Experimental Studies of Heterostructure Devices: Resonant Tunneling Transistors and GaAs/AlAs/GaAs Capacitors

This thesis is concerned with the experimental study of two kinds of heterostructure devices. The resonant tunneling transistor (RTT) is the subject of the first part of the thesis. The RTT is a new class of electronic device that has a controllable negative differential resistance (NDR) as its distinguishing characteristic. Since the first realization of a device of this type, in 1985, about 6 types of transistor structures have been reported that exhibit controllable NDR. We report the development of two types of RTTs, which are series integrations of GaAs/AlₓGa₁₋ₓAs double-barrier heterostructures with field-effect transistors. Samples were produced by metalorganic chemical vapor deposition (MOCVD). Several fundamental applications of these devices are also presented. The first device is an integration of a resonant tunneling double-barrier heterostructure with a vertical field-effect transistor. The composite device is referred to as a DB/VFET. The device exhibits NDR in its source-drain I-V curve at 77 K, which is controllable with gate bias. Novel device features include the observation of NDR at large voltages (greater than 10 V) in one bias direction. One device exhibits NDR at room temperature. Typical 77 K peak-to-valley current ratios were about 5. Frequency multiplication and microwave oscillations at 0.8 and 3.3 GHz have been observed in this device. This device is discussed in Chapter 3 and Chapter 5. The second device is an integration of a double-barrier heterostructure with a planar field-effect transistor, in this case a metal-semiconductor field-effect transistor (MESFET). The composite device is referred to as a DB/MESFET. It also exhibits NDR in its source-drain I-V curve, but is qualitatively different from the DB/VFET in its behavior. A variety of output characteristics may be obtained by varying the double-barrier and MESFET parameters. Logic operations are of interest for this device, and a flip-flop circuit is demonstrated with a single DB/MESFET. This device is described in Chapters 4 and 5. In Part II of the thesis, studies of a different heterostructure are reported. GaAs/AlAs/GaAs single-barrier capacitor structures, characterized by relatively thick AlAs barriers (1000 - 4000 Å) are the subject of this part of the thesis. Samples were grown by MOCVD. A variety of electrical and optical measurements were performed on these structures. These included capacitance-voltage (C-V), current-voltage (I-V), deep-level transient spectroscopy (DLTS), and photoresponse measurements. This structure, a fundamental part of many heterostructure devices, exhibits novel C-V and I-V behavior that can be attributed to significant densities of electron trap states near one of the GaAs/AlAs interfaces, or in the AlAs. Estimates of the deep-level concentration can be made from both C-V and I-V measurements, which have been confirmed with DLTS measurements. DLTS confirmed that the trap levels are localized. These studies are described in Chapter 6. Photoresponse measurements of the structures are interesting, and are described in Chapter 7. These studies explain the observation of zero-bias photocurrent consistent with electron transport from the back of the sample to the front.