B. Van Zeghbroeck

105-GHz bandwidth metal-semiconductor-metal photodiode

The authors report the fabrication and characterization of a metal-semiconductor-metal (MSM) Schottky-barrier photodiode with a measured impulse response shorter than 5 ps and a bandwidth of 105 GHz at a bias voltage of 0.5 V. It is shown that the experimental results are in good agreement with numeric calculations. Because of its high performance and process compatibility with GaAs FETs, this type of photodiode is very well suited for monolithic micro- and millimeter-wave optoelectronic circuits.<<ETX>>

5.2-GHz bandwidth monolithic GaAs optoelectronic receiver

A high-speed monolithic optoelectronic receiver consisting of a photodetector, a transimpedance amplifier and a 50- Omega output buffer stage has been fabricated using an enhancement/depletion 0.35- mu m recessed-gate GaAs MESFET process. The interdigitated metal-semiconductor-metal photodetector has a dark current of 0.8 nA, a responsivity of 0.2 A/W, and a capacitance of 12 fF. The bandwidth of the receiver is 5.2 GHz with an effective transimpedance of 300 Omega into a 50- Omega load, which corresponds to a transimpedance bandwidth product of 1.5 THz- Omega .<<ETX>>

Parasitic bipolar effects in submicrometer GaAs MESFET's

We report the observation for the first time of parasitic bipolar action in GaAs MESFETs. It manifests itself in the form of increased transconductance at higher drain voltage, abrupt change in output conductance (kink effect) around 4-V drain-source voltage, and a gate-voltage-dependent substrate current. These effects are explained by electron-hole pair generation in the high-field region at the drain. The holes generated are injected into the substrate where they form the base region of a parasitic lateral bipolar transistor. The effect also explains a new breakdown mechanism for short-channel enhancement-mode MESFETs.

A novel high-speed silicon MSM photodetector operating at 830 nm wavelength

A novel high-speed silicon photodetector that operates at a wavelength of 830 nm is reported. It consists of a Metal-Semiconductor-Metal (MSM) detector that is fabricated on a 5-/spl mu/m thick silicon membrane. The detector has a measured -3 dB bandwidth of 3 GHz at 10 V, which is almost one order of magnitude larger than the reported bandwidth of conventional silicon MSM detectors as measured at 830 nm. The DC responsivity is 0.17 A/W, corresponding to an internal quantum efficiency of 60.5% and an external quantum efficiency of 25.4%. The large bandwidth and good responsivity at the wavelength of interest, combined with its low operating voltage and compatibility with most silicon integrated circuit technologies, make this detector a promising candidate for monolithic optoelectronic receiver circuits for use in short distance optical communication systems and computer interconnects.<<ETX>>

A Josephson sampler with 2.1 ps resolution

A Josephson sampler with 2.1 ps resolution is reported. The sampler was made with Nb edge junctions, and consists of a sampling junction to which a Faris pulser is coupled directly. Two experiments are connected to the sampler: a two-junction interferometer and another Faris pulser. A new and simple electronic delay allows a flicker-free display on an oscilloscope of the waveform sampled. A current sensitivity of 0.8 μA was achieved. It was possible to measure the switching transitions of the two-junction interferometer over its whole vortex boundary, including vortex-to-vortex transitions which occur at low bias currents. To our knowledge, this is the fastest Josephson sampler made to date.

Photocurrents in a metal-semiconductor-metal photodetector

Photocurrents in a metal-semiconductor-metal (MSM) photodetector have been analyzed in a one-dimensional structure using both time-dependent and steady-state continuity equations. Analytical solutions are presented for the carrier concentrations as well as for the currents, which form a valuable tool for the investigation of the detector behavior under various bias conditions. Applying these expressions to a GaAs device, we have studied the influence of carrier diffusion, recombination, and drift on the photocurrents as a function of the applied bias voltage. We also show that the switching time at low bias voltage is dominated by a voltage-independent diffusion time constant which is of particular interest when using the device as an optoelectronic sampling gate. On the other hand, carrier recombination is found to have minimal influence on DC characteristics and pulse response.

Submicrometer GaAs MESFET with shallow channel and very high transconductance

A 0.5-µm GaAs MESFET with a 25-nm thin channel, 400- mS/mm maximum transconductance, and 580-mS/V.mm K value is presented. This extremely high K value was obtained using an electron-beam fabricated recessed-gate MESFET structure on a highly doped (9.1017cm-3) MBE-grown channel layer with 2600-cm2/V.s mobility. The use of thin channels and a buried p-layer also reduced the output conductance and other short-channel effects dramatically. As a result, these scaled MESFETs are very promising for high-speed digital logic circuits.

GaN/SiC heterojunction bipolar transistors

Abstract We report on the evolution of the fabrication and characterization of high-temperature and high-power GaN/SiC n–p–n heterojunction bipolar transistors (HBTs). The HBT structures consists of an n-type GaN emitter and a SiC p–n base/collector. Initially, the HBTs were fabricated using reactive ion etching (RIE) to define both the emitter and base areas. However, the poor etch selectivity between GaN and SiC made it difficult to stop at the thin base layer. Furthermore, the RIE caused damage at the heterojunctions, which resulted in large leakage currents. Selective area growth was therefore employed to form the n-GaN emitters. GaN/SiC HBTs were first demonstrated using the 6H-polytype. These transistors had an extraordinary high dc current gain of >10 6 at room temperature and were able to operate at 520°C with a current gain of 100. However, in more recent work, this performance could not easily be reproduced due to the presence of a parasitic deep defect level in the p-type 6H–SiC. The possibility of obtaining higher quality 4H–SiC than 6H–SiC, without this defect level, seemed promising since much of the materials development is focused on 4H–SiC, due to its larger energy band gap and superior electron mobility. GaN/4H–SiC HBTs are demonstrated with a modest dc current gain of 15 at room temperature and 3 at 300°C.

High-temperature GaN/SiC heterojunction bipolar transistor with high gain

A new high temperature heterojunction bipolar transistor (HBT) with a current gain as high as 100,000 has been fabricated. This HBT utilizes GaN for the emitter and SiC for the base and collector. The devices exhibit near ideal current-voltage characteristics, as demonstrated by their high current gain along with the absence of any observable Early effect, with the exception of high leakage currents at voltages above 10 V. High temperature operation has been demonstrated up to 260/spl deg/C with minimal degradation in output, except for an increase in leakage currents.<<ETX>>

4H-SiC bipolar junction transistor with high current and power density

H silicon carbide bipolar transistors were fabricated using a double-mesa process. The devices exhibit a maximum common emitter current gain of 17.4, a maximum current density of 42 kA/cm 2 and maximum DC power dissipation density of 1.67 MW/cm 2 . The current gain was measured to decrease to 65% of its room temperature value at 300 C. The record high current and power density of the devices makes them attractive for high-power RF applications. 2002 Elsevier Science Ltd. All rights reserved.