Integrated Photoreceivers For Long Wavelength Communications Systems - Is The Technology Ready?

summary of some results obtained for InGaAs FET's with different gate technologies, is givenin Table 1. 116 To expand on this second point we show in Fig. 2 the variation with composition of both low field electron mobility1 ^ and peak electron drift velocity13 for the InGaAsP alloy system. The ternary alloy can be seen to have the highest low field mobility, in excess of 12,000 cm2/Vs for lightly doped material (in excess of 8000 cm2/Vs for electron concentra­ tions in the 0.5 - 1.Oxlol7cm~3 range). This material also exhibits the highest peak electron drift velocity, vp ~ 2.4x10? cm/s. Both these characteristics contribute to making InGaAs attractive for FET applications. In fact on the basis of these transport characteristics alone, InGaAs compares very favorably to other semiconductors currently being investigated for high performance FET's as illustrated in Fig. 3 where the electron drift velocity characteristics for InGaAs, GaAs, InP and Si are compared.Discrete components for photo receiver's A. PhotodiodesLow capacitance, (<1.0pf) low dark current (<10nA) photodiodes fabricated from InGaAs have already become the standard for use in long wavelength hybrid pin/FET photoreceivers. The achievement of dark currents in the 1-10 nA range requires careful fabrication starting with good quality, lightly doped material. Further, lightly doped material is essential for low diode capacitance as illustrated in Fig. 4 where we show the variation of diode deple­ tion depth with reverse bias for InGaAs material of different carrier concentration.For a diode of given dimension, the depletion depth increases with increasing applied voltage up to a voltage at which the junction breaks down. The variation of breakdown voltage VB with carrier concentration is indicated by the dashed boundary on the right side of the figure. Diode dark current is found to increase rapidly as junction breakdown is approached, due in the case of InGaAs to the rapid increase in tunneling currents. Conse­ quently, operation at low dark current requires that the reverse bias voltage be kept well below breakdown.For a junction area compatible for use with fibers, A = 1.0x10