Fig. 1. Schematic structures for (a) Device R and (b) Device N. (c) Top view of GaN-based MSM PD showing interdigital electrodes. Schematic energy band diagrams of (d) Device R and (e) Device N.

Fig. 2. (a) Dark current and photocurrent in terms of the applied bias for Devices R and N. Photocurrent for (b) Device R and (c) Device N in terms of different laser powers. (d) Two-dimensional electric field profiles and (e) current distributions for Devices R and N at the applied bias of 3 V under the 266 nm UV illumination. The bias is applied at the left electrode, such that the positive bias is biased at the left electrode in the first quadrant and the negative bias is biased at the left electrode in the second quadrant. The other electrode is grounded when the devices are measured. The wavelength for the illumination laser is 266 nm. The positive direction for the electric field is defined to point to the right side in (d).

Fig. 3. (a) Spectral responsivity, (b) and (c) energy band diagrams near the GaN layer surface, and (d) two-dimensional electric field profiles for Devices R and N at the applied bias of 0 V under 266 nm UV illumination. Ec, Ev, Efe, and Efh denote energies of the conduction band, the valence band, and quasi-Fermi levels for electrons and holes, respectively. The positive direction for the electric field is defined to point to the right side in (d).

Fig. 4. Spectral responsivity for (a) Device R and (b) Device N at different applied biases.

Fig. 5. Time-dependent photo-response characteristics for (a) Device R and (b) Device N when devices are biased to 3 V. The laser wavelength is 266 nm.

Table 1. Absorption Coefficient of GaN Under Different Wavelengths

Table 2. Performances for the Reported GaN-Based and AlGaN-Based PDs in the Literature