Operational Silicon Bipolar Inversion-Channel Field-Effect Transistor ...

DEVICE LETTERS. VOL. EDL-I. NO.9, SEPTEMBER 1986

ELECTRONIEEE

513

Operational Silicon Bipolar Inversion-Channel Field-Effect Transistor (BICFET)

Metalic

Abstract-A new transistor structure, the bipolar inversion-channel field-effect transistor (BICFET), has been recently proposed. Potential advantages include a very high frequency response and ability to be scaled to small dimensions.The first operationaldevicesof this type are described and are shown to possess characteristics of the general form predicted.

Emitter

n-type Substrate

I, IKTRODUCTION

Collector

R

ECENTLY a new transistor, the bipolar inversion-channel field-effect transistor (BICFET), has been proposed [11, [ 2 ] .Potential advantages include a transconductance and current gain at least equal to the bipolar junction transistor, a transconductance-to-input capacitance ratio better than bipolar or MOS transistors, and ability to avoid scaling limitations 111. The basic BICFET structure is shown in Fig. l(a) for an ntype substrate. In the proposed operating mode, the collector is biased with respect to the emitter so that an inversion channel tends to be induced at the semiconductor surface underlying the emitter. The inversion channel is supplied with holes from the p source with the supply rate determining the degree of inversion maintained. This, in turn, determines the electric field strength at the surface and the voltage across the semiinsulating barrier layer. The emitter is designed so that the electron emission current depends sensitively on this voltage, allowing large electron emission currents between the emitter and collector to be controlled by the much smaller supply rate of holes to the inversion channel. In the structureasproposed, thicknesses of the semiinsulating layer of order of 300 A were envisaged with thermionic emission the transport mechanism across this layer. A sophisticated processing sequence was also proposed to get the required sensitivity of this emission current to the voltage across the barrier layer. Our approach to the fabrication of an experimental BICFET has been to use an alternative technique to obtain this sensitivity. This technique has the advantages of already being reasonably well characterized [3], [4] and of being technologically less complex. In this approach, the semi-insulating layer consists of a good quality insulator sufficiently thin to allow current flow across it by tunneling rather than by thermionic emission.

k-4 n-type epitaxial

n- S u b s t r a t e

+

-Tunnel

Oxide

1

f

Aluminium

Vc>O

(b) Fig. 1. Schematic diagram of the BICFET: (a) as originally proposed; and (b) as implemented experimefitally using a tunneling emitter contact.

11. DEVICESTRUCTURE

Fig. l(b) shows the structure of the BICFET as actually implemented. It incorporates the major features of the structure as proposed [ 11, [2] with some modifications. One is the use of an n/n epitaxial substrate to minimize collector resistance. The second is the use of a thin tunneling oxide for the semi-insulating region. Apart from the previous advantages, this also solves the technological problem of providing the precise alignment required between source and emitter region. A slight increase in the tunneling oxide thickness converts it to a good insulating layer. Hence the emitter metal canoverlapthesource diffusion without the detrimental effects predicted for the originally proposed approach [l], provided the oxide is slightly thicker in these regions. Since the emitter overlaps the region of semiconductor of length X Manuscript received April 16, 1986; revised June 26,1986. This work was connecting the source to the active emitter region as shown in supported by the Australian Research Grants Scheme. Fig.l(b), there will also bean inversion channel induced The authors are with the Joint Microelectronics Research Centre, Univeralong this region once the collector voltage becomes high sity of New South Wales, Kensington 2033, Australia. enough to induce an inversion layer in the active emitter IEEE Log Number 8610330. 0741-3106/86/0900-0513$01.00

