Naresh Chand

GaAs bipolar transistors grown on (100) Si substrates by molecular beam epitaxy

We have investigated the properties of GaAs homojunction bipolar transistors grown on (100) oriented Si substrates by molecular beam epitaxy. In a structure with a base thickness of 0.2 μm, a small‐signal common emitter current gain β of about 10 at a current density of 10 kA/cm2 has been obtained. Current densities as high as 105 000 A/cm2 were obtained in these devices without degradation demonstrating the excellent stability of this material. Since the minority‐carrier lifetime is quite sensitive to defects in the base region, these measurements demonstrate the high quality of GaAs on Si substrates. From the collector current dependence of current gain, an ideality factor n=1.5 for the emitter junction was obtained, indicating that space‐charge recombination is an important mechanism in these devices. We also demonstrate that the entire GaAs on Si wafer is phase ordered by observing the orientation effect in the mesa etching. These results show that excellent minority‐carrier properties can be obtained...

Collector‐emitter offset voltage in AlGaAs/GaAs heterojunction bipolar transistors

Factors affecting the collector‐emitter offset voltage in AlGaAs/GaAs heterojunction bipolar transistors have been examined both experimentally and theoretically. In the offset region the collector current, instead of being zero as reported in the literature, is negative and is equal to or less than the base current. The majority of our double heterojunction transistors have negligible offset voltage (50 mV). The prime reason of unusually large offset voltages as observed in some of our devices is found to be the poor quality of the base‐collector (b‐c) junction which is affected by the growth and fabrication processes. The effect of the conduction‐band spike on the offset voltage depends on the compositional grading and in properly graded devices is negligible. For zero offset voltage, high quality of the b‐c junction is important such that its turn‐on voltage is either equal to or higher than that of the emitter‐base junction.

Temperature dependence of current gain in AlGaAs/GaAs heterojunction bipolar transistors

Details of the temperature dependence of current gain in AlGaAs/GaAs heterojunction bipolar transistors (HBT’s) are reported for the first time. Unlike homojunction transistors, the gain initially increases with decreasing temperature from 425 K followed by a decrease towards 77 K at a temperature which is dependent on AlAs mole fraction in the emitter. Variation of gain with temperature is much smaller than what is normally seen in homojunction transistors adding another advantage in favor of the HBT’s. Hole injection current into the emitter and parasitic recombination currents are shown to be the reasons for the observed temperature dependence of gain.

Diffusion of As and Ge during growth of GaAs on Ge substrate by molecular‐beam epitaxy: Its effect on the device electrical characteristics

Owing to surface‐exchange reaction and diffusion of As and Ge species during crystal growth by molecular‐beam epitaxy, the electrical characteristics of n‐GaAs/p‐Ge heterojunctions are found to be affected significantly. The junctions grown at temperatures higher than 500 °C become thyristorlike npnp structures. The junctions grown below 500 °C exhibited normal current–voltage characteristics with the p/n junction apparently displaced into the Ge.

Pnp GaAs/Ge/Ge phototransistor grown by molecular beam epitaxy: Implications for bipolar and hot‐electron transistors

In order to exploit the potential of GaAs/Ge heterojunctions for high‐speed heterojunction bipolar transistors and hot‐electron transistors, a Pnp GaAs/Ge floating base phototransistor, sensitive to 1.1–1.55‐μm wavelength range, was made. The gallium‐doped Ge substrate was first exposed to the As2 flux in the growth chamber of a molecular beam epitaxy system to form a pn junction in Ge. This was followed by the growth of a p‐type Be‐doped GaAs layer which served as the emitter. The fabricated mesa devices showed an optical gain of 85 at 1.15 μm incident wavelength. The gain was found to be nearly independent of the incident light power indicating good quality of the GaAs/Ge heterointerface.

