Influence of Electrical Modes on Radiation Sensitivity of Hydrogen Sensors Based on Pd-Ta₂O₅-SiO₂-Si Structures ^†

Abstract

The influence of the circuit’s electric modes on the radiation sensitivity of hydrogen sensors based on the metal-insulator-semiconductor field-effect transistor with structure Pd-Ta₂O₅-SiO₂-Si (MISFET) was investigated. There were measured the hydrogen responses of output voltages V of the MISFET-based circuits at different gate voltages before and after the electron irradiations. The voltages V as functions of hydrogen concentration C were determined for different ionizing doses D. Models of influence of the electric modes on the radiation sensitivity of sensors were based on experimental dependencies of V(C, D). The recommendations for the optimal choice of MISFET-based circuit’s electric modes were formulated.

1. Introduction

The hydrogen sensors based on the metal-insulator-semiconductor field-effect transistor (MISFETs) have been studied by many investigators [1,2,3,4,5,6,7]. The studies have shown that the conversion characteristics (the sensor output signals V as function of the hydrogen concentration C) depend on MISFET’s technological parameters [3,5] chip temperature [6] and external factors (other gases, irradiation) [4,7].

This work deals with the integrated hydrogen gas sensors with MISFET sensing element based on Pd-Ta₂O₅-SiO₂-Si structure. The characteristics of these sensors have been already investigated at normal levels of background radiation. However, there are cases, when the gas analysis devices must be used for a long time at raised radiation levels. For examples, in nuclear reactors, in uranium mines, into oil underground deposits search tools based on hydrogen-methane-radon discharge from soil, into the upper atmosphere monitoring systems or into space systems. In spite of low dose rates in above examples, the long-time irradiation (during 200–500 days) may result in degradation of performance characteristics of semiconductor sensors. So the question is: how MISFET-based sensors’ performance characteristics should change under irradiation?

The radiation effects in MIS-structure-based devices are studying since the 1960-ies [7,8,9,10,11,12,13]. It was found the basic radiation effects in MIS structures: increase the concentration of trapping centers under the ionizing radiation, change charges in the dielectrics Q_t and on its border with the semiconductor Q_ss. As a consequence the electrical characteristics of MISFETs are changing. The studies have shown that changes of characteristics depend on temperature, structural-technological characteristics of MISFETs, on total ionizing dose D (TID) and its rate P [9,10,13]. The aim of this work is to investigate the influence of the measuring circuit’s electrical modes on the radiation sensitivity of MISFET-based hydrogen sensors, and to develop models for forecasting the performances of gas-analysis MISFET-based devices under action of ionizing radiation.

2. Materials and Methods

The testing MISFET element based on Pd-Ta₂O₅-SiO₂-Si structure was fabricated on single chip (2 × 2 mm²) together with (p–n)-junction temperature sensor, heater-resistor and Pd-resistor by means of conventional n-MOS-technology using laser evaporation Pd-films. The integrated sensor chip layout with outputs’ pads and the temperature-stabilization circuitry supported chip temperature 130 °C by feedback loop using thermo-sensor and heater are shown in Figure 1.

The parameters of MISFET are the following: acceptors concentration N_a = 5 × 10¹⁵ cm^–3; length L and width z of the channel are 10 µm and 3.2 mm; dielectric capacitance C₀ ≈ 30 nF/cm²; transconductance b₀ ≈ 2.0 mA/V². In the first stage of experiments the I_D–V_G characteristics (V_G is the gate voltage) of MISFETs were measured using the measuring system MERA-3 and of circuit No. 1 (Figure 2a; position 2). These characteristics were used to determine the initial values of the threshold voltage V_T0, the transconductance b₀, the charges in the oxide Q_t0 and at SiO₂-Si interface Q_ss0 as in [13]. In the next stages of the experiments before and after the electrons’ irradiations there were measured the hydrogen responses of output voltages V at step-rising concentration C (e.g., Figure 3a) for two types circuits (Figure 2a,b) at different electrical modes, as well as I_D–V_G characteristics. The sensors were 4 times exposed to electron radiation (6 MeV energy) by the following TID(Si): 50 Gy, 100 Gy, 250 Gy and 850 Gy. The dose rate P(Si) ≈ 2.0 Gy/s.