+

O 1986 IEEE

IEEE ELECTRON DEVICE LETTERS, VOL. EDL-7, NO. 9 , SEPTEMBER 1986

514

region. The disadvantages of this approach compared to a perfectly aligned structurearea slight increase in input capacitance, a decrease in the transconductance due to the resistance associated with current flow along the distance X, and a decrease in the frequency response of the device due to the corresponding transit time. The BICFET’s were fabricated on (100) oriented nin epitaxial wafers, with the epilayer 10 pm thick and 5-Q.cm resistivity. After growth of an oxide layer, windows for source diffusion were opened and boron diffused. Removal of the boron glass was followed by the growth of a 3000-A field oxide. Windows were then opened in the source and emitter regions with the emitter window extending over the boron diffusion for the source. A 400-A-thick oxide was grown and then removed from the source window and a portion of emitter window. The tunneling oxide (16 thick by ellipsometry) was then thermally grown as the result of an 850°C treatment for 20 min in dry N2. Thin oxides result from treatments in such nominally inert atmospheres by several mechanisms [ 5 ] . As well as trace quantities of oxidants (02, H20) in the source gas, there is a background pressure of such oxidants in the tube probably due to H20 ingress through the tubewalls [ 5 ] . Backstreaming during wafer loading and “native oxide” (b) layers on the wafer prior to loading are other contributors. Fig. 2 . Experimentalcommonemitter output characteristics of a BICFET with an active emitter area of 5 X 25 pm2. The vertical axis is collector After thin oxide growth, photolithographically defined A1 of 1 IC while the horizontal axis is collector voltage VCEwith the steps current pm thickness was deposited in the source windows as well as representing differentvalues of source currentIs: (a) characteristics at high over the wafer rear and sintered. Finally, the AI emitter currents and voltages ( I c = 1 d i d i v . ; V , = 5 Vldiv.; I S = 10 pAlstep; current gain = 100ldiv.); (b) at small currents and voltages ( I c = 50 PA/ contact was deposited. +

A

111. DEVICECHARACTERISTICS Fig. 2(a) shows the experimental common emitter output characteristics of a BICFET with the structure of Fig. l(b). Nominal values of X and L e for this device are 2.5 and 5 pm, respectively, with an emitter width of 25 pm. The device has a maximum current gain of about 120 with a breakdown voltage above 60 V. This breakdown is nondestructive provided currents are controlled.It is attributed to the breakdown of the reverse biased source-collector junction as in the basecollector junction of a conventional bipolar transistor. Fig. 2(b) shows these characteristics at small values of VcE, where the BICFET’s have been predicted to behave differently from bipolar transistors [l]. As predicted, there is a voltage VcE(cut-in) below which the collector current I C is negative. This voltage has a value in the present case of about 0.6 V. The surface under the emitter isnot inverted until voltages larger than this resulting in small electron emission currents. On the other hand, the source-collector diode is forward biased giving a relatively large current flowing from the source to the collector and a negative I C . With this difference, the characteristics also agree well with those predicted in an earlier analysis of the tunneling emitter contact [3]. The maximum current gain of 120 agrees with earlier theoretical predictions [3] and photodetector measurements [4].The rapid increase in gain with increasing I C apparent in Fig. 2(b) has also been discussed previously [4] and is attributed to surface state effects [3]. A final feature concerns the voltage across the thin oxide to which the emission current is very sensitive. Hence the

div.; VcE = 0.2 Vidiv.; I s = 10 pAistep;current gain = Sidiv.). Note < 0.6 V. that IC is negative for ,/I

collector current rather than collector voltage is the most appropriate indicator of its value. The saturation of IC at high collector voltage shows that almost all this voltage appears across the source-collector depletion region. Hence, oxide breakdown does not determine voltage handling capability. However, the oxide voltage increases as IC increases. Experimentally, increasing I C above about lo4A/cm2 for the present devices causes an irreversible reduction in the current gain of the device. In view of recent work on the breakdown of thin oxides [6], one explanation could be an increase in surface states due to excessive oxide fields. Note that the bonding structure in thin oxide films may be more resistant to stressing than in thicker films [6]. IV. COMPARISON TO OPTIMIZED BICFET Exceptional high-frequency performance has been calculated for an optimized BICFET with operating frequencies of 500 GHz predicted [ 2 ] . It is of interest to see whether the practical structure of Fig. l(b) retains this potential. One obvious limitation is the inoperative channel length X . The transit time along this channel would alone limit operating frequencies to a few gigahertz for the present devices with X = 2.5 pm. A self-aligned approach (e.g., utilizing the different oxidation rates of doped and undoped regions) would remove this limitation. A second apparent limitation is the relatively high sourceemitter input capacitance due to the use of a tunneling oxide

