918 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 5 . MAY 1993

A Silicon Double Switching Inversion-Controlled Switch for Multiple-Valued Logic Applications

Y . K. Fang, Ching-Ru Liu, Kuin-Hui Chen, and Jun-Dar Hwang

Abstract-A new device with the structure of metal/thin in- sulator/crystalline silicon (n+-p)/thin insulator/metal (MIS- SIM) has been demonstrated to possess a double switching characteristics, which is expected to generate multiple stable states easier than the conventional resonant tunneling devices with multiple negative resistance for multiple-valued logic ap- plications. Based on current-voltage measurements with or without light irradiation, and under negative gate-biased con- dition, the operation mechanism of the MISSIM structure is proposed and illustrated in detail.

I. INTRODUCTION ECENTLY, resonant tunneling (RT) devices with R multiple negative resistance (MNR) have been stud-

ied for a variety of applications, including ultra-high speed analog-to-digital converters, parity bit generators, and multiple-valued logic circuits [ 11-[4]. In these applica- tions, the multiple-valued logic is the most attractive for its ability to reduce circuit complexity. However, in this application, to ensure that the multiple stable states can exist in MNR characteristics, the largest valley current shall not exceed the smallest peak current, i.e., it requires nearly equal peak currents (see Fig. 1 (a) and (b)) [3]- [ 5 ] . It is difficult to obtain this type of MNR from a gen- eral double-barrier resonant tunneling structure. Hence, many approaches have been proposed to avoid this diffi- culty, e.g., to use two double-barrier diodes in parallel with external bias [3], or to use a triple-well resonant tun- neling diode in which two resonance voltages can be con- trolled separately, and obtained a nearly equal peak cur- rents at 219 K [4], thus leading to the realization of multiple-valued logic function in a single diode. How- ever, these approaches make the structures of the MNR devices complicated or hard to manufacture. In this study,

' t

r---- I t I /

Fig. 1 . (a) Multiple negative resistance characteristics with nonequal peak currents cannot generate multiple stable states. (b) Multiple negative resis- tance characteristics with nearly equal peak currents generate multiple sta- ble states. (c) Multiple stable states can be easily defined in a double- switching MNR characteristics.

This paper reports for the first time that the double switching NR characteristics can be generated by a metal/ thin insulator/crystalline silicon (n+)/crystalline silicon (p)/thin insulator/metal (MISSIM) structure, which has been developed from the modification of MISS switch diode, called the inversion-controlled switch [6], [7], with

we propose another way to obtain multiple stable states, i.e., from the S-type double switching characteristics. As illustrated in Fig. l(c), these stable states can be easily defined just by a proper choice of load line; they do not need to limit the peak currents to be nearly equal again. Thus the structure of the device can be simplified.

an additional injection emitter to generate the second Manuscript received March 4 , 1992. This work was supported by the

National Science Council, Republic of China, under Contract 81-0404- E006-111. The review of this paper was arranged by Associate Editor K .

The authors are with the VLSI Technology Laboratory, Department of

switching operation. The MISS is selected for its simple structure and quick switching 'peed 161. in the

Shenai.

Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China.

manufacture of MISSIM samples, the amorphous silicon

replace the conventional thermally grown SiO, as thin conducting insulator. Both amorphous silicon (a-Si : H)

grown in the plasma-enhanced CVD system was used to

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0018-9383/93$03.00 0 1993 IEEE

FANG er al.: A SILICON DOUBLE SWITCHING INVERSION-CONTROLLED SWITCH 919

and Si02 have been found in preparing a successful MISS switch diode, except for the different thicknesses that were used, i.e., 20-70 A for Si02 and 200-2000 A for a-Si : H [7]. Using thicker a-Si : H layer as insulator can obtain higher reproducibility for its low-temperature deposition process. The low-temperature process inhibits the outdif- fusion of the doped impurities of the substrate, while the uniformity and thickness control of the a-Si:H layer grown in the PECVD system are suitable for deposition of a thicker a-Si:H layer, especially in preparation of these MISSIM samples which need double deposition of thin insulator.

