A New Triple-Well Resonant Tunneling Diode with Controllable Double-Negative Resistance

A newly structured triple-well resonant tunneling diode (RTD) is proposed as a triple-valued logic device. The superiority of this new RTD for triple-valued logic application i s demonstrated with numerical simulation based on the transfer matrix method. This new RTD exhibits significant double-negative differential resistance, and the two current peak voltages are independently controlled with well thicknesses. Comparing the numerical calculation and experimental data, it is shown that agreement between the calculation and experiment on the two current peak voltages is excellent. I . INTRODUCTION ECENTLY, resonant tunneling in multibarrier hetR erostructures [ 13, [2] has attracted considerable interest, and several resonant tunneling diodes (RTD) [3]-[lo] and transistors [11]-[15] have been proposed as functional devices. One of the most hopeful applications of their negative differential resistance (NDR) is multiplevalued logic, which can greatly reduce the number of devices needed for one function [16]. For this application, multiple NDR where all peak currents are nearly equal is necessary. The simplest structure for an RTD with multiple NDR is a double-barrier heterostructure in which resonant levels are created in one quantum well. In this structure, however, the peak currents due to upper resonant levels are significantly larger than the peak current due to the lowest resonant level. This is caused by excess current whose origin is not yet determined. Thus, it is difficult to obtain the required multiple NDR by using the double-barrier structure. One certain method to avoid this difficulty is to arrange two double-barrier RTD's in parallel, where the NDR characteristics of the two diodes can be controlled with external bias [17]. This circuit, howtriple-well RTD, where the required double NDR can be realized, and the two current peak voltages are independently controllable with well thicknesses. The method of our numerical calculations is shown in Section 11. In Section 111, the operation of the triple-well RTD as a triple-valued logic device is demonstrated. Comparison between the calculations and experiments is discussed in Section IV. 11. FORMULATION In this section, we show the method of our calculation to analyze the resonant tunneling in multibarrier heterostructures. Our theoretical approach is based on the transfer matrix method [ 181, where the potential distribution of a resonant tunneling barrier (RTB) is approximated by a series of small steps as shown in Fig. 1. The wave function \kj ( z ) in the ith section is given in a plane wave form as \ k I ( z ) = A, exp (ik,z) + B, exp ( i k , z ) where k, is the complex wavenumber. Continuity of wave function and probability flux are required at all boundaries. When incident electrons come from the left side of RTB, the wave functions \kL and \kR on both sides of RTB are given by the following expressjons: \k,(z) = A, exp ( ikLz) + B, exp ( -ik,z) , z C zo \ ~ R ( z ) = AR exp ( i k ~ z ) , z > ZN and coefficients AL, BL, and AR are connected by transfer matrix TI as follows: a? exp ( i ( k j k i l ) z i l ) a,: exp ( i ( k i k i l ) z i l ) a ; exp ( i ( k , + k , l ) z , _ l ) CY: exp ( i ( k i + I C , ~ ) ~ ~ , ) K = ever, is more complex than that which uses a single diode. In this paper, we propose a new RTD structure, i.e., the where m: is the effective electron mass in the ith section. The transmission probability T through RTB is given as follows: I A R l 2 Manuscript received May 11, 1988; revised June 28, 1988. The authors are with the Central Research Laboratory, Hitachi, Ltd., T = g . ! ? ! . k L JA,12 Kokubunji, Tokyo 185, Japan. IEEE Log Number 8823793. 0018-9383/88/1100-1951$01.00