Origin of the enhancement of negative differential resistance at low temperatures in double-barrier resonant tunneling structures

An explanation of the increased peak-to-valley (PTV) current ratio for double-barrier resonant tunneling structures (DBRTS's) operated at low temperatures is proposed. We found that this phenome- non is an inherent property of DBRTS's, not caused by the suppression of thermionic current over barriers. The energy distributions of electrons at different temperatures result in the variations of peak and valley currents. OUBLE-BARRIER resonant tunneling structures D (DBRTS's), demonstrated first by Chang et al. in 1974 ( 13, have attracted much attention recently. The structures can produce negative differential resistances (NDR's) and thus provide a wide variety of applications such as microwave oscillators (2), (3), frequency multipliers (4), and multivalued logics (5), (6). Structures grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have been reported with large peak-to-valley (PTV) current ratios even at room temperature (7)-(9). It has been shown that the NDR characteristics are more pronounced at low temperatures (7)-(18), but the reason is not very clear yet. The thermionic current over barriers (lo) and the scattering effects ( 171 have been considered to explain this phenomenon. In this paper, a different approach which utilizes different electron distributions at different temperatures is used to explain this phenomenon. It is found that this temperature dependence is essentially an inherent behavior of DBRTS's. The contributions from thermionic current and scattering effects are only secondary. Fig. l(a) shows a typical DBRTS. The device has a well width of L,, symmetrical double barriers each with width Lb, a barrier height Vo, and a Fermi level Ef above the conduction-band edge. The typical current-voltage character- istics at different temperatures are shown in Fig. le). Only one bias polarity is presented and the scale is not indicated. When the temperature is reduced from T2 to TI, the peak