Electron transport and shot noise in ultrashort single-barrier semiconductor heterostructures V. Ya. Aleshkin Institute for Physics of Microstructures, Nizhny Novgorod GSP-105, 603600, Russia L. Reggiani Dipartimento di Ingegneria dell’ Innovazione and Istituto Nazionale di Fisica della Materia Universita ` di Lecce, Via Arnesano s/n, 73100 Lecce, Italy A. Reklaitis Semiconductor Physics Institute, Goshtauto 11, 2600 Vilnius, Lithuania Received 19 April 2000; published 31 January 2001 We present a theoretical study of electron transport and shot noise in ultrashort single barrier semiconductor structures. Calculations are applied to a simple GaAs semiconductor model in the presence of ballistic and thermalized carriers. The coupling between space charge and the dependence of the transmission coefficient on energy is found to provide the positive feedback that enhances shot noise and ultimately leads to a current instability of S type. When the strength of this feedback is weak, shot-noise suppression is observed. The occurrence of enhanced shot noise is explained in terms of a negative lifetime related to carrier escape through the collector contact. The model also predicts shot-noise enhancement in single barrier structures with constant transparency in the region of current saturation. Theoretical results are in qualitative agreement with existing current-voltage experiments and confirm recent Monte Carlo simulations evidencing shot-noise enhancement in GaAs/AlGaAs semiconductor heterostructures. Shot-noise enhancement is found to be a precursor indicator that the device is approaching an instability regime in analogy with the case of phase transitions. DOI: 10.1103/PhysRevB.63.085302 PACS numbers: 72.70.+m, 72.20.-i, 73.23.Ad I. INTRODUCTION Semiconductor heterostructures are very interesting non- linear devices showing several kind of electrical instabilities if a sufficiently high bias is applied. In particular they exhibit S-shaped current voltage ( I -V ) characteristics that can be conveniently used for fast switching, microwave generators, and amplifying devices. 1,2 The single barrier heterostructure is the prototype of these devices, and as such received great interest since it appeared in as heterostructure hot electron diode. 3–10 The microscopic mechanisms responsible for the I -V characteristics have been associated with the tunneling and thermionic regimes that control the transport at different applied voltages. Accordingly, several theoretical modelings, mostly based on detailed Monte Carlo simulations, have been applied to understand and predict the salient features of the transport properties of these devices. 5–8,11,10 Despite this interest in the I -V characteristics, the study of the noise prop- erty of these structures has received some attention only re- cently in the context of the problem of shot-noise suppres- sion and/or enhancement. 12–15 It is very interesting and useful to study noise in switching devices, because noise is the same tool, which indicates how far the system is from the instability region. Since the transport properties of these de- vices are controlled by carrier number, shot noise is the pri- mary noise source of interest. Shot noise is the electrical fluctuation due to discreteness of the charge that provides direct information on the corre- lation of different current pulses and as such is receiving increased attention from the scientific community. A conve- nient analysis of shot noise is usually performed by introduc- ing the dimensionless Fano factor  0 defined as  =S I (0)/(2 qI ), S I (0) being the spectral density of current fluctuations at low frequency, I the current flowing in the device, and q the elementary quantum of charge determining I. In the absence of correlation between current pulses it is  =1, and this case corresponds to full shot noise. Devia- tions from this ideal case is a signature of existing correla- tions between different pulses and the two possibilities of suppressed i.e.,  1) and enhanced i.e.,  1) shot noise are in principle possible. Shot-noise suppression is associated with a negative cor- relation between current pulses as due to Coulomb interac- tion and Pauli principle, and has been theoretically predicted and experimentally evidenced in a variety of electron devices 16 and mesoscopic structures. 17–26 Shot-noise en- hancement is associated with a positive correlation between current pulses and has been experimentally evidenced in double barrier resonant diodes. 27–30 Theoretical models based on the existence of a negative differential conductivity NDC region in the I -V characteristic has been also pro- posed for its explanation. 29,31 Recently, shot-noise enhance- ment has been observed in Monte Carlo calculations of a single barrier GaAs/AlGaAs structure when the distance be- tween the emitter and the barrier is comparable with the scat- tering length. 15 The positive feedback between tunneling and space charge has been proposed as the mechanism respon- sible for the positive correlation between current pulses. However, a quantitative theoretical basis supporting this in- dication is still lacking. PHYSICAL REVIEW B, VOLUME 63, 085302 0163-1829/2001/638/08530211/$15.00 ©2001 The American Physical Society 63 085302-1