DC AND AC CHARACTERISTICS OF GaAs / InGaAs / AIGaAS REAL SPACE TRANSFER TRANSISTORS

DC and AC performances of a GaAs/InGaAs/AIGaAs negative resistance field-effect transistor 
(NERFET) are demonstrated by molecular beam epitaxy (MBE) for the first time. The 
negative differential resistance (NDR) resulted from the observation of the hot electron real 
space transfer effect in InGaAs channel. By Hall measurements, the structure shows carrier 
mobility as high as 4300 (13500) cm2/v-s at 300 (77)K, which is suitable for high frequency 
operation. For DC performance, the largest peak-to-valley current ratio of the device is about 
5 at room temperature. For AC performance, S-parameter measurements of high frequency 
and microwave characteristics indicate a projected maximum frequency of oscillation of 
fmax⁡=2.7GHz and a current gain cutoff frequency (fT) occurs at 1.8GHz.


INTRODUCTION
Negative resistance field-effect transistors (NERFET) are a three-termi- nal transistor based on the effect of real space transfer (RST) of hot elec- trons, When the heating voltage, V DS, is applied between source and drain electrodes, the emitter electrons are accelerated and become hot.If bias is sufficiently large, electrons may have enough energy to spill over the potential barrier and transfer to the collector (substrate) terminal.The device exhibits an NDR in the source-drain and an efficient control of the injection current by the drain voltage.  Retly, most of the efforts have been made on DC characteristics of A1GaAs/GaAs structure, 4 however, the investigation of AC characteristics are still rare in the literature.Considerable interests have been attracted in finding the small-signal circuit and frequency characteristics in order to improve device performance.
In this work, we use GaAs/InGaAs/A1GaAs-based material to character- ize the DC and AC performances of RST.The use of InGaAs channel pro- vides potential applications for optoelectronic integrated circuits and high-speed microwave devices in light of its F-to-L valley separation, high mobility, and large peak electron velocity.

EXPERIMENTAL
The cross-section of the studied GaAs/InGaAs/A1GaAs RST devices grown by MBE is shown in Fig. l(a).The substrate was an n+-GaAs layer and used as the current collector.Following, a 2000/-thick undoped Alo.45Gao.55Asbarrier layer was grown.Then, a 100/-thick pseudomor- phic channel, Ino.16Gao.84Asquantum well, was grown.Finally, an 1800/-thick n+-GaAs (Nd=2Xl018cm-3) cap layer was deposited.The device length (L) between source and drain are 5 gin.After MBE growth, the isolation regions and the active areas were defined by wet chemical etching using 3NH4OH:lH202:100H20 solution.Au/Ge was evaporated onto the cap layer and annealed to form source and drain contacts.I-V characteristics of the RST transistors were measured by HP4145B semi- conductor parameter analyzer in the dark.

RESULTS AND DISCUSSION
Fig. 1 (b) shows the energy band diagram of the RST transistor.The Source (or drain) behaves as the hot electron emitter.The third layer, the substrate (collector), is separated by a A1GaAs potential barrier.The substrate cur- rent (Isub) as a function of substrate bias (Vsub) at 300K is shown in Fig. 2 with source (or drain) grounded.The Isub for Vsub < 2.5V is in the nanoampere range.This shows that the A1GaAs barrier layer functions properly and the ohmic contacts give low leakage current.On the other hand, from Hall measurements, the proposed structure shows carder mobilities of 4300 cm2/v-s at 300K and 13500 cm2/v-s at 77K with sheet carrier density of 4 and 3xl012cm-2, respectively.These characteristics show that the structure is suitable for high frequency and high power oper- ations.Fig. 3 shows the drain current (ID) under different substrate biases.
NDR first occurs at about VDs=2V.In the NDR region, the I D shows step-like changes with sudden drop in current.This is believed to be due to the formation of a positive feedback loop that makes the device switch.
The largest peak-to-valley current ratio at 300K is about 5.As the VDS becomes larger than about 12V, electron flow from the substrate to the drain becomes dominant and the drain current rises.This is the so-called NERFET operation.ID-VDs and Isul:,-VDs characteristics for Vsub=8V at room temperature are also shown in Fig. 4. Fig. 4 is also taken at the same condition as in Fig. 3.As the VDS increases, the Isut, begins to increase with VDS starting at about 2V.The large increase in Isul, indicates a significant transfer of channel electrons.Above 12V, the Isu/: decreases, which agrees with the increase of I D shown in Fig. 3. AC characteristics are implemented by on-wafer measurement with low-inductance Microtech Cascade probes and calibration standards.The RST transistors are operated in a common-source configuration with the input signal applied to the drain and the output signal measured from the substrate.All studied RST transistors are measured by a HP87210A net- work analyzer with the frequency range from 130MHz to 10GHz after the standard calibration process is finished.From S-parameter measurements, a projected maximum oscillation frequency (fmax) of 2.7GHz and unity current gain cutoff frequency (fT) of 1.8GHz are indicated in Fig. 5.  4 1D versus VDS and Isu/versus VDS.The source is grounded and Vsub=8V fast decrease in maximum available gain (gmax) at frequency above 1.8GHz is believed to be caused by circuit parasitic loss, On the other hand, fT and fmax can be derived from the well-known algebraic transfor- mations of these S-parameters.The experimental data are found to be very consistent with the theoretical analyses as shown in Fig. 6.

CONCLUSIONS
RST transistors using GaAs/InGaAs/A1GaAS have been successfully fab- ricated by MBE.The device shows high carder mobility (up to 13500 cm2/v-s) and peak-to-valley current ratio (up to 5) at room temperature.
Microwave S-parameter measurements of fT up to 1.8GHz and fmax up to 2.7GHz are obtained for the device length of 5ktm.Based on these charac- FIGURE (a) Schematic cross-section of GaAs/InGaAs/A1GaAs RST transistor (b) Energy band-diagram of the device

FIGURE 6
FIGURE 6 Comparison between theoretical analyses and experimental data for the h21 and gmax 1D versus VDS at room temperature.The source is grounded.Six I-V curves with different biases (the step is 0.5V from Vsut,=7V to 9.5V) are shown