Broken symmetry and the formation of hot-electron domains in real-space-transfer transistors

Abstract

Real-space-transfer (RST) transistors are studied theoretically. In a symmetric configuration, with no voltage applied between the channel electrodes, we find anomalous steady states in which the RST is driven by the fringing field from the collector electrode. Some of these states are unconditionally stable and hence accessible experimentally. Our study elucidates the formation of hot-electron domains, which is shown to be a discontinuous process that passes the control of electron heating from the drain to the collector. Multiply connected current-voltage characteristics are predicted. PACS numbers: 73.50.Fq, 73.40.Kp, 73.50.Lw The concept of real-space transfer (RST) refers to a process in which electrons in a narrow semiconductor layer are heated by a parallel field and spill over an energy barrier into adjacent layers [1,2]. A number of highspeed electronic and optoelectronic devices have been proposed based on this principle [3][4][5][6][7]. The generic RST transistor [8] is a three-terminal device (see the inset to Fig. 1) consisting of a channel with two contacts, S and Z>, and an individually contacted collector C, separated from the channel by a heterojunction barrier. In the normal operation, the channel electrons are heated by the lateral field due to a voltage V D applied between S and D. The resultant RST leads to highly nonlinear effects, including a strong negative differential resistance (NDR) with sharp steps, indicative of an internal switching and the formation of high-field domains [4,9,10]. These processes are not well understood, even though RST transistors have been extensively studied, both experimentally and theoretically [11].The RST transistor is symmetric with respect to reflections in the midplane normal to channel. Hence states of the device at an external bias [Vo.Vc] are related to those at [-Vo,(Vc~ VO)]-In particular, states at Vo -0 must either be symmetric or possess brokensymmetry partners. An important discovery of the present work is the existence of a number of such states, some of which are not only stationary but also stable with respect to small perturbations. In these "anomalous" states [12], the electron heating is due to the fringing field from the collector electrode. Our study shows that the formation of hot-electron domains at Vo > 0 represents a transition to a collector-controlled state that is continuously related to one of the anomalous states at V D =0.The tool of our study is a numerical simulation of the hot-electron transport in a three-terminal RST structure. Details of the program and computational methods are described elsewhere [13,14]. The analysis is based on the solution of a set of coupled partial differential equations, including Poisson's equation and expressions [15] for current continuity and energy balance, in a two-dimensional sample, subject to boundary conditions at the three electrodes. Models for the local hot-electron mobility n(T e ) and the energy relaxation time r E (T e ) are chosen so that in a uniform electric field F, one obtai...

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