RU2790304C1 - Flight diode with variable injection for generation and detection of terahertz radiation - Google Patents

Abstract

FIELD: micro- and nanoelectronics.

SUBSTANCE: invention relates to the field of micro- and nanoelectronics and can be used for the manufacture of generators and receivers of terahertz radiation. Transit diode with variable injection for generation and detection of terahertz radiation, in which regions of strong single-type doping are formed on a semiconductor substrate with undoped span gaps of the same width between them; improves the matching of the diode with the antenna, increases the generation power and increases the sensitivity of radiation detection.

EFFECT: proposed design of the transient diode ensures its matching with the antenna, increases the generation power and the detection sensitivity.

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Description

The invention relates to the field of micro- and nanoelectronics and can be used for the manufacture of generators and receivers of terahertz radiation.

When moving into the frequency range of electromagnetic radiation from 0.5 to 10 THz, both from the side of optical generators and from the side of electronic devices, a sharp drop in power is observed [1]. The creation of compact and efficient sources operating in this frequency range at room temperature and having a power of 5 mW and more sufficient for practical purposes is an urgent task.

The proposed device is based on the use of a solid-state structure as a transient diode, which can be fabricated, in particular, using a well-developed planar silicon technology [2].

The description of the device is given with reference to the attached drawings, where the numbers indicate: 1 - heavily doped areas of the semiconductor; 2 - metal contacts; 3 - span unalloyed gap, and on which are presented:

Fig. 1. Top view of a transient diode with one transient gap, formed on a semiconductor substrate and having two heavily doped regions with metal contacts lying on them, connected to an antenna and a constant voltage source;

Fig. 2. Qualitative explanation of the injection variable. The calculated band diagram of the diode [4] is presented, namely, the position of the edge of the conduction band of the semiconductor. A potential barrier arises in the transit gap as a result of the leakage of electrons from heavily doped regions with a doping level of 10 ¹⁹ cm ^-3 . When a voltage is applied between the contacts, the top of the barrier shifts to one of the contacts, which serves as an electron injector, in this case it is the right contact, while the second contact plays the role of a drain. A change in the diode voltage leads to a change in the height of the potential barrier, and hence to a change in the injection current;

Fig. Fig. 3. Calculated frequency dependences of the active ReY (solid curve) and reactive ImY (dashed curve) parts of the transit gap admittance without taking into account the contact capacitance with respect to the static admittance (differential conductivity) Y ₀ ; ω - circular frequency, t ₀ - flight time;

Fig. 4. Top view of a transient diode with three transient gaps formed on a semiconductor substrate and having heavily doped regions. On the extreme areas there are metal contacts connected to the antenna and a constant voltage source.

The most important property of the structure for generating terahertz radiation is the presence of negative conductivity, in other words, the negative active part of the admittance, in the linear mode of a weak signal at the generation frequency. In this case, the energy for generating oscillations is drawn from the energy of a constant voltage applied to the structure. Oscillations are self-excited from thermal current and voltage fluctuations, but only when the element is properly matched to an external circuit, such as a waveguide or antenna.

The transit diode consists of a semiconductor substrate with heavily doped regions, metal contacts are deposited on the extreme regions, as shown in Fig. 3 and FIG. 4. Between the regions of heavy doping are undoped span regions approximately 100 nm long, and current flows through them as a result of thermal emission.

When the diode is operating, a constant voltage V ₀ is applied to the metal contacts from an external source. The resulting alternating current of the terahertz frequency is fed to the antenna. To some extent, the structure resembles a diode electron lamp, in which a ballistic, without scattering, current flow mode is realized, which is essential for the appearance of negative conductivity.

It is impossible to achieve a ballistic regime in a semiconductor diode with a transit gap of 100 nm at room temperature, but there is an effect that ensures the occurrence of negative conductivity even in the case of strong scattering - this is the effect of variable injection. It was first considered in [3] in relation to a BARITT (Barrier Injection Transit-Time) diode. Injection through the tunnel contact of the p-n junction was proposed, but the tunnel current was weak and resulted in low power. In addition, the high capacitance of the contact made it impossible to obtain a high generation frequency.

Thermionic injection was considered in [2] (Fig. 2): the voltage across the span changes the height of the potential barrier, which leads to a change in the flux of electrons entering the span from one contact. Compared to a BARITT diode, this design has more current and less capacitance between the contacts, which provides significant advantages.

The active ReY and reactive ImY parts of the span admittance are shown in Fig. 3 [4]. The drift velocity saturation mode was taken into account. For silicon, the saturation rate v _s =10 ⁷ cm/s, which sets the length of the transit gap approximately equal to 100 nm for generation at the terahertz frequency. Areas with negative conductivity ReY(ω)<0 determine the frequency ranges at which generation can occur. It should be specially noted that self-excitation of oscillations arising from thermal current and voltage fluctuations occurs only when the transient diode is correctly matched to the external circuit, i.e. antenna and/or waveguide.

