Infrared Hot-electron Transistor Research Articles

Structure and Process of Infrared Hot Electron Transistor Arrays

An infrared hot-electron transistor (IHET) 5 × 8 array with a common base configuration that allows two-terminal readout integration was investigated and fabricated for the first time. The IHET structure provides a maximum factor of six in improvement in the photocurrent to dark current ratio compared to the basic quantum well infrared photodetector (QWIP), and hence it improved the array S/N ratio by the same factor. The study also showed for the first time that there is no electrical cross-talk among individual detectors, even though they share the same emitter and base contacts. Thus, the IHET structure is compatible with existing electronic readout circuits for photoconductors in producing sensitive focal plane arrays.

Structural and optical characterization of infrared hot electron transistor

We present structural, optical, and transport characterization of long wave infrared hot electron transistor (IHET) based on doped quantum wells of InGaAs/AlGaAs. The atomic resolution images and x-ray diffraction patterns verified a lattice matched and band-gap engineered device structure of IHET. Measured values of the photocurrent were less than the theoretically expected values and indicated a loss of photocurrent between the base of the IHET and the collector. A higher filter height due to high unexpected dopant in the filter barrier was suggested as a possible cause of the current loss. Photoluminescence data in the near infrared showed the existence of such a dopant.

Optimization of high detectivity infrared hot-electron transistors at low temperature

In this paper, we report a 10 /spl mu/m-cutoff infrared hot-electron transistor (IHET) with extremely high detectivity, D*>7.5/spl times/10/sup 12/ cm/spl radic/Hz/W at 4.2 K. This large D* is accomplished by using InGaAs material in the quantum wells and by using a low filter barrier at the collector for the large photocurrent transfer ratio. We have verified the detector performance by explicitly performing noise characterization at low temperatures. We found that the noise of the quantum well infrared photodetector, which forms the emitter of the IHET, is dominated by 1/f noise and bias-independent noise. The 1/f current noise is due to the conductance fluctuation in impurity-assisted tunneling via DX centers in the quantum well barriers. The filter barrier of the IHET blocks the impurity-assisted tunneling and hence its noise at the collector and thus improves the detector sensitivity.

Noise characteristics of an infrared hot-electron transistor

We report on the noise characteristics of dark current in an infrared hot-electron transistor at 77 K. The measured emitter noise power can be described as the sum of generation-recombination noise and thermal noise. The measured collector noise in our device is shot noise. This result is explained with the binomial statistics for the number of injected electrons to pass through the base and the energy filter. Since the probability for the electrons contributing to the collector current is much less than 1 in our noise measurement range, the binomial statistics can be approximated as Poisson statistics and the noise of the collector current behaves like shot noise.

Very long-wavelength GaAs/AlxGa1−xAs infrared hot electron transistor

We have demonstrated a bound-to-continuum state GaAs/AlxGa1−xAs infrared hot electron transistor which has a peak response at λp=16.3 μm. This device utilizes a bound-to-continuum quantum well infrared photodetector as a photosensitive emitter and a wide AlxGa1−xAs barrier between the base and the collector as an energy discriminating filter. An excellent photocurrent transfer ratio of αp=0.12 and very low dark current transfer ratio of αd=7.2×10−5 is achieved at a temperature of T=60 K.

Long-wavelength λc=18 μm infrared hot-electron transistor

A long-wavelength λc=18 μm infrared hot-electron transistor (IHET) with low dark current is demonstrated. In order to achieve long-wavelength absorption, a low barrier height is required, which in turn results in a large dark current. Therefore, operation of a normal long-wavelength quantum-well infrared photodetector (QWIP) structure is limited to very low temperatures and biases due to the thermally activated dark current. In the IHET, a high-energy pass filter placed after 30 periods of GaAs/AlGaAs quantum wells blocks the temperature-activated dark current while allowing high-energy photoexcited electrons to pass and be collected as photocurrent. A comparison of the dark current to the 300 K background photocurrent shows that the QWIP structure without the high-energy pass filter demonstrates background-limited infrared photodetection (BLIP) only at T≤35 K. Furthermore, in order to avoid saturating a typical readout circuit, detector operation of the QWIP is restricted to biases less than 0.08 V at 35 K. In contrast, the filtered dark current in the IHET is reduced by two to four orders of magnitude such that BLIP performance can be achieved for temperatures up to T=55 K without saturating the readout circuit. Because of the preferential current filtering effect, the noise equivalent temperature difference of the IHET can be improved by a factor of 100 at T=55 K. The dark-current-limited detectivity was found to be D*=1×1010 cm Hz1/2/W at λp=15 μm, Ve=−0.2 V, and T=55 K.

