The device uses an. AlGaAsiGaAsiAlGaAs quantum well resonator as a hot-electron ... was 9.8 ns. This work was supported by the MITI's Project of Basic Tech-.
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-33, NO. 11, NOVEMBER 1986
VIB-2ResonantTunneling in aDoubleSuperlatticeBarrier Heterostructure*-Mark A.Reed, J . W.Lee,andH-L.Tsai, Central Research Laboratories, Texas Instruments, Incorporated, Dallas, TX 75265. The use of superlattices to emulate alloy material (e.g., a AlAsi GaAs superlattice in place of a A1,Gal -,As alloy) is referred to as “wave function engineering.” The use of these superlattice structures as quantum well barriers has recently been shown in a series of opticalmeasurements [l].However,theelectricaltransport properties of these structures, as well as transport through the confined quantum well states, has not been explored. We report investigations of resonant tunneling in double barrier heterostructures in which the tunnel barriers havebeen replaced by short period superlattices, and have shown for the first time transport through quantum well states bounded by superlattices. These results demonstrate the first utilization of short period binary superlattifes as effective tunnel barriers. These structures, consisting of 50-A GaAs quantup wells and 50;A GaAsiAlAs superlattice barriers (3 AMs, 10 Ai2 GaAs, 10 A), have exhibitedpeak-tovalley tunnel current ratios in the negative differential resistance (NDR) region of 2 : 1 at 300 K. The resonant peak position will be compared to predicted barrier heights froman averaged alloy composition model, an envelope function approximation, and a modified Kronig-Penney model. The superlattice structure doesnot exhibit the asymmetry around zero bias in the electrical characteristics normally observed in the conventional Al,Gal -,As barrier structures, suggestive of reduced roughness at the inverted interface by superlattice smoothing. The thermal activation of inelastic currents in the superlattice barrier structure will be compared to a conventional Al,Gal -,As barrier resonant tunneling structure. *Supported in part by ARO and ONR. [ l ] H . Sakaki, M. Tsuchiya, and J . Yoshino, Appl. Phys. Lett., vol. 47, p . 295, 1985.
VIB-3 A Bistable Multivibrator Using A Resonant-Tunneling Hot-Electron Transistor-N. Yokoyama, K . Imamura, H. Ohnishi, S . Muto, T . Mori, and S . Shibatomi, Fujitsu Limited,10-1, Morinosato-Wakamiya, Atsugi 243-01, Japan. This paper reports on a bistable multivibrator using two resistors and a three-terminal resonant-tunneling device. Preliminary tests have demonstrated that the multivibrator circuit can be used as a static memory element, and that the minimum trigger pulse width to interchange the states is less than 9.8 ns. Resonant-tunneling structures (quantum well resonators) are attracting much interest recently due to some intriguing characteristics, including high-speed charge transport and broadregions of negative resistance. In 1985, we proposed and fabricated a threeterminal resonant-tunneling device named RHET [l], which stands for resonant-tunneling hot-electron transistor. The device uses an AlGaAsiGaAsiAlGaAs quantum well resonator as a hot-electron injector. The main feature of RHET is that there is a peak collector current with respect to the base voltage, and therefore a negative transconductance. This paper demonstrates another attractive feature of this device, whichmay be usedinmemory and/or logic applications. The device used for this study was the same as that used in the Erior work [l]. The device has a-quantum well, consisting of 564 GaAs sandwiched by two 50-A A1,Gal -,As (x = 0.33), 1300A GaAs base, and 3000-A Al,Gal -,As (x = 0.20) collector barrier. Collector current was measured as a function of base current with a constant collector voltage of 2 V in the common emitter configuration.Withincrease in basecurrent,collectorcurrent monotonically increases.However,collectorcurrent rapidly increases at a base current of 0.85 mA. The differential current gain was measured as more than 2000. With decrease in base current, the collector current monotonically decreases and drops at a base current of 0.15 mA. Thus, there is a large hysteresis in collector
1865
current. This is considered to be due to presence. of negative resistance region in the base current withrespect to the base voltage. Bistable multivibrator circuits were built by connecting a 51-kQ load resistor to the collector and a 1.6-kQ resistor to the base. The voltage supplied to the load resistor was4 V in the common-emitter configuration. A trigger pulse generator was connected to the 1.6kQ resistor, with a dc offset voltage of 1 V. The output voltage (collectorvoltage) was monitored with an oscilloscope using an FET probe. With a positive trigger pulse, RHET goes to a conductive state, so that the output voltage is low at 1.2 V. With a negative trigger pulse, RHET goes to a poor conductive state, and hence, the output voltage is high at 2.0 V . These two states are maintained even after removal of these pulses. Preliminary tests showed that a minimum trigger pulse width to interchange the states was 9.8 ns. This work was supported by the MITI’s Project of Basic Technology for Future Industries. [l] N. Yokoyama et al., Japan. J. Appl. Phys., vol. 24, p. L853, Nov. 1985;also in Proc.12thInt.Symp. GaAs and Related Compounds (Japan), Sept. 1985.
