ULTRAFAST SEMIINSULATING INP-FE-INGAAS-FE-INP-FE MSM PHOTODETECTORS - MODELING AND PERFORMANCE

The limits of the high-speed and frequency response of lateral InGaAs MSM photodetectors are theoretically explored. The results are experimentally verified. The impulse response is modeled by calculating the time-dependent sweep-out of photocarriers making proper use of the generalized Ramo theorem. The inherent parasitics and the two-dimensional character of the problem are fully taken into account. The calculations clearly yield the distinct contributions of electrons and holes to the response due to different field dependence of their drift velocities. For a typical geometry of 1 mum wide fingers separated by 1 mum, a bias voltage of 10 V, and illumination with 1.3 mum light, a FWHM of the impulse response of 17 ps is obtained. The comparatively slower motion of holes is reflected by a small tail of the impulse response which limits the electrical -3 dB bandwidth to 10 GHz at 10 V bias. The effect of parasitics of the detector chip is treated for the first time by a full-wave analysis yielding the frequency dependent S11-parameters. The results demonstrate that the frequency response of unmounted devices is free from degradation due to parasitics up to 50 GHz. Our fabricated detectors are based upon Fe-doped semiinsulating epitaxial InGaAs and InP layers. The photoactive material InGaAs:Fe is capped by InP:Fe to enhance the Schottky barrier height. The dark current is about 30 nA at 10 V for devices with a polyimide passivated mesa. Response measurements at 0.62 and 1.3 mum wavelength yield a FWHM of 13 and 17 ps, respectively. To our knowledge, these are the best values reported so far for InGaAs MSM photodetectors. For the first time a heterodyne technique at 1.3 mum wavelength is utilized to characterize InGaAs MSM devices. The measurements show a -3 dB +/- 0.5 dB rolloff at 13 GHz. Our experimental results are thus in excellent agreement with the theoretical predictions.