Ambient temperature or moderately cooled semiconductor hot electron bolometer for mm and sub-mm regions V.N. DOBROVOLSKY 1 , F.F. SIZOV *1 , Y.E. KAMENEV 2 , and A.B. SMIRNOV 1 1 Lashkariov Institute of Semiconductor Physics, 41 Nauki Ave., 03-028 Kiev, Ukraine 2 Usikov Institute of Radiophysics and Electronics, 12 Akad. Proskury Str., 61-085 Kharkov, Ukraine A model of semiconductor hot electron bolometer (SHEB), in which electromagnetic radiation heats only electrons in nar- row-gap semiconductor without its lattice slow-response heating, is considered. Free carrier heating changes the genera- tion-recombination processes that are the reason of semiconductor resistance rise. It is estimated, that Hg 0.8 Cd 0.2 Te detector noise equivalent power (NEP) for mm and sub-mm radiation wavelength range can reach NEP ~10 –11 W at Df = 1 Hz signal gain frequency bandwidth. Measurements performed at electromagnetic wave frequencies n = 36, 39, 55, 75 GHz, and at 0.89 and 1.58 THz too, with non-optimized Hg 0.8 Cd 0.2 Te antenna-coupled bolometer prototype confirmed the basic concept of SHEB. The experimental sensitivity S v ~2 V/W at T = 300 K and the calculated both Johnson-Nyquist and genera- tion-recombination noise values gave estimation of SHEB NEP ~3.5×10 –10 W at the band-width Df = 1 Hz and n = 36 GHz. Keywords: hot electrons, bolometer, THz, narrow-gap semiconductor. 1. Introduction Now, terahertz region is often mentioned as the challenging one as it spans the transition from radio-electronics to pho- tonics and because of its importance for scientific and com- mercial applications. The cryogenic millimetre and sub-mi- llimetre detectors being developed now are very sensitive (NEP ~10 –16 –10 –20 W/Hz 1/2 ) and have variety of applica- tions, particularly in astronomy, but they are difficult to be assembled into large multielement arrays like modern in- frared arrays [1–3]. For many applications, these cryogenic single detectors, as a rule, require long integration time while events may happen quickly to be detected. At the same time, THz technologies need compact, simple, rela- tively sensitive and fast uncooled detectors for low-cost systems which are reasonably sensitive in the whole THz region (n ~0.2–5 THz). The purpose of this investigation was to consider a sim- ple uncooled or slightly cooled direct semiconductor hot electron bolometer which can be used in wide mm and sub-mm ranges. Because of low level signals, the limitations introduced by the noise of direct detectors and electronics are the prob- lem for mm and sub-mm detectors. This problem, as a rule, is resolved by application of cryogenic detectors, operating at T = 4.2 K and at lower temperatures in these spectral bands. There is a reason why the measurements at a low level of signals in the regions pointed out, is a difficult task, par- ticularly at μW levels. Thermal detectors seem the most ap- propriate uncooled detectors, although they are much less sensitive (NEP ~10 –9 –10 –10 W/Hz 1/2 ) compared to cryogenic detectors. However, they should satisfy practical require- ments to be sensitive good enough (NEP < 10 –10 ) and fast for detecting low (<1 ìW) sub-mm or mm wave signals [3]. To be used in different areas of human activity (e.g., mm and sub-mm imaging, security, biological, drugs and explosion detection, etc.) it is desirable to have lightweight and space saving, moderately cooled, reasonably sensitive, and broadband detector operating, for example, in active real-time THz imaging systems. However, little instrumen- tation is now commercially available for mm and sub-mm measurements, and it is generally expensive [4]. The re- quired THz active imaging or spectroscopic systems for the whole mm and sub-mm region would be reasonable to be developed on the basis of uncooled fast and direct detection detectors because of their space saving and real-time opera- tion, as, for example, for short sub-mm wavelength region where direct detection is preferred [5]. Another aspect of any mm and sub-mm sensor is “cou- pling” of such kind of radiation to a detector and such cou- pling is realized with the help of various types of antennas [6,7]. Low quanta energy in sub-mm region requires for oper- ation of the direct semiconductor quantum detector, the temperatures, as a rule, lower than 4.2 K. The longest wavelength detection in this case is l co = 220 μm (stressed Ge:Ga photoconductors [5]). The low temperature semi- conductor and superconductor bolometers [8], and super- conducting hot electron bolometer mixers [9,10] also re- 172 Opto-Electron. Rev., 16, no. 2, 2008 OPTO-ELECTRONICS REVIEW 16(2), 172–178 DOI: 10.2478/s11772-008-0003-6 * e-mail: sizov@isp.kiev.ua