Semiconductor structures with high impedances for use in biasing for applying voltage bias to part of a device. The semiconductor structure comprises a continuous structure having a plurality of regions of a first semiconductor type (n type or p type) material arranged alternately with at least one region of the opposite type. The structure may be formed from polysilicon and may also include a plurality of intrinsic regions arranged between the n and p type regions. The structure forms a composite diode and provides a high impedance.