VIB-4 Hot-Electron Transport in the Base of Heterojunction Bipolar Transistors-J. R.Hayes, Bell CommunicationsResearch, 600 Mountain Ave., Murray Hill, NJ, and A. F. J . Levi, J . H . English, and A. C. Gossard, AT&T Mountain Ave., Murray Hill, NJ.
Bell Laboratories, 600
In the last few years there has been considera.ble interest in the development of GaAsi(GaA1)As heterojunction bipolar transistors (HBT’s) for high-speed applications. It has been suggested that the performance of HBT’shaving abruptemitters isinfluenced by “ballistic” electron transport across the heavily doped p-type GaAs base. If true, this has a direct bearing on the ultimate high-speed performance of this device. To resolve thisissue we haveused “hot-electronspectroscopy”to obtain direct informationon the dynamics of injected hot-electron transport in the base. The device structure, used forthisstudy, was adouble HBThaving ahotelectroninjectorattheemitteranda compositionallygraded (A1Ga)As collector. The structure was designed such that the application of a forward bias between the collector andthe grounded base could adjust the graded heterojunction barrierenergy in a controlled way. By varying this barrier energy we were able to obtajn spectroscopic informationon the carrier distribution at the collector edge of the base region. A typical device used in thisstudyhadan (A1Ga)As emitter doped n-type to 2 X loi7~ m - a~GaAs , base doped p-type to 2 X 10I8 and an (A1Ga)As collector doped n-type to 2 X 10’’ cm-3 compositionally graded over ashort distance at the base edge so that “hot-electron spectroscopy” could be realized. Such transistors had current gains of 50 (100 A cm-*) at room temperature K where increasing to 500 at 100 K remaining constant down to 4.2 the measurementwas conducted.Sincehot-electron coolingis dominated by emission processes, operating the device at reduced temperature does not change the transport dyns.mics but only adds spectralresolution tothe measured distribution by suppressing thermal smearing effects. No evidence of the initial injected distribution was obtained in transistors having transit region widths greater than 800 A , there being only a broad thermal distribution charac:erized by an effective electron temperature greater than the lattice temperature.
VIB-5 InGaAdInAlAs HotElectronTransistor Operating at 77 K-U. K. Reddy, J . Chen, W. Kopp, C. K. F’eng, D. Mui, and H. MorkoC, University of Illinois at Urbana-Champaign, Coordinated Science Laboratory, 1101 W. Springfield Avenue, Urbana, IL 61801. The transport of hot electrons in thin semicmductor structures has recently received a great deal of attention because of the pos-
1866
IEEE VOL. DEVICES, ELECTRON TXANSACTIONS ON
ED-33, NO. 11, NOVEMBER 1986
sibility of collision free transport that canlead to ultrafast devi1:cs. effects of tunneling effective mass and barrier height. The conducWe have fabricated the first InyGal- ,As/In,All ...As hot-elec,tron tion band discontinuities derived from tunneling tracked those detransistor and obtained evidence for the existence of ballastic trc. ns- rived from thermionic emission, indicating that the same energy port in the InGaAs base layer at 77K as opposed to 4K solely used band is involved in both processes. The tunneling effective masses for GaAsiAlGaAs system. were low: = 0.07 and 0.12 for (A1,Ga)As barriers with 40- and 60Hot-electron transistors were fabricated usingi-In,Al, -,As :mr- percent AI. These results can nor be explainedby tunneling through riers and n'-In,Gal - ,As for the basewhich were grown by mo w - the longitudinal X valley since the effective mass thereis very large. u1ar;beam epitaxy on InP substrates. Electrons tunnelingthroui, 11 a It is likely that the transverse X valleys are involved in the tunnel75-A-thicki-InAlAs emitter barrier enter the n.+-InGaAs base 1.e- ing process along with some mechanism for momentum transfer. gion as hot electrons with energies close to the emitter-base !lias [l] J . Batey and S. L. Wright, J . Appl. Phys., vol. 59, p. 200, 1986. potential. Some of these hot electrons traverse the InGaAs base [ 2 ] F. Stem and S . Das Sarma, Phys. Rev. B , vol. 30, p. 840, 1984. plus the InAlAs collector barrier withoutanyscattering and are collected by the negatively biased collector. Using this negatil, ely biased collector as theelectron energy selector,wehave d e w mined the energy distribution of the hot electrons at 77K. In olcler VIB-7 ImprovedPhysics of OhmicContacts to Semiconducto determine the fraction of electrons ballistically traversing the and S . J . Eglash,** Integrated base and the collector barrier we measured the gain at co1lel:lor tors*-K. Shenai, R. W. Dutton, voltages correspondjng to the ballistic peakin output conductar