3. Results Are Presented in Figure 3and Figure 4

4. Discussion

To describe the results there was used the 4-component model of V vs. (C; D) as in [13]:

V (C; D) = V₀ (D) − ΔV_C (C; D) = V₀₀ − ΔV_te (D) + ΔV_ss(D) − ΔV_C (C; D); ΔV_te = ΔV_teM [1−exp(−k₁D)];
ΔV_ss = ΔV_ssM [1−exp(−k₂D)]; ΔV_C (C; D) =ΔV_CM [1−exp(−kC)]·{1 − exp[−k₃(D₃ − D)]}

(1)

In circuit No. 1 the voltage V is equal to V_D, the voltage V_G is the controlling parameter:

V = E − 0.5β·(U)2 at 0 < U ≤ Um; V = 1/β + U − [(1/β + U)2 − 2E/β]0.5 at U >Um= [(1 + 2βЕ)0.5 − 1]/β;
U(C; D) = VG − VT0 = VG − VT00 + ΔVC + ΔVte − ΔVss; β = bR.

(2)

In circuit No. 2 the voltage V is equal to V_G, the current I_D is the controlling parameter:

V = U(I_D) + V_T00 − ΔV_C − ΔV_te + ΔV_ss; U = {2[I_D/b − (φ_T)²]}^0.5, if V_D ≥ U; U = I_D /(bV_D) + 0.5V_D, if U > V_D

(3)

The MISFET’s parameters and the model parameters are presented in [13] and in

Table 1. Average values (dispersion < 10%) of MISFET’s, circuit’s and the model parameters.

Parameters	VT00, V	b0, mA/V2	ΔVteM, V	ΔVssM, V	k (1/%)	k1, Gy–1	k2, Gy–1	k3, Gy–1	D3, kGy(Si)
Values	1.35	2.0	1.65	1.5	14	6 × 10–4	3 × 10–5	2 × 10–4	15
TID, Gy(Si)	ΔVTCM, V	∆Vte, V	∆Vss, V	VT0, V	V0, V		SCM, %/V		b, mA/V2
					VG = 1.5 V	VG =2 V	VG = 1.5 V	VG = 2 V
50	0.47	0.06	0.002	1.32	4.96	4.50	2.76	9.34	2.0
100	0.47	0.10	0.005	1.26	4.94	4.44	3.29	9.87	2.0
250	0.47	0.23	0.012	1.13	4.86	4.24	4.85	12.1	1.9
850	0.45	0.66	0.04	0.73	4.28	3.39	8.9	14.5	1.7

. The hydrogen and radiation sensitivities S_C(C) = |dV/dC| and S_D(D) = |dV/dD| are decreasing in both circuits. Results of experiments and calculations have shown how the conversion functions V(C) and the hydrogen sensitivity S_C depend on TID for different gate voltages and drain currents. In circuit No. 1 the sensitivity S_C increases with dose’s rising, when MISFET is working in the saturation region of the I_D (V_D) characteristic (U ≤ U_m). Calculated optimum parameters (V_G = 1.36 V and R = 3.2 kΩ) correspond to the maximum hydrogen sensitivity S_CM ≈ 46 V/% for TID < 850 Gy (Si). In circuit No. 2 the voltage V₀ (D) and dose swipe ΔV(D) are increasing with the growth of I_D, but the sensitivity S_C is not depending on I_D.

5. Conclusions

TID effects in the MISFET-based hydrogen sensors depend on electrical modes, which are determined by the type of circuit and its electrical parameters. In circuit No. 1 the conversion characteristics V (C, D) depend on the selected operating voltage V_G and value resistor R, which determine the initial value of the output voltage V₀ (D) and the hydrogen sensitivity S_C. The maximum hydrogen sensitivity S_CM is proportional to kβ·ΔV_TCM·U(D). So the sensitivity S_C is rising with the growth of TID, if parameter U is less than U_m. There were determined the optimum parameters V_G and R, corresponding to the maximum sensitivity S_CM for D < 850 Gy(Si). In circuit No. 2 the initial value of output voltage V₀ (D) depends on the current drain I_D. With the growth of I_D the voltages V₀ (D) and the swipe of dose characteristic ΔV(D) are increasing. The sensitivity S_C is equal to MISFET’s hydrogen sensitivity |dV_T/dC|, which is not depending on I_D. Under irradiation doses of D ≤ 700 Gy(Si) the hydrogen sensitivity does not change.