Very Long Wavelength Intersubband Infrared Hot Electron Transistor

We have demonstrated the first very long wavelength ( 16 µm) infrared hot electron transistor (IHET). This device utilizes a bound to continuum GaAs/Alx Ga1-x As (x=0.11) quantum well infrared photodetector (QWIP) as a photosensitive emitter, a wide quantum well as a base, and a thick Alx Ga1-x As (x=0.11) barrier between the base and the collector as an energy discriminating filter. This energy filter blocks the lower energy tunneled electrons, which drain through the base while higher energy photo electrons pass to the collector. Therefore, the detectivity of the device at the collector is much higher than the detectivity at the emitter.

Low dark current infrared hot-electron transistor for 77 K operation

Recently, thermal imaging using quantum well infrared photodetector (QWIP) focal plane arrays has been demonstrated. However, the operating temperature needs to be kept around 60 K due to the large dark current occurring at higher temperatures. In order to achieve thermal imaging at 77 K, we have designed and demonstrated two infrared hot-electron transistor structures, whose dark current is two to three orders of magnitude lower than that of a QWIP. The resultant dark current falls within the limit of the charge handling capacity of a readout circuit, and the infrared detection is demonstrated to be background limited at 77 K. The noise equivalent temperature difference of the detectors is estimated to be 14 and 26 mK, respectively.

Background limited infrared hot-electron transistor at 77 K

The authors report a background-limited photodetector (BLIP) infrared hot-electron transistor (IHET) with current density compatible with the read-out circuit at 77 K. To reduce J/sub D/ (dark current density), a novel GaAs/Al/sub x/Ga/sub 1-x/As quantum-well infrared photodetector (QWIP), in which the aluminum molar ratio of the barriers increases in three steps, has been designed. This barrier is able to suppress the dark current due to thermally assisted tunneling by providing a thicker effective when the structure is under bias. The remaining J/sub D/ is also largely eliminated by designing a bandpass filter placed adjacent to the QWIP.

Excitation hot-electron spectroscopy of quantum well structures

By using hot-electron transistor structures, the authors showed that the hot-electrons in the base suffer little or no energy loss within a distance less than 1500 AA. They utilize this hot-electron property to construct a highly sensitive 10 mu m infrared hot-electron transistor.

High detectivity InGaAs base infrared hot-electron transistor

An infrared hot-electron transistor with a thin (300 Å)InGaAs base layer is constructed. By adopting a thin base material with a large Γ-L valley separation, the photocurrent transfer ratio is improved by a factor of four in comparison with the GaAs base transistor. As a result, the detectivity of the transistor is increased to 1.4×1010 cm√Hz/W at 77 K with a cutoff wavelength of 9.5 μm, two times as large as the companion state-of-the-art GaAs quantum well photoconductor. Combined with the lower dark current, the voltage responsivity and the noise equivalent temperature difference of a detector array can be improved by more than an order of magnitude.

Photovoltage amplification and quasi-photovoltaic operation of an infrared hot-electron transistor

Under proper external bias, we showed that photovoltage amplification can be achieved from an infrared hot-electron transistor, and hence the transistor structure can be used to increase the voltage responsivity of a multiple quantum well photoconductor. In addition, the ouput voltage of the transistor can be very small when there is no signal light source, similar to a photovoltaic detector. As a result, the transistor is able to dc couple to a high-gain voltage amplifier for further signal amplification without reducing its detectivity.

Application of superlattice bandpass filters in 10 μm infrared detection

Recently, experimental evidence has revealed that the energy distribution of the dark current in a typical multiple quantum well GaAs infrared detector is extremely broad, in contrast to the narrowly distributed photocurrent. In this letter, we present the current transfer ratio of an infrared hot-electron transistor with a superlattice collector filter. From the current transfer characteristics, we demonstrate that the superlattice is able to collect electrons with specific energy against a broad background. The energy filtering characteristics can be attributed to the underlying band structure of the superlattice. When the filter is applied to infrared radiation detection, the detectivity of the transistor is improved.