MORAVVEJ-FARSHI AND GREEK: OPERATIONAL SILICON BICFET

515

about 20 times thinner than proposed. However, this increased input capacitance reduces the calculated intrinsic cutoff frequency at a given transconductance by only a factor of 3 from that of a fully optimized BICFET duetothe low dielectric constant of the oxide and the decreased sensitivity to source-collector capacitance. The transconductance of an optimized BICFET is given by q I c / k T where k T / q is the thermal voltage [2]. An expression derived for the theoretical transconductance of the present structure has the same form at low values of ICbut suggests an inferior transconductance at high-current densities. The measured transconductance of the device of Fig. 2 was five times smaller than qIc/kTat high IC values (1-5 mA). Combined with the increased input capacitance, this gives a calculated 15 times reduction in the intrinsic cutoff frequency for a given value of IC. This is not a serious limitation in itself due to the extremely high intrinsic cutoff frequency (lo4 GHz) predicted for an optimized BICFET [2]. However, these high cutoff frequencies were calculated on the basis of operating current densities of order of lo6 A/cm2 while the safe operating area of the present devices lies below lo4 A/cm2. In principle, the safe operating current level could be increased merely by decreasing tunneling oxide thickness (about an order of magnitude increase for each 2-A decrease). Hence the major improvements required in the present devices to approach the highfrequency potential of the BICFET approach are believed to be the implementation of self-alignment between the source and the emitter and a decreased tunneling oxide thickness. ~

V . CONCLUSION Experimental bipolar inversion-channel field-effect transistors (BICFET’s) have been shown to possess characteristics

similar to those recently predicted theoretically [11, [ 2 ] . The experimental devices were fabricated much more simply than originally proposed [I] by taking advantage of the current gain possible from tunneling contacts [3], [4]. The simpler experimental approach was shown to be capable of producing devices with performance approaching the full potential of the BICFET approach.

ACKNOWLEDGMENT The authors acknowledge the assistance of W, F. Guo in characterizing the present devices. The Joint Microelectronics Research Centreis supported under the Commonwealth Special Research Centres Scheme.

REFERENCES [l] G. W. Taylorand J. G. Simmons,“The bipolar inversion channel field-effect transistor (BICFET)-A new field-effect solid-state device: Theory and structures,” IEEE Trans. Electron Devices, vol. ED-32, no. 11, pp. 2345-2367, Nov. 1985. [2] G. W. Taylorand J . G. Simmons, “Small-signal model and highfrequencyperformance of theBICFET,” IEEE Trans. Electron Devices, vol. ED-32, no. 11, pp. 2368-2377, Nov. 1985. [3] M. A . Greenand J. Shewchun,“Current multiplication in metalinsulator-semiconductor (MIS) tunnel diodes,” Solid-state Electron., vol. 17, p. 349, 1974. J. Shewchun,“Frequency [4] M. A . Green, V. A. K. Temple,and response of the current multiplication process in MIS tunnel diodes,” Solid-State Electron., vol. 18, pp. 745-752, 1975. [ 5 ] E. A. Irene,“Theeffectoftraceamounts of water on the thermal oxidation of silicon in oxygen,” J. Electrochem. Soc., vol. 121, p. 1613,1974. N. Kawamara, “Reliability of 6-10nm [6] Y. Hokari,T.Baba,and thermal SiOz films showing intrinsic dielectric integrity,”IEEE Trans. Electron Devices, vol. ED-32, no. 11, pp. 2485-2491, Nov. 1985.