layer comprising a group III-nitride first conductivity type semiconductor substrate or template; a second group III-nitride semiconductor layer disposed over the group III-nitride semiconductor substrate or template; a third group III-nitride semiconductor layer disposed over the second group III-nitride semiconductor layer; a fourth group III-nitride semiconductor layer disposed over the third group III-nitride semiconductor layer; a pn junction at an interface between the third and fourth group III-nitride semiconductor layers (103, 104); and a polarization hetero-junction between the second group III-nitride semiconductor layer and the third group III-nitride semiconductor layer, the polarization junction having fixed charges of a polarity on one side of the polarization junction and fixed charges of an opposite polarity on an opposite side of the polarization junction; wherein, when unbiased, the pn junction comprises a first electric field that opposes the flow of carriers across the pn junction and the polarization junction comprises a second electric field that opposes the flow of oppositely charged carriers across the polarization junction; wherein the first to fourth layers are disposed between a first and a second contact electrode, the first contact electrode being disposed over the fourth group III-nitride semiconductor layer and the second contact electrode being disposed below the first layer; and wherein; the third group III-nitride semiconductor layer is a first conductivity type GaN layer; the fourth group III-nitride semiconductor layer is a second conductivity type GaN layer; and either: the second group III-nitride semiconductor layer is a first conductivity type InAIN layer wherein the InAIN layer is an In x Al 1-x N layer, wherein x is between 0.10 and 0.25; or the second group III-nitride semiconductor layer is a first conductivity type AlGaN layer wherein the AlGaN layer is an Al y Ga 1-y N layer, wherein y is greater than 0.40. a first layer comprising a group III-nitride first conductivity type semiconductor substrate or template; a second group III-nitride semiconductor layer disposed over the group III-nitride semiconductor substrate or template; a third group III-nitride semiconductor layer disposed over the second group III-nitride semiconductor layer; a fourth group III-nitride semiconductor layer disposed over the third group III-nitride semiconductor layer; a pn junction at an interface between the third and fourth group III-nitride semiconductor layers (103, 104); and a polarization hetero-junction between the second group III-nitride semiconductor layer and the third group III-nitride semiconductor layer, the polarization junction having fixed charges of a polarity on one side of the polarization junction and fixed charges of an opposite polarity on an opposite side of the polarization junction; wherein, when unbiased, the pn junction comprises a first electric field that opposes the flow of carriers across the pn junction and the polarization junction comprises a second electric field that opposes the flow of oppositely charged carriers across the polarization junction; wherein the first to fourth layers are disposed between a first and a second contact electrode, the first contact electrode being disposed over the fourth group III-nitride semiconductor layer and the second contact electrode being disposed below the first layer; and wherein; the third group III-nitride semiconductor layer is a first conductivity type GaN layer; the fourth group III-nitride semiconductor layer is a second conductivity type GaN layer; and either: the second group III-nitride semiconductor layer is a first conductivity type InAIN layer wherein the InAIN layer is an In x Al 1-x N layer, wherein x is between 0.10 and 0.25; or the second group III-nitride semiconductor layer is a first conductivity type AlGaN layer wherein the AlGaN layer is an Al y Ga 1-y N layer, wherein y is greater than 0.40. the second group III-nitride semiconductor layer is a first conductivity type InAIN layer wherein the InAIN layer is an In x Al 1-x N layer, wherein x is between 0.10 and 0.25; or the second group III-nitride semiconductor layer is a first conductivity type AlGaN layer wherein the AlGaN layer is an Al y Ga 1-y N layer, wherein y is greater than 0.40.