A. Kastalsky

Hot‐electron memory effect in double‐layered heterostructures

We studied a memory effect due to the real‐space hot‐electron transfer between two conducting GaAs layers separated by a graded AlGaAs barrier. Application of a lateral electric field to one of the layers enhances its electron temperature and leads to a charge injection into the other layer which is kept floating. As the heating voltage is removed, the injected electrons remain in the second layer, giving rise (at low temperatures, 77 K) to a persistent potential difference. Kinetics of the charge‐up process are studied at different heating voltages.

Negative-resistance field-effect transistor grown by organometallic chemical vapor deposition

Abstract Results of studying the process of real space hot electron transfer in the Negative-Resistance Field-Effect Transistor (NERFET) grown by organometallic chemical vapor deposition (OMCVD) are presented. In the modified AlGaAs/GaAs heterostructure we obtain a significant reduction in the parasitic leakage and a drastic improvement of negative differential resistance (NDR) in the drain circuit—with the peak-to-valley current ratio as high as 160 at room temperature. The effect of NDR is proved to be due to real-space transfer of hot electrons from the high mobility channel into the second conducting layer, with the current of transferred electrons depending exponentially on the second conducting layer potential. Analysis shows that the same device can operate as a Charge-Injection Transistor with an available power gain of about 5.

Microwave generation in NERFET

Microwave generation in a NEgative Resistance Field-Effect Transistor (NERFET) is reported for the first time. This device is based on a GaAs/AlGaAs heterostructure which exhibits negative differential resistance due to a transfer of hot-electrons out of a source-drain channel and into a conducting substrate. In an untuned microwave circuit at 77 K, the NERFET was found to generate wide-band noise at frequencies up to 2.3 GHz. In a tunable resonant circuit, stable microwave oscillations were observed at frequencies as high as 1.45 GHz. While further experiments are needed to determine the performance limits of the NERFET, the preliminary results presented here demonstrate the potential of this new device as a high-frequency element.

Anomalous photomagnetoresistance effect in modulation‐doped AlGaAs/GaAs heterostructures

A novel negative photoresistance effect at Al0.3Ga0.7As/GaAs interface in the presence of a magnetic field B is discovered and explained. At low temperatures (T=4.2 K) illumination of the sample leads to a persistent electron accumulation in the GaAs channel (the well‐known persistent photoconductivity effect). In the presence of B≳0.3 T the dependence of the longitudinal resistance (as measured by the four‐probe method) shows an anomalous behavior in that the resistance increases sharply with the increasing concentration n of carriers provided by light. In the same range of concentrations the longitudinal resistance at fixed n is proportional to B2. It is shown that the observed behavior of the resistance is associated with photoexcitation of electrons from donor vacancy (DX) centers in the highly doped AlGaAs region resulting in the creation of a second conducting layer of high charge density and low mobility.

New features of real-space hot-electron transfer in the NERFET

Abstract We report two new effects of negative differential resistance in the modulation doped AlGaAs/GaAs Negative Resistance Field Effect Transistor. Both effects are well controlled by a third electrode. Presence of a top n+ AlGaAs layer causes a hot-electron real-space transfer to this layer and prevents the electron transfer to the collector. Creation of the high-field domain in the source-drain channel is needed to activate the the hot-electron injection to the collector. The real-space transfer to the top n+ AlGaAs layer combined with the quenching of the high-field domain by the collector bias are responsible for the appearance of the observed new negative differential resistance effects.

Switching in NERFET circuits

NERFET is a three-terminal device possessing a pronounced negative differential resistance due to the effect of real-space hot-electron transfer. The device structure, substantially modified compared to our earlier reports, gives an improved performance at room temperature. In this work, logic operation of a circuit formed by two NERFETs is demonstrated for the first time. Sharp switching of output voltage is observed. Using this circuit, we have demonstrated the operation of an inverter and a bistable switch. Under certain bias configurations the circuit also exhibits a tristable behavior which can be used for ternary logic.

Degradation‐free modulation‐doped field‐effect transistors grown by organometallic chemical vapor deposition

Using AlGaAs and GaAs grown by organometallic chemical vapor deposition (OMCVD), we have fabricated high‐transconductance modulation‐doped field‐effect transistors which do not exhibit the severe electrical degradation in the dark at 77 K that is ordinarily observed in such devices. Such degradation has been attributed to defects in the n+‐AlGaAs. Experiments on ohmic test devices using different OMCVD grown structures suggest that the improvement is due to a drastically reduced charge polarization in the n+‐AlGaAs layer.

Unipolar transistor based on charge injection

W e d e s c r i b e a new type of t r a n s i s t o r b a s e d on h o t-e l e c t r o n t r a n s f e r between two conducting layers in a GaAlAs/GaAs h e t e r o j u n c t i o n s t r u c t u r e. One of t h e s e l a y e r s i s t h e FET channel. The o t h e r l a y e r (implemented a s a h e a v i l y doped GaAs sub-s t r a t e) i s separated from the channel by an AlXGal-,As graded barrier, with x varying from 0.34 near the channel t o 0. 1 n e a r t h e s u b s t r a t e. Because of t h e b a r r i e r asymmetry, a p p l i c a t i o n of a p o s i t i v e b i a s t o t h e s u b s t r a t e r e s u l t s i n a low s a t u r a t e d c u r r e n t. A p p l i c a t i o n of a source-to-drain field leads to a heating of channel electrons and an exponential enhancement of c h a r g e t r a n s f e r i n t o t h e s u b s t r a t e. T h i s s i t u a t i o n i s a n a l o g o u s t o t h a t i n a vacuum diode. In o u r case t h e s u b s t r a t e s e r v e s a s a n a n o d e and t h e FET channel represents a h o t-e l e c t r o n cathode, whose e f f e c t i v e t …

Current-controlled hot-electron instability in the four-terminal heterostructure device

Abstract We demonstrate new effects of the current-controlled (S-shaped) negative differential resistance (NDR) in different circuit configurations of four-terminal modulation-doped AIGaAs/GaAs heterostructure devices, with the magnitude of the voltage drop varied by external electrodes. We observed two S-shaped instabilities having different physical origins. One effect arises due to an avalanche in the top AIGaAs layer underneath the reverse-based gate region. Another phenomenon relies on the creation, during the avalanche, of nonequilibrium hot electrons at the conducting channel (quantum well). This dramatically enhances electronic flow over the heterostructure barriers and gives rise to a strongly pronounced current-controlled NDR with a peak-to-valley ratio of ∼ 10.

The application of OMCVD to high performance two-dimensional electron gas structures

Recent advances in organometallic chemical vapor deposition (OMCVD) have made it an attractive alternative to molecular beam epitaxy (MBE) for the growth of device quality, multilayer, ultrathin structures requiring abrupt interfaces. The motivation for this development has been the potential for large area deposition, the excellent surface morphology obtainable, the wide range of growth rates possible and the wide variety of compounds that can be grown. In this paper we discuss our demonstration of reproducible growth by OMCVD of high quality GaAs/AlGaAs heterostructures for obtaining a 2-dimensional electron gas. We then present our results for modulation doped field effect transistors (MODFETs), inverted MODFETs, negative resistance field effect transistors (NERFETs), charge injection transistors (CHINTs) and a novel memory cell in all of which a 2-dimensional electron gas channel is utilized. We show that OMCVD has made possible the demonstration of devices which either were unachievable or possessed inferior characteristics when MBE material was used.