The Observations of the IR Sensitivity Enhancement , Negative Differential Resistance and Hysteresis in a Microelectronic Gas Discharge Device with GaP

In this paper, some interesting features of a microelectronic gas discharge device with GaP semiconductor are reported. The device is a complicated plasma system with a metal anode and a GaP cathode. A discharge occurs in the micro-scaled gap, when a voltage larger than the breakdown value is applied between these electrodes. Since large region of applied voltages are scanned in the microelectronic gas discharge device, both Townsend and glow regimes are clearly observed and a complete electrical and optical responses of the device has been clarified. Following the increase of U gradually, different light emission intensities occur as a result of discharge current I. An IR light source is also used in order to test the IR excitation of the microstructure. Although it has been believed that the GaP is sensitive to UV and visible regions, it has been proven for the first time that the IR sensitivity of GaP can be enhanced by using microelectronic gas discharge device, when an appropriate parameter set is applied. Moreover, a negative differential resistance regime, which is important for the high frequency microwave applications, has been observed at moderate voltages. In addition to the negative differential resistance regime, certain hysteresis behaviour is also observed in the sweep up/down cases of U. DOI: http://dx.doi.org/10.5755/j01.eee.20.1.6173

1 Abstract-In this paper, some interesting features of a microelectronic gas discharge device with GaP semiconductor are reported.The device is a complicated plasma system with a metal anode and a GaP cathode.A discharge occurs in the micro-scaled gap, when a voltage larger than the breakdown value is applied between these electrodes.Since large region of applied voltages are scanned in the microelectronic gas discharge device, both Townsend and glow regimes are clearly observed and a complete electrical and optical responses of the device has been clarified.Following the increase of U gradually, different light emission intensities occur as a result of discharge current I.An IR light source is also used in order to test the IR excitation of the microstructure.Although it has been believed that the GaP is sensitive to UV and visible regions, it has been proven for the first time that the IR sensitivity of GaP can be enhanced by using microelectronic gas discharge device, when an appropriate parameter set is applied.Moreover, a negative differential resistance regime, which is important for the high frequency microwave applications, has been observed at moderate voltages.In addition to the negative differential resistance regime, certain hysteresis behaviour is also observed in the sweep up/down cases of U. Index Terms-Gas discharge devices, semiconductor devices, semiconductor materials, air gaps, hysteresis.

