Solid-State Electronics Vol. 29 , No . 12, pp. 1207-1211 , 1986 Printed in Great Britain 0038-110 I/86 $3.00 + 0.00 Pergamon Journals Ltd HIGH-FREQUENCY CURRENT OSCILLATIONS IN SUBMICRON BIPOLAR STRUCTURES N. BANNOV, V. GRUZINSKIS, A. REKLAITIS and V. RYZHIJ Semiconductor Physics Institute of the Lithuanian Academy of Sciences, Vilnius, U.S.S.R. (Received 21 February 1985; in revised form 18 November 1985) Abstract- The current oscillations with frequency of the order of plasma frequency in the submicron GaAs bipolar diodes are obtained by Monte Carlo simulation. It is shown that current oscillations are the result of carrier bunching, which occurs due to either the two-stream instability or optical-phonon emission. 1. INTRODUCTION The rapid progress in the technology of fabricating submicron size semiconductor structures stimulated the charge-carrier dynamics investigations in such structures. A number of theoretical (see Refs[I-7] also review Ref.[8]) and experimental(9-13] .works show that the charge dynamic in sufficiently thin and perfect semiconductor layers can qualitatively differ from that in the bulk semiconductors. The low efficiency of the carrier scattering in the layer has the decisive influence on the charge dynamics in the thin layers, because the transit time through the layer can be of the same order or even less than the mean time between the scattering events. The result of this is the experimentally observed[13] high velocity of carriers in GaAs n +-i-n + structures, several times exceed- ing the maximum drift velocity of electrons in this material. Moreover, as is shown by Monte Carlo calculations[l4] the charge-carrier velocity distribu- tion function in the thin layers is dependent on the coordinate and in some coordinate regions it has several distinctive peaks. A number of papers[I5- 24] are devoted to the calculation and analysis of the diodes and transistors operating in the ballistic (noncollisional transport) or near-ballistic (a small number of collisions during the flight through the active layer) regimes. However, the majority of authors don 't take into account the charge-carrier velocity distribution function pecu- liarities. Namely, these function peculiarities cause a number of qualitatively new high-frequency current instabilities in the submicron semiconductor structures[6, 7, 21-24]. Thus, in Refs[21 , 22] it is shown that in the symmetrical n + -n -n + structure at the steady voltage in the ballistic transport con- ditions the high-frequency current pulsation can arise. The current oscillations in monopolar diode structures can occur also when the optical-phonon spontaneous emission is the dominating scattering mechanism[6, 23 , 24]. The instability of the opposite motion of the electrons and holes in the bipolar diodes is shown in Refs[7, 23] . The presence of a hole-plasma component in bipolar structures leads to more strong current instabilities than in mono- polar ones. Therefore, one can expect the bipolar structures to be more efficient for high-frequency current generation than the monopolar structures. In this paper the results of the numerical simu- lation of current instabilities in bipolar semiconduc- tor diodes at a steady applied voltage are presented. 2. STRUCTURES AND CALCULATION PROCEDURE We shall ):onsider layered GaAs structures consis- ting of two high-doped n +- and p +-type layers (con- tacts) with low-doped layer (basis) between them. It is assumed that the contact regions are quasineutral and the whole external voltage cP drops on the basis layer. This assumption is valid when the doping density in contact layers is high enough. The current in the basis is assumed to be considerably lower than the contact saturation current, i.e. the contacts oper- ate in the regime of current, limited by space charge. The current flow through the structure at these conditions is fully determined by the processes in the basis region. The modeling of carrier dynamics in such structures is performed by numerical solution of a Boltzmann and Poisson equation set. Let us discuss the structure parameters and the GaAs model used in the carrier dynamics simulation. The length L of the basis, the lattice temperature T and the external voltage drop cP on the basis are the parameters which determined the model choice. The limits of the parameter values used in the calculations are: L 0.7-lllm; T 4.2-15 K; and cP 43-65 mV. At such short basis lengths L and low temperatures T the carrier energy in the basis cannot considerably exceed the value of ecP (where e is the electron charge). Therefore, the upper valleys for electrons and the split-off band for holes as well as nonparabolicity of bands can be ignored. The evaluation of the scatter- ing probabilities by possible scattering mechanisms in S.S.E. 29 12-A 1207