We have observed a negative differential resistance (NDR) in a single-barrier tunneling structure in which electrons tunnel from a doped semiconductor emitter layer into a quantum well (QW) layer and subsequently drift laterally to a specially designed contact. Pronounced NDR is seen already at room temperature and at 77 K the peak to valley (PTV) ratio in current is more than 2:1. Our results lend support to a recent hypothesis by Luryi [Appl. Phys. Lett. 47, 490 (1985)] that the NDR in double-barrier tunneling structures is not related to a resonant enhancement of the tunneling probability at selected electron energies, but rather originates from tunneling into a system of electron states of reduced dimensionality. For comparison we have also fabricated a QW structure with two tunneling barriers, in which the parameters of the emitter barrier and the QW are identical to those in the single-barrier structure. In the double-barrier structure we have obtained current densities as high as 4×104 A/cm2 and a NDR with PTV ratios of 3:1 at 300 K and 9:1 at 77 K.
Extremely low alloyed and nonalloyed ohmic contact resistances have been formed on n-type InAs/In0.53Ga0.47As/In0.52Al0.48As structures grown on InP(Fe) by molecular-beam epitaxy. To insure the accuracy of the small contact resistances measured, an extended transmission line model was used to extrapolate contact resistances from test patterns with multiple gap spacings varying from 1 to 20 μm. For a 150-Å-thick InAs layer doped to 2×1018 cm−3 and a 0.1-μm-thick InGaAs layer doped to 1×1018 cm−3, a specific contact resistance of 2.6×10−8 Ω* cm2 was measured for the nonalloyed contact, while a resistance less than 1.7×10−8 Ω* cm2 is reported for the alloyed contact. Conventional Au-Ge/Ni/Au was used for the ohmic metal contact and alloying was performed at 500 °C for 50 s in flowing H2. Using a thermionic field emission model, the barrier height at the InAs/InGaAs interface was calculated to be 20 meV.
The base-collector junction of GaAs/AlGaAs single heterojunction bipolar transistors has been observed to emit light at avalanche breakdown. The spectral distribution curve exhibits broad peaks at 2.03 and 1.43 eV, with the intensities dependent upon the reverse current. These observations suggest that electrons, excited to the upper conduction band by the field, lose their energy by impact ionizing electron-hole pairs and producing the 2.03 eV light, which corresponds to the threshold energy for electron impact ionization. The band-edge emission is the result of direct-gap free-carrier recombination and self-absorption of the high-energy transition.
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