Using the developed MISSIM structure to generate multiple stable states for multiple-valued logic applica- tions possesses the following advantages: a) the structure is very simple and easy to prepare, b) based on econom- ical silicon material, low cost can be expected, c) the de- sign parameters of the structure are not so critical as those used in design of GaAs resonant tunneling devices, thus reproducibility is higher.

11. DEVICE MANUFACTURE A N D MEASUREMENT

The devices, as schematically shown in Fig. 2 , were fabricated on ( 1 1 1 ) oriented n+/p silicon substrate with a resistivity of 3.14 and 12.4 Q . cm, respectively. The thickness of the substrat! are 10 and 150 pm, respec- tively. At first, a 300-A intrinsic amorphous silicon (a-Si : H) layer was grown on both sides of the substrate in a plasma-enhanced chemical vapor deposition system [8]. The growth temperature and eressure are 250°C and 1 torr, respectively. Next, 5000 A of gold were evapo- rated on both tops of the a-Si : H layers. Then the current- voltage characteristics were measured on Tektronix 370A curvetracer.

111. EXPERIMENTAL RESULTS AND DISCUSSIONS A . Operation Mechanism

Since both thin a-Si : H insulators are conducting when the anode of the MISSIM device is positively biased with respect to the grounded cathode, the c-Si (n+)/c-Si (p) junction is reversed and no current is transported. How- ever, if the sample is biased negatively, the n f / p junction is forward, but the current is still very low due to the ex- istence of surface depletion regions in both c-Si(n') and c-Si(p) layers 171 as illustrated in Fig. 3(a). When a suf- ficiently high voltage is applied, say V = V, (the first switching voltage), the surface depletion regions under both metal electrodes extend to the n+/p junction. Im- mediately after the extension of surface depletion regions, the electrical fields in a-Si : H insulators are insufficient to allow the relatively larger injected holes or electrons to pass through both thin insulators, since the current is tun- neling-limited. Consequently, an incremental voltage in- crease will cause holes and electrons to accumulate at both a-Si : H/c-Si (n') and a-Si : H/c-Si (p) interfaces, thus moving the surface regions from depletion toward inver-

n+ c-si k! 300A a-SkH

Fig. 2. Schematical diagram of MISSIM structure.

sion, as illustrated in Fig. 3(b). In this state, the presence of the inversion layer can limit the width of the surface depletion to approximately its small thermal equilibrium value [7], except that the surface depletion region in the c-Si (p) layer possesses a wider zone due to its lower dop- ing. Both the surface potential 4, and the voltage across the c-Si(n+) layers decrease 161, [7]. Furthermore, the voltage across the a-Si : H layer under the anode electrode is higher than that of the a-Si : H layer under the cathode electrode, since the width of the c-Si (p) surface depletion region is wider than that of the narrow c-Si(n+) surface depletion region. A higher electrical field exists in the a-Si : H thin layer under the anode metal, allowing a larger electron tunneling current to go through the nf /p junc- tion, thus keeping the junction forward even more. The forward n f / p junction provides more holes to arrive at the a-Si : H/c-Si (n'). The positive feedback occurs. The re- generative feedback on the left MISS structure (i.e., an- ode/thin insulator/c-Si (n+)/c-Si (p)) causes the device to display a negative-resistance region [6], as indicated by curves a and b in Fig. 3(b). This is the generation of the first switching and the device transfers from the high impedance state into the median impedance state. In this state, the left MISS structure is in the ON state, and the applied voltage almost loads on the right MISS structure (i.e., c-Si (n+)/c-Si (p)/thin insulator/cathode). To gen- erate the second switching, a larger voltage is applied to enhance the inversion of electrons on the surface of c-Si(p) layer and to shrink the surface depletion region. This in turn increases the voltage across the a-Si : H in- sulator under the cathode electrode, and the second re- generative feedback occurs on the right MISS structure as indicated by curves b and c in Fig. 3(c). After that, the device transfers from the median impedance state into the low impedance state. Fig. 4 shows a photograph of typi- cal current-voltage characteristics at room temperature. A very significant double-switching phenomenon is found.

