We report a new quantum-type photoconductivity in a forward-biased p + -n junction with a superlattice in the n layer. This novel phenomenon is characterized by several striking features: a high photocurrent gain ( -7 x 10 3 ), accompanied by a blue shift in the spectral response and a reversal in the direction of the photocurrent, when the forward bias exceeds the built-in potential. Photoconductive gain is caused by the large difference in the tunneling rates of electrons and holes through the superlattice layers, due to their large mass difference (effective-mass filtering). This is the first time that photoconductive gain is observed in a p-n junction.PACS 73.40.Lq Photoconductivity is a widespread phenomenon in semiconductors. If the lifetime of the photogenerated carriers exceeds their transit time, photoconductivity is accompanied by current gain. 1 The gain and associated gain-bandwidth product are therefore controlled by bulk properties such as carrier mobilities and lifetimes. In this Letter we report a new photoconductivity of quantum mechanical origin, accompanied by a very large photocurrent amplification, in a forward-biased superlattice p-n junction.The heterostructure, grown by molecular-beam epitaxy, consists of a p + -n junction grown on a 1-^m-thick n + ( = 2x 10 18 cm" 3 ) Gao.47Ino.53As buffer layer, lattice matched to a (100) n + InP substrate. The undoped n layer (n = 1 x 10 15 -1 x 10 16 cm" 3 ) comprises 100 23-A-thick Al 0 48 In 0 .52As barriers alternated with 100 49-A-thick Gao.47Ino.53As wells. Transmissionelectron-microscopy studies indicate that the interfaces are abrupt to better than two monolayers. The top p + region is 1 /xm thick and doped to -2xl0 18 cm" 3 . The mesa diodes (area = 1.3xl0~4 cm 2 ) had excellent forward and reverse I-V characteristics. From capacitance-voltage measurements the carrier concentration (rt-type) in the superlattice region was found to decrease monotonically from 2xl0 16 /cm 3 (at a distance of 0.2 /xm from the p + -n heterointerface) to 4 xl0 14 /cm 3 near the interface with the n + layer. The built-in potential determined from both C-Kand I-V measurements is 0.65 V. The optical power incident on the detector was kept low ( -1 nW); the shortcircuit photocurrent was measured by a 181 PAR current-sensitive preamplifier followed by a 5604 PAR lockin. Figure 1 shows the responsivity (photocurrent/ divided by incident optical power) at 70 K as a function of forward bias voltage at a wavelength of 1.2 /xm and a chopping frequency of 1 kHz. The responsivity exhibits a relatively flat portion at low bias and then a rapid (roughly exponential) falloff at voltages greater than 0.4 V. This represents the standard behavior of the photocollection efficiency in forward-biased p-n junctions. 2 The responsivity reaches a minimum at a forward bias equal to the built-in potential (0.65 V). Above this voltage the responsivity takes off rapidly and increases by about five orders of magnitude within 0.2 V, indicating the presence of a current gain mechanism. The hi...