quantum transistors and circuits break through the barriers

Quantum Transistors and Circuits

Break Through the Barriers

uantum effects are unavoidable in devices with

layerthicknesses smallerthan 100nanometers But the term “quantum device” has been re- served for those devices that actually rely on quantum effects for their operation [l]. The classic example is the tunnel diode, invented by Leo Esak~ in the late 1950s, which relies on electron tunneling through the bandgap of a p-n junction.

When AlfredCho and John Arthur pioneered the development of molecular- beam epi taxy (MBE) a t A T & T Bell Laboratories in the late 19605, they in- itiated a technology that now permits

device designers to grow ultrathin (down to approximately 5 A ) layers of various materials with atomically sharp interfaces [2]. This led to the development of “vertical” quantum devices, in which the current flows perpendicular to the layers instead of along them. This vertical technology is now producing resonant tunneling diode5 and transistors that have the potential to become practical devices.

Resonant tunneling transistors, in particular, have shown considerable potential for reducing circuit complexity in several analog and digital applications. In the long run, they might also open the door for revolutionary computing ar-

18 8755-3996/91/0500-18$1.00.1991 IEEE Circuits & Devices

Steady progress may allow transistors

based on resonant tun ne1 i ng to significant 1 y

i n crease c i rcu i t performance and

c u I component c ou r i t in some digital

and analog applications

by F. Capasso, S . Sen, L. M. Lunardi, and A. Y. Cho

chitectures based on multiple-valued logic.

Functional Devices The resonant tunneling transistor is an example of the rapidly growing family of functional devices. The concept of “functional device” was introduced by J . A. Morton of Bell Laboratories more than 20 years ago in a remarkable pioneering paper [3]. The key characteristic of such devices, in Morton’s words, is that “they promise to reduce greatly the number of elements and process steps per function when their capabilities are properly matched to an old or new system function.” Morton provided a few examples of

functional devices, including the tunnel diode. The charge-coupled device (CCD), invented and developed in the 1970s, is another early functional device. It merits the designation because it can perform a wide range of electronic functions including image sensing and signal processing.

There is no question that Morton was ahead of his time. His vision, which relied strongly on dramatic progress in growth t e c h n i q u e s , m a t e r i a l s c i e n c e , and semiconductor physics, is only now gradually starting to become a reality. In pa r t i cu la r , t he adven t of advanced epitaxial growth techniques such as MBE, metallorganic chemical vapor deposition

May 1991 19

1 .o TRANSMISSIVITY

50

‘. 2 10

0 0.5 1.0 1.5 2.0 VOLTAGE (V)

(d)

(MOCVD), and bandgap engineering [4] have made possible the development of a new class of materials and a new class of heterojunction devices, both of which have unique optical and electronic properties. In- vestigation of the novel phenomena that arise when layer thicknesses become com- parable to the de Broglie wavelength of electrons, i.e., the quantum size effect, has proceeded in parallel with the development of novel devices such as quantum-well (QW) lasers.

Resonant Tunneling Diodes The resonant-tunneling diode, first demonstrated in 1974 by Esaki, Leroy Chang and Raphael Tsu at IBM, consists of two potential barriers in series, separated by a potential well (Fig. la) [5] . The kinetic energy of an electron’s motion perpendicular to the layers is quantized, just as one would expect for a particle in a box. In the plane of the layers, however, the electron is free and it behaves semiclassically. As a result, two- dimensional energy subbands En(k) are formed:

E, ( k ) = E!+ h2k 2/ 2m*

where Eo, is the nth energy level (shown in Fig. la) given by the quantization of the perpendicular kinetic energy; and the second term is the kinetic energy of the elec-

1. (a) Energy band diagram of resonant tunneling double barrier. (b) Transmissivity as a function of perpendicular energy for a double barrier. (c) Band diagram of a double barrier with applied bias. (d) Current-voltage characteristic of a resonant tunneling diode wn- sisting of AlAs barriets and an InGaAs quantum well. (Part (d) wurtesy of PrvfessorC. G. Fonstadt )

tron’s free motion parallel to the layers, with wavenumber k and effective mass m*.