The I / V characteristics of Fig. 4 can be utilized in the circuit which is shown in the insert of Fig. l(c). This cir- cuit is analogous to a successful example in multiple-val- ued logic application [3]. Under a supply voltage V,, and load resistor RL, the load line inserts the I / V curve at five different points. The Ql-Q3 are stable operating points. Since these points are located on the positive slope parts of the I / V curve. The output voltages are V , , V,, and V3 corresponding to the Q , , Q2, and Q3 points, respectively.

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 5, MAY 1993

I M I N P 1 M I

" U ' inversiot I ' (c)

Fig. 3 . Bistable current-voltage characteristics, relative widths of surface, and junction depletion zones and energy band dia- gram of MISSIM structure. E,, E , , E, denote Fermi level, conduction band edge, and valence band edge, respectively. (a) High impedance state. (b) Median impedance state. (c) Low impedance state.

These states are stable and can retain the last voltage in- formation impressed on it. Thus the circuit can be used as a memory element in a three-state logic circuit where Vl-V3 are the voltages corresponding to the three logic states. By applying a short voltage pulse, this circuit can

voltage of the Q3 state is the smallest (V3 < V2 < VI). Thus the circuit significantly reduces the number of com- ponents in a three-state memory which has been con- structed with four transistors and six resistors [ 5 ] .

be switched from one state to another. The operation mechanism of the proposed circuit is similar to the re- B. Light Irradiated and Gate-Biased Condition

ported resonant tunneling MNR devices [3]. Except in the resonant tunneling MNR device, the voltage of the Q3

state is the largest ( V , > V2 > VI) , while in our case, the

To support the above mechanism, the device was irra- diated by a light source (632.8 nm) with 0, 150, and 300 pW/cm2 incident power from the anode side. As shown

FANG et rrl A SILICON DOUR1 b SWITCHING INVERSION-CONTROLLED SWITCH 92 I

Fig. 4 . Photograph of measured current-voltage characteristics of MIS- SIM device for anode biased negatively with respect to grounded cathode at room temperature.

Fig. 6. Photograph of measured three-terminal MISSIM current-voltage characteristics under negative gate bias (V, = - 1 V/step) with respect to cathode at room temperature. The outermost curve is under zero gate volt- age.

Fig. 5 . Photograph of measured current-voltage characteristics under dif- ferent power light irradiation from the anode side at room temperature. The outermost curve is under dark, and the innermost curve is under 300 pW/cm’ light irradiation.

in Fig. 5 , the first switching voltage is decreased with increasing incident light power, but the second switching voltage is not affected by the incident light power. Clearly, the electron-hole pairs generated in the c-Si (n’) surface depletion region by incident light enhance the establish- ment of the first regenerative feedback early, thus de- creasing the first switching voltage. However, the tunnel- ing of electrons in the MISS structure under the cathode electrode is kept the same, because no light has been ab- sorbed so that the second switching voltage is not af- fected.

In addition, if a gate electrode is formed on the c-Si (n’) layer and biased negatively, both the first and second switching voltages are decreased with increasing gate voltage, as illustrated in Fig. 6. This can be realized from the band diagram in Fig. 3(a)-(c). Since the negative bias applied at the gate lowers the p-n junction barrier, it, in turn, enhances the diffusion transport of electrons and holes, and shrinks the surface depletion region of both

c-Si (n’) and c-Si (p) layers. So the establishments of the regenerative feedbacks are promoted and both switching voltages of the device are decreased.

IV. CONCLUSION

We have demonstrated that the MISSIM structure pos- sesses a controllable double-switching characteristics which is expected to generate multiple stable states for multiple-valued logic application easily. Also the opera- tion mechanism of the structure has been proposed, based on the current-voltage measurements under various con- ditions. Thus we have shown that a double-switching MNR characteristics device can be prepared using silicon material with a simple structure easy to manufacture.