A pnp AlGaAs/GaAs heterojunction bipolar transistor

Preliminary results of the first successful operation of pnp AlGaAs/GaAs heterojunction bipolar transistors grown by molecular beam epitaxy are reported. A common emitter current gain of 50 was obtained for a base width of 0.1 μm. For collector currents larger than a certain value, a negative output differential resistance (NDR) was seen at temperatures between 360 and 83 K. The onset of collector current for NDR was found to increase with decreasing temperature from 40 mA at 360 K to 200 mA at 83 K for emitter‐base junction area of 10 μm×50 μm. The NDR was seen for both common base and common emitter characteristics and for both constant input currents and voltages. Increased surface recombination current at the edges of the emitter‐base junction at large electric fields, large currents, and high temperatures appears to be the cause for NDR.

The effect of grading on the electrical behavior of Np AlGaAs/GaAs heterojunction diodes

Abstract We have studied the effect of compositional grading on the characteristics of N AlGaAs p GaAs heterojunction diodes, and have found that the elimination by grading of the heterojunction spike leads to a factor of 10 difference in the magnitude of current density. The behavior of heterojunction diodes has also been compared to GaAs homojunction diodes. Ideality factors for graded diodes were as good as n = 1.12, while those for the abrupt diodes were n = 1.25−1.30. At low forward-bias voltages the I–V characteristics are dominated by a recombination mechanism which leads to a temperature-independent logarithmic slope with applied bias. We also observe a difference in the built-in voltage between the graded-, abrupt- and homojunction diodes. By comparing the built-in voltage between abrupt heterojunction diodes and homojunction diodes, a value of ΔE c = 0.64 ΔE G for the conduction-band-edge discontinuity was obtained in excellent agreement with recent measurements on other device structures. An understanding of the current conduction mechanisms of these pN heterojunction diodes is essential for the understanding of GaAs/AlGaAs heterojunction bipolar transistors.

Measurement of the minority‐carrier lifetime and injection efficiency in AlGaAs/GaAs heterojunction bipolar transistors

To obtain an insight of the factors limiting the current gain, minority‐carrier lifetime (τn), base transport factor (αT), and emitter injection efficiency (γ) have been studied in npn AlGaAs/GaAs heterojunction bipolar transistors using a base width modulation technique. It is found that in addition to γ, αT is also a function of current injection level. At low injection αT increases at a fast rate with increasing injection. It is felt that, in addition to the surface recombination current at the periphery of the emitter‐base space‐charge region which affects γ, surface recombination of the minority carriers in the base of a mesa device is also important as it greatly reduces αT and hence the current gain.To obtain an insight of the factors limiting the current gain, minority‐carrier lifetime (τn), base transport factor (αT), and emitter injection efficiency (γ) have been studied in npn AlGaAs/GaAs heterojunction bipolar transistors using a base width modulation technique. It is found that in addition to γ, αT is also a function of current injection level. At low injection αT increases at a fast rate with increasing injection. It is felt that, in addition to the surface recombination current at the periphery of the emitter‐base space‐charge region which affects γ, surface recombination of the minority carriers in the base of a mesa device is also important as it greatly reduces αT and hence the current gain.

Reduced light sensitivity and persistent photoconductivity in novel modulation doped heterostructures incorporating an n·GaAs/AlGaAs superlatt1ce as the high bandgap material

A short period 15 Å/15 Å GaAs/AlxGa1−xAs superlattice where only the GaAs layers are doped with Si donors has been incorporated in a GaAs/AlxGa1−xAs molecular beam epitaxial modulation doped heterostructure in place of the doped high bandgap ternary alloy. The removal of the donors from the AlxGa1−xAs eliminates the component of light sensitivity resulting from the deep persistent photoconductivity (PPC) traps present in this material. This results in a reduction in overall light sensitivity and an elimination of the PPC effect at 77K.

Prospects of High-Speed Semiconductor Devices

Concepts, technologies, and performance of high speed devices are reviewed. Among the devices treated are the modulation doped GaAs/AlGaAs heterojunction transistors (MODFET), heterojunction bipolar transistors (HBT), resonant tunneling devices (RTD) and hot electron transistors (HET) with semiconductor or metal type base. A contrast is drawn between the intrinsic switching speed and the extrinsic speed which is dominated by, but often overlooked, parasitic resistances and capacitances. It is shown that because of parasitics, not just the carrier velocity but the device current needs optimization.