Abstract:
A study on the spectral response of GaAlAs/GaAs heterojunction photodiodes has been performed by concentrating on an important role of the P+ top layer in defining the bandwidth of the spectral response of photodiodes. The energy band diagrams of the top layer, both of the isotype of P+ GaAlAs layers and the anisotype P+ - GaAlAa/n- - GaAs, are considered, and then the spectral response of a number of GaAlAs/GaAs heterojunction photodiodes with different top layers are theoretically calculated. Next, three different structures of GaAlAs/GaAs heterojunction photodiodes were designed and fabricated by Liquid Phase Epitaxy (LPE) Technology with the constant band gap absorbing layers of 1.674 eV and 1.924 eV and linear graded band gap of 1.924 to 1.55 eV. The constant band gap structures apparently respond to the incident photon whose energy is in between of the surface layer and the active layer owing to the window effect. As a result, the corresponding photocurrent occurs in the range between ~725 nm and ~875 nm and between ~655 nm and ~875 nm in Ga0.8Al0.2As and Ga0.6Al0.4As top layer photodiodes, respectively. On the contrary, although graded band gap GaAlAs top layer photodiode can absorb the light energy between 1.924 to 1.55 eV, the spectral response can be measured from ~650 nm to ~875 nm, similar to that of Ga0.6Al0.4As top layer photodiodes but its sensitivity is lower. By comparison with the theoretical calculation, this result can be explained by considering that the photoexcited carriers in absorbing layers are caused to drift through the junction by the built-in electric field. Therefore, by a suitable architectural design of the top layer, the phototdiode with optimal spectral response can be realized.
