Zero Bias rf Detection Diodes based on Triple Barrier Resonant Tunneling Structures

Scientists: Gregor Keller, Werner Prost, Franz-Josef Tegude and Michihiko Suhara


In the field of THz frequency signal generation resonant tunneling diodes (RTD) have proven their excellent high frequency performance. The devices benefit from their large current densities beyond 1000 kA/cm2 at moderate parasitic capacitances. These features can be used to design quantum structures with an additional third barrier [1]. By asymmetric design of the second well these elements provide nonlinearity in the I/V-characteristic, so that these elements are promising candidates for rf signal detection.


The principle of function of the investigated device is shown in fig 1. The wells are designed to have a misalignment for the different discrete energy levels. In forward direction tunnel current flow is possible because the energy levels in both wells are in resonance. This is not possible in reverse direction so that the current flow is blocked [2]. The investigated structure is grown by MBE on semi-insulating InP. The quantum structure is embedded into heavily Si-doped contact layer with doping concentrations up to 3.7e19 cm-3, to provide good ohmic contact to the intrinsic structure. The quantum structure is formed with one 1.7 nm InAlAs and two 1.7 nm AlAs barriers. The quantum wells are formed by an InGaAs/InAs/InGaAs layer stack. The 1.17 nm thin InGaAs layers are used to smooth the interfaces, because AlAs and InAs are not lattice matched to the InP substrate. The InAs used for the two wells has a thickness of 2.42 nm in the first and 1.21 nm for the second well.

Fig. 1. Conduction band edge for reverse and forward operation


The dc current voltage characteristics of the fabricated devices were measured on-wafer with a semiconductor parameter analyzer at room temperature. The measurements show high current densities, up to 230 kA/cm2 at peak voltage of 0.45 V. With an s-parameter analyzer sensitivity measurements are performed on-wafer, with coplanar contacted diodes. Zero bias sensitivity measurements of the detected output voltage (Vdet) are performed at a frequency of 30 GHz with 1 M? resistive load. For this the mismatch between the rf-source and the device under test is unaccounted. The input power is measured at the end of the cable, so the power loss at the coplanar probe tip is not taken into account, too. The device shows no compression for power levels up to -10 dBm. Due to the low thermionic current for higher voltages also higher input levels should be possible. To calculate the available sensitivity for impedance matched devices the following equation is used [3]:

Here G is the return loss caused by the impedance mismatch between source and diode. The result for a 1 µm2 device, in dependence of the input power, is presented in the figure 2.

Fig. 2. Detected voltage and matched sensitivity of a 1 µm2 device at 30 GHz

By down scaling to devices with smaller active area the matched sensitivity can be improved due to smaller intrinsic capacitance. The different lines represent structures with a different number of contact fingers but the same active area for each finger.

Scaling the active area of the devices at 30 GHz with -35 dBm input power

Modelling for Circuit Simulation

NBased on RTD large signal models an approach for TBRTD can be achieved. For this, the forward and backward characteristic is modeled with the same equations but a different set of parameters like presented in the following equation:

The current in backward direction is basically dominated by the thermionic component. For low voltages a current component from tunneling electrons, but with small peak current density and low peak voltage can also be observed. In forward direction the characteristic is mainly given by the tunnel current, with low influence of the thermionic component, especially for low voltages. The Gaussian component of the tunnel current is used for the negative differential region, only. To perform first simulations a constant capacitance scaling with the active area has been used. The voltage depending modeling of this intrinsic capacitance is in progress so that further detailed results can be expected. With large signal S-Parameter simulations, the matched sensitivity up to 1.2 THz is analyzed for diodes with different active area (Fig. 3). The simulations show matched sensitivities above 1 kV/W up to 1.1 THz for a device with 0.25 µm2 active area.

Fig. 3. Simulated matched sensitivity for -35 dBm input power


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