The concept of the real-space-transfer (RST) diode to obtain negative differential resistance was conceived by Gribnikov in 1972,’ and independently by Hess et al. in 1979.2 Analytical modeling was presented by Shichijo et al. in 1980.3 Computer simulations using the Monte Carlo method were performed by Glisson in the same year.4 The first experimental evidence of the negative resistance from a RST diode was shown by Keever et al. in 1981.5 Demonstration of an RST oscillator was made by Coleman et al. in 1982.6 This device is still under investigation and has not been produced commercially.


The requirement of a real-space-transfer diode is a heterojunction whose two materials have different mobilities. In addition, for an «-channel device, the material having lower mobility must also have a high conduction-band edge Ec. A good choice is the GaAs-AlGaAs heterostructure. Although modulation doping is not a requirement, the heavy doping in the AlGaAs further decreases its mobility and at the same time, the absence of doping in the GaAs layer increases its mobility. Modulation doping results in high mobility ratio and, thus, has been used commonly for the RST diode. An example of the structure is shown in Fig. 11.1. The thickness of the intrinsic GaAs is not important as the main channel is confined to the AlGaAs-GaAs heterointerface. Typically a GaAs thickness of ~ 1 (im is used. The AlGaAs layer has to be much thicker than the main channel for efficient real-space transfer. In this case, since the main channel is thin


REAL-SPACE-TRANSFER DIODEAn example for the RST diode in which GaAs-AIGaAs heterostructure and modulation doping are employed

(= 100 A), the AlGaAs can be about 1000 A. The doping in this AlGaAs layer ranges between 1017 to 1018 cm-3. A thin layer of intrinsic AlGaAs (~ 50 A) is typical for modulation doping to ensure that the heterointerface is separated from the heavily doped region to avoid impurity scattering. The /7+-regions can be formed by diffusion of the impurity from the alloyed contacts which commonly is made of AuGe.

Fine control of the layer thickness and doping profile necessitates MBE or MOCVD growth. The example shown here has only one layer of channel, but a multichannel structure can be built with repeated heterojunctions on top of one another.


The real-space-transfer effect is similar to the transferred-electron effect (see Chapter 7), and it is sometimes difficult to separate them experimentally in a heterostructure. The transferred-electron effect is due to the properties of a single, homogenous material. When carriers are excited by a high applied field to a satellite band in the momentum-energy space, the mobility is decreased and the current is lowered, resulting in negative differential resistance. In the real-space-transfer effect, transfer of carriers is between two materials (in real space), rather than two energy bands (in momentum space). In low fields, electrons (in an /7-channel device) are confined to the material (GaAs) with low Ec and higher mobility. The high-field energy-band diagram is shown in Fig. 11.2. Carriers near the anode acquire enough energy from the field to overcome the conduction-band discontinuity and flow to the adjacent material (AlGaAs) of lower mobility. This current can be considered as thermionic — emission current with the electron temperature replacing the room temperature. Thus, a higher field results in a smaller current, the definition of negative differential resistance. Typical I-V characteristics are shown in Fig. 11.3. The critical field for this real-space transfer has been shown to be between

1. 5-3 kV/cm, while that for the transferred-electron effect is typically 3.5 kV/cm for GaAs. One has to bear in mind that these critical fields are obtained from two different types of channels (heterointerface vs. bulk), and cannot be used alone to separate the effects. Another property of the real-space transfer is that there is

better control over factors such as conduction-band discontinuity, mobility ratio, and film thicknesses so that device characteristics can be varied and optimized.

The modeling of the RST diode is complicated, and there are no equations derived explicitly for the exact I-V characteristics. Qualitatively, the following expressions can be used to get an insight of the origin of the negative resistance. Assume that the total carrier density per unit area is Ns, distributed between the GaAs modulation-doped channel layer L | (m^) and AlGaAs layer Z2 (ns2),


Подпись: AlGaAs The fraction of carriers excited to the AlGaAs layer is defined as


Energy-band diagram showing the conduction-band edge Ec of the RST diode under bias Electrons in the mam channel acquire energy from the field to overcome the barrier to spill over to the AlGaAs layer


Typical current-voltage (field) characteristics of a RST diode



and is a function of the applied field. It starts at zero at low field and approaches the ratio of Z2/(Zi + Lj) at high field. The total current is given by

I = AqnsXn^ + Aqns2n2^


Подпись: (11.3)= AqZN^^-iM^-ЯJR]

where A is the cross-section area of the channel. The differential resistance is given by


Подпись: (11.4)= AqNsЯl-

and it can be shown to be negative for a proper choice of n |, n 2 and dRiel’S. In the GaAs-AlGaAs modulation-doped system, n ~ 8000 cm[2]/V-s and p. 2 is less than 500 cm2/V-s at room temperature. Experimental data show that the current peak-to-valley ratio is not very high, with a maximum value around 1.5. Computer simulations show that a ratio of more than 2 can be achieved.


One of the advantages of the RST diode is high-speed operation. The response time is limited by the movement of carriers across the heterointerface between the two materials, and is much faster than in a traditional diode where the transit time of carriers between the cathode and anode is the dominating factor. So far the application is demonstrated only by oscillators. The real-space-transfer effect is also applied in a three-terminal device (see Chapter 32-RST Transistor).

The general applications of negative differential resistance are listed in Appendix C2.