I. INTRODUCTION
Gallium phosphide (GaP) is one of a III-V semiconductors with an indirect wide-band-gap (WBG) of 2.26 eV at room temperature and used in many electrical, optical and electronics applications such as luminance diode [1], sensor nanodevices [2], acousto-optical modulator [3] and optical limiter [4].In the recent studies, GaP has been an interesting material due to its allowance to work for terahertz-scale generation and detection via pulsed lasers at 1040 nm [5].Thus, GaP can be used as an emitter to enhance the energy of terahertz pulses with very high pump powers [6].One of the usage areas of GaP is the production of photodetector tubes, because GaP avalanche photodiodes (APDs) exhibit high responsivity in the wavelength range from 400 to 500 nm [7].From the gas discharge field, it is clear that there are two modes of a gas discharge, namely Townsend (TD) and glow (GD) [8].The TD refers to a weak discharge with a low space charge production (i.e.current).The applied electric field is not deteriorated and the maximum discharge light emission DLE radiated from the excited species is found near the metal anode [9].On the other hand, the GD has a stronger discharge mechanism with a higher space charge production [10].A swarm of positive charge is accumulated about the cathode forming a cathode fall.This is observed as a highly illuminated layer near the semiconductor cathode.The TD is ignited at very low currents [10], whereas GD is ignited at higher currents.Discharge gap d, electrode structure, filled gas type and pressure are key parameters to characterize the discharges.In most of the studies, the applied voltage is almost indistinguishable from the breakdown voltage UB.In TD, a large number of electrons create the current before the breakdown [11].The electrons can be produced via the associative ionization [11], the desorption from barriers or the electron emission from metastable species [12].Both theoretical and experimental findings prove that the applied voltage U, which enables to produce a current between the electrodes, is called as the breakdown voltage UB and the Paschen curve can be obtained as function of the multiplication of p and d [11], [13], [14].An application of a MGDD with the WBG semiconductors is the manufacturing of microwave components.High-frequency and high-power components are vital for the microwave applications [15].It has been known that GaP is better than the GaAs and pure Si materials in microwave power with respect to trap concentration [16].Therefore, intensive The Observations of the IR Sensitivity Enhancement, Negative Differential Resistance and Hysteresis in a Microelectronic Gas Discharge Device with GaP researches have been realized in this direction over the last decade.It is also known from the literature that the negative differential resistance (NDR) affects the feature of microwave devices [16].In this manner, NDR formation was successfully included in many semiconductor devices.For instance, Gunn diodes are common in microwave and millimeter wave signal applications in that context [17].The previous experimental studies [18]- [22] have proven that some conditions should be ascertained in order to observe NDR: There should be upper satellite valleys near to the lower valley in conduction band, where the carriers can be excited with certain mobility [22].In fact, the carrier mobility in this valley can be much lower than in the other one.The second important condition is that the energy difference between the upper satellite valleys and the lower one should be larger than kBT.Here T should be the device operation temperature.However, this difference should not be larger than the band gap.It will be shown that all these conditions are found to be valid in our high resistivity material GaP.In addition, GaP semiconductor indicates a hysteresis behavior, when the applied voltage is sweeped up and down.Such a hysteresis effect can be explained by the existence of bistable potential well inside the gas device.In this paper, the main features of gas discharge device operated in the air filled micro-cell are explored.The cathode is formed by a GaP semiconductor with high resistivity.To our knowledge, it the first time that GaP indicates a clear enhanced sensitivity to external IR radiation in such a MGDD and also creates an NDR beyond a certain applied voltage U.The paper is organized as follows: In Sec.II, a brief explanation of the experimental setup and some measurement properties are presented.The main results and discussion are given in the next section.Finally, the conclusions part includes the important remarks on the observations.

II. EXPERIMENTAL
The experimental set-up of the microelectronic gas discharge device (MGDD) is shown in Fig. 1 and Fig. 2. While Fig. 1 shows only the parts of the discharge device, the total measurement system is shown in Fig. 2. A dc voltage up to 2000 Volt is applied to the device, gradually and a photomultiplier tube is used to record the intensity value of discharge light emission (DLE).A Stanford (PS 325.2500V-25 W) is used as a digital high power supply.The measurements from the discharge device are received via a connection to the computer.An insulating mica foil is placed between GaP cathode and anode in order to form a micro-range air gap between the electrodes.A basic circuitry has been formed to measure the discharge current I (see in Fig. 1).The gap spacing d and the diameter D of the GaP photodetector can be adjusted to a value between 143 m and 525 m and 15 mm and 22 mm, respectively.The voltage values can be sweeped up and down between 200 and 2500V in order to scan the entire plasma regime.Ohmic contact to n-type GaP photodetector is obtained by evaporating a transparent film of Au at about 350 °C.The range of pressure is adjusted from 28 to 690 Torr during the experiments.I is measured using a Digital Multimeter (Keithley 199) through a 10 k resistor connected in series to MGDD.Gas discharge gap is located between the GaP photodetector and a glass disc (Fig 1).Gas discharge is created between these two electrodes and current-voltage characteristics of the GaP cathode has been measured at room temperature.The CVCs can be recorded for different voltages with the increasing or decreasing rates of 5 Vs −1 .
The photocathode of MGDD can be illuminated by an incandescent lamp with 250 W from the front side of semiconducting cathode.By doing so, the cathode photoconductivity is increased up to a certain level.During the experiments, the illumination intensities Ln have been adjusted between 10 −6 Wcm −2 and 10 −2 Wcm −2 by the use of filters.