The matching requirements are as follows:

1) amplitude matching provides the condition for the active parts of admittances:

- ReY > ReY _a

where Y is the admittance of the span gap without taking into account the capacitance of the diode contacts, and

Y _a - admittance of an external circuit, such as an antenna.

It is obvious that the condition is feasible only for a negative value of ReY.

2) phase matching provides a condition for the reactive parts of admittances:

-ImY-ωC=ImY _a

where C is the capacitance of the diode contacts, and

ω - circular frequency.

This condition cannot be met with a large capacitance C. In other words, it can be said that a large capacitance C shunts the span and the oscillations cannot be brought out.

The structure of a diode with one span gap is schematically described in [2]. To estimate the contact capacitance C, consider a realistic structure that can be fabricated using modern silicon technology (Fig. 1). The structure has an undoped span of 100 nm between heavily doped regions. The width of the gap is about 100 µm. Considering that the depth of heavy doping is 30 nm and the thickness of the metal layer is 30 nm, the estimate of the diode capacitance is ~0.04 pF. The reactive part of the admittance ωС created by this capacitance at a clock frequency of 1 THz is only 0.24 Ohm ^-1 , which makes it difficult to match the diode with an antenna designed for the terahertz frequency range. Note that the trivial method of reducing the capacitance of a diode by reducing its width leads to a decrease in the generation power.

To reduce the output capacitance of the diode, you can use the sequential arrangement of two or more span gaps in the diode (Fig. 4). This reduces the output capacitance of the diode and simultaneously increases the generation power.

Indeed, the total diode admittance Y _tot becomes equal to

_Ytot =(Y+iωC)/N

where N is the number of spans.

In fact, the decrease in capacitance is more significant in magnitude, due to the fact that the intermediate regions of heavy doping do not have metal electrodes.

If in the operating state of a diode with one span gap, a constant voltage V ₀ is applied to it, then a constant voltage NV ₀ should be applied to the diode with N span gaps, while the magnitude of the flowing current is maintained, and its power increases N times. The generation power increases by the same factor. The number of gaps N can be increased to increase power as long as the amplitude matching condition 1) above is not violated. This determines the maximum achievable generation power.

The considered diode can also serve as a radiation detector due to the effect of current rectification, which can be qualitatively described by the following formula:

where V is a constant voltage applied to the structure;

- амплитуда переменного напряжения, поступающего с антенны;

- amplitude of the alternating voltage coming from the antenna;

ω is the circular frequency of the alternating voltage coming from the antenna;

ϕ(ω) - phase shift of the alternating current relative to the alternating voltage coming from the antenna, depending on the frequency of the signal;

<…> - time averaging.

Numerical coefficients are omitted here. The first term describes the direct current, the second - the alternating current coming from the antenna, the third - the rectified alternating current, which determines the detection.

To achieve the maximum sensitivity of the diode in the detection mode, it is again necessary to fulfill the conditions for its matching with the antenna 1) and 2) above. Good matching can be achieved by increasing the number of spans N.

Thus, the proposed design of the transient diode with variable injection ensures the matching of the diode with the antenna, increases the generation power, and increases the sensitivity of radiation detection.

Information sources

1.S.S. Dhillon, M.S. Vitiello, E.N. Linfield et al. "The 2017 terahertz science and technology Roadmap (Topical Review)". J Phys. D:Appl. Phys., 50, p. 043001, 2017.

2. V. Vyurkov, A. Miakonkikh, A. Rogozhin, M. Rudenko, K. Rudenko, V. Lukichev. Barrier-injection transit-time diodes and transistors for terahertz generation and detection, Proc. of SPIE Vol. 11022, 1102202 (2019) https://doi.org/10.1117/12.2522493.

3. Coleman D.J., Jr., "Transit-Time Oscillations in BARITT Diodes," J. Appl. Phys. 43, 1812-1818(1972).

4.V.V. Vyurkov, R.R. Khabutdinov, A.B. Nemtsov, I.A. Semenikhin, M.K. Rudenko, K.V. Rudenko, and V.F. Lukichev. Analytic Model of Transit-Time Diodes and Transistors for the Generation and Detection of THz Radiation, Russian Microelectronics, 2018, Vol. 47, no. 5, pp. 290-298. DOI: 10.1134/S1063739718050104.

Claims (1)

Transit diode with variable injection for generation and detection of terahertz radiation, characterized in that regions of strong doping of the same type are formed on the semiconductor substrate with undoped span gaps of the same width between them, and output metal contacts of the diode are superimposed on the extreme doped regions with a retreat from the edge of the doping zone , which improves the matching of the diode with the antenna, increases the generation power and increases the sensitivity of radiation detection.

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