A method of manufacturing a bipolar transistor, comprising: forming an isolation region in a substrate, said isolation region being adjacent to an active region of said bipolar transistor, said isolation region having a first side wall adjacent to the active region, a second side wall, opposite the first side wall, adjacent to the collector terminal of the bipolar transistor and a base, adjacent to a portion of the collector region of the bipolar transistor; and forming a dielectric on the first and second side walls of said isolation region and the base of said isolation region, the dielectric being formed to have a first thickness (W1) on the first side wall, a second thickness on the base (W2) and a third thickness (W3) on the second sidewall wherein the first thickness is thin relative to the third thicknesses; and said third thickness is thick relative to the second and first thicknesses, the method further comprising: depositing a semiconductor material in said isolation region so as to form a field plate, wherein said field plate adjoins the dielectric formed on the first and second side walls and on the base of said isolation region and is adjacent to a base-collector junction of said active region ; wherein said dielectric defines a gate dielectric, wherein said gate dielectric and said semiconductor material define a gate terminal of said bipolar transistor, the method being characterised in that the first thickness (W1) is thin relative to the second thickness (W2) and in that said gate terminal is arranged to be biased independently of a collector, base or emitter of said bipolar transistor. forming an isolation region in a substrate, said isolation region being adjacent to an active region of said bipolar transistor, said isolation region having a first side wall adjacent to the active region, a second side wall, opposite the first side wall, adjacent to the collector terminal of the bipolar transistor and a base, adjacent to a portion of the collector region of the bipolar transistor; and forming a dielectric on the first and second side walls of said isolation region and the base of said isolation region, the dielectric being formed to have a first thickness (W1) on the first side wall, a second thickness on the base (W2) and a third thickness (W3) on the second sidewall wherein the first thickness is thin relative to the third thicknesses; and said third thickness is thick relative to the second and first thicknesses, the method further comprising: depositing a semiconductor material in said isolation region so as to form a field plate, wherein said field plate adjoins the dielectric formed on the first and second side walls and on the base of said isolation region and is adjacent to a base-collector junction of said active region ; wherein said dielectric defines a gate dielectric, wherein said gate dielectric and said semiconductor material define a gate terminal of said bipolar transistor, the method being characterised in that the first thickness (W1) is thin relative to the second thickness (W2) and in that said gate terminal is arranged to be biased independently of a collector, base or emitter of said bipolar transistor. <EOS>