III. RESULTS AND DISCUSSION
In order to initiate a discharge in the MGDD, slightly higher breakdown voltage UB is required due to the indirect gap feature of GaP. Figure 3 shows the so-called Paschen curve (i.e.UB versus p) for different electrode distances d.
When d gets larger, higher voltages are required for the ignition of the discharge between the electrodes as also stated in previous studies [13], [14].UB increases as function of p, however the inclination of this increment decreases with p.The discharge current and the light emission (LE) intensities under different IR illumination intensities are given in Fig. 4(a,b) as function of U. Note that the interelectrode distance d = 330 m for these observations.When d gets lower than this critique value, such an NDR behavior has not been encountered.It should be stated that the observation of NDR in GaP is very important in the sense that this material can be used as a microwave component in high frequency devices, when an appropriate interelectrode distance is adjusted.
Figure 5 shows the unstable (lower) and stable (upper) discharge currents at two different voltages.In order to produce Fig. 4, the averages of many temporal recordings such as Fig. 5 are used and current averages are defined.While the lower plot fluctuates around I = 3.5 × 10 -5 A, the upper one presents much stable appearance with a smooth increment up to I = 1.18 × 10 -4 A. Strictly speaking, while the Townsend regime is just below the unstable regime, the glow discharge regime appears at higher currents with high stability beyond the NDR region.The illumination intensities, which are denoted by L1, L2 and L3 excite the carriers and supply energy to them for higher energy bands.However this process does not only depend on the semiconductor properties, since the gaseous narrow gap media of the MGDD also contributes to the carrier dynamics (i.e.current).
By comparing the inset with Fig. 6, the sole parameter change is pressure p.In other words, low pressure rates are much stable towards the external illuminations.The detailed differences between the plots can be summarized as follows: While the illumination intensities (i.e.Ln) cause different currents such as I = 5 × 10 -6 A and I = 10 -5 A for 1400 V at high p, they do not affect the currents such as I = 6 × 10 -6 A for the same voltage (i.e.U = 1400 V) at low pressures.These current values prove that p is a key parameter for the IR excitation of GaP specimen.
According to the literature, the effect of IR illumination in such a MGDD with GaP has not been studied before.In fact, there are only a few studies on the IR transmission spectra as optical features of the material [4], [23].These recent measurements have been taken from either thin film or nano-particle states of GaP.On the other hand, most of the studies deal with the transmission features in UV or visible regions [7].But there is not any measurement on the high resistivity planar GaP material in terms of IR sensitivity.According to our findings, if an appropriate pressure rate is adjusted, the optical properties of GaP can be enhanced and IR sensitivity of the material is increased optically.

IV. CONCLUSIONS
The exploration of an MGDD with GaP cathode has proven that both the gaseous microdischarge gap and GaP semiconductor material dominate the electrical features of the system.The detailed measurements indicate the importance of the charge carriers, which are responsible for the current formation in the system.While the Townsend regime is encountered at 10 -5 A, the glow regime is observed at 10 -4 A for this cathode material.The breakdown voltage UB, which is responsible for the discharge process increases with the interelectrode distance, thereby the discharge can occur at higher voltages.In addition, the other important parameter -pressure affects the optical responses of the discharge system, when the device has been operated under different IR illumination intensities for the first time to our knowledge.Strictly speaking, high pressures cause an IR enhancement, while low pressures make the system much stable to external illuminations.It has also been observed for the first time that the device indicates the negative differential resistance NDR and hysteresis phenomena with GaP cathode, when an appropriate parameter set is adjusted.It has been believed that these reported properties of GaP can contribute at the manufacturing of high frequency microwave devices and IR-detection systems in addition to the UV and visible spectral region applications.

Figure 6 Fig. 6 .
Figure 6  gives two different responses towards three different illumination intensities in cases of two different pressures such as p = 35 Torr and p = 690 Torr.The GaP material is found to be excited optically via an IR illumination, when suitable interelectrode distance and pressure are applied.