REFERENCES

[ I ] F. Capasso and R . A . Kiehl, “Resonant tunneling transistor with quan- tum well base and high-energy injection: A new negative differential resistance,” J . Appl . Phys. , vol. 58, pp. 1366-1368, 1985.

[2] F. Capasso, “New high speed quantum well and variable gap super- lattice devices, ” in Picosecond Electronics and Optoelectronics. G . A. Mourou, D. M. Bloom, and C. H . Lee, Eds. Berlin: Springer, 1985. pp. 112-130.

[3] F. Cappasso, S. Sen, A . Y . Cho, and D. Sivco. “Resonant tunneling devices with multiple negative differential resistance and demonstra- tion of a three-state memory cell for multiple-valued logic applica- tions.” IEEE Electron Device Lett., vol. EDL-8, no. 7, pp. 291-299, 1987.

141 T. Tanoue, H . Mizuta, and S . Takahashi, “ A triple-well resonant- tunneling diode for multiple-valued logic application,” fEEE Electron Device Left . , vol. 9 , no. 8, pp. 365-367, 1988.

IS] C. Rine, Ed., Computer Science und Multiple-Vulued Logic. Am- sterdam, The Netherlands: North Holland, 1985, 415 pp.

[6] S. M. Sze, Ed., Physics of Semiconductor Devices, 2nd ed. New York: Wiley. 1981, 549 pp.

171 H. Kroger and H. A . R . Wegener, “Steady-state characteristics of two terminal inversion-controlled switches,” Solid-State Electron., vol. 2 I , pp. 643-654, 1978.

[8] Y. K . Fang, S. B. Hwang, Y . W. Chen, and L. C. Kuo, “A vertical- type a-Si : H back to back diode for high speed color image sensor,” f E E E Electron Device Lett., vol. 12, p 172, 1991.

922 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 5, MAY 1993

Y. K. Fang was born in Tainan, Taiwan, Repub- lic of China, on October 10, 1944. He received the B.S. and M.S. degrees in electronics engi- neering from National Chaio Tung University in 1957 and 1959, respectively, and the Ph.D. de- gree in semiconductor engineering from the Insti- tute of Electrical and Computer Engineering, Na- tional Cheng Kung University, in 1981.

From 1960 to 1978, he was a Senior Designer and Research Engineer in the private sector. From 1978 to 1980. he was an Instructor, then an As-

sociate Professor and a Professor in 1981 and 1986, respectively, in the Electrical and Computer Engineering Department, National Cheng Kung University.

Dr. Fang is a member of Phi Tau Phi.

Ching-Ru Liu was born in Taiwan, ROC, on March 12, 1966 He received the B S and M S degrees in electrical engineering from National Cheng Kung University, Taiwan, in 1988 and 1990, respectively

Since 1990 he has been working toward the Ph.D degree in the Institute of Electrical Engi- neering, National Cheng Kung University. His current study is to research and develop the amor- phous-crystalline silicon heterojunction devices for electrooptical system application. t

Kuin-Hui Chen was born in Taiwan, ROC, on November 20, 1964. He received the B.S. and M.S. degrees in electrical engineering from the National Cheng Kung University, Taiwan, in 1988 and 1990, respectively.

He is working toward the Ph.D. degree in elec- trical engineering at Cheng Kung University. His current research interest is in the heterojunction between compound and amorphous materials.

Jun-Dar Hwang was born in Taiwan, ROC, on February 18. 1960. He received the B.S. degree in electrical engineering from National Taiwan Institute of Technology in 1986 and the M.S. de- gree from the National Cheng-Kung University, Taiwan, in 1990.

He is currently working toward the Ph.D. de- gree in electrical engineering at Cheng-Kung Uni- versity. His research interests is in SIC material devices.