The energy levels, Eon, correspond to a half-integer number of electron de Broglie wavelengths across the width of the quantum well. The barriers are thin enough that electrons can tunnel through them into and out of the quantum well. This structure is often compared to a Fabry-Perot optical in- terferometer. The two barriers play the role of partially transparent mirrors through which light is coupled into and out of a resonant cavity. As we might expect, the transmissivity for electrons through the double barrier shows resonant peaks when the perpendicular kinetic energy of the incident electrons equals Eon. At these resonant energies, the transmissivity for a symmetric double barrier reaches 100 percent, even though the transmissivity for a single barrier might be less than 1 percent (Fig. Ib). This striking resonant enhancement of electron transmission is the result of constructive in- terference between multiply reflected waves.

This description in terms of Fabry- Perot interferometry is somewhat ideal- ized. In many realist ic devices the resonant enhancement of the transmission is considerably weakened by scattering processes. Multiple reflected waves do not interfere constructively if they have been reflected more than a few times because

scattering events destroy their phase coherence.

The current through a resonant-tunneling diode is measured when a voltage is applied across the double barrier through the heavily doped contact layers. The applied bias voltage lowers the resonant energy of the cavity relative to the energy of the incident electrons (Fig. 1 c). Once the resonant energy has fallen below the range of incident energies, below the conduction-band edge in the emitter, there is a sharp drop in the current as the applied voltage is increased (Fig. Id). This negative differential resistance is a useful feature for circuit applications such as high-frequency oscillators. Such applications have been pioneered by T.C.L.G. Sollner at Lincoln Laboratory [l]. E. R. Brown and his colleagues at Lincoln Laboratory recently demonstrated oscilla- tion frequencies up to 675 GHz in resonant tunneling diodes with AlSb barriers and InAs wells [6]. Clifton Fonstadt and his coworkers at MIT observed one of the highest peak-to-valley ratios seen to date: 30:l at room temperature, in a structure they fabri- cated having an InGaAs well and AlAs barriers (Fig. Id) 171.

Transistors: the Main Event Although resonant-tunneling diodes may find application in oscillators, logic circuits require devices with input-output isolation to achieve gain and to avoid loading of the input stages. This isolation is best accom- plished with a transistor. The early demise of the tunnel diode as a potentially useful device for integrated circuits was due not only to its limited reliability and the diffi- culty of reproducing its I-V characteristic, but also to its two-terminal nature. No one was able to incorporate the Zener tunneling concept into a transistor. On the other hand, a resonant-tunneling double barrier can be readily used in a transistor. Furthermore, resonant tunneling diodes and transistors made on an epitaxially grown wafer with atomic control of layer thicknesses, made possible by MBE, have an I-V characteristic that is intrinsically more reproducible than a tunnel diode’s. This reproducibility is cru- cial for digital circuits.

In general terms, a resonant-tunneling transistor is a device that uses an applied control voltage to modulate the difference between the energy levels of the quantum well and the energy of the incident electrons. The resonant-tunneling current through the

20 Circuits & Devices

double barrier can be made to peak at one or more values of the control voltage, corresponding to the different energy levels. A transistor with such a current-voltage characteristic has multiple on and off states, corresponding respectively to the peaks and valleys of the I-V curve. This makes such devices attractive for implementing multiple-valued logic. Al- though multiple-valued logic has been the subject of considerable theoretical inves- tigation, all proposed and demonstrated circuit architectures to date have relied on conventional two-state devices or tunnel diodes. Multiple-valued logic may one day reduce the interconnection complexity of integrated circuits.

The first resonant tunneling transistors were demonstrated at Fujitsu Laboratories by N. Yokoyama and his group (Fig. 2a) 181. In one of these transistors, electrons tunnel resonantly through a quantum well in the emitter and then move through the thin (300 A) base with hardly any scatter-

2. Energy band diagrams of various resonant tunneling transistors under operating conditions. (a) Hot- electron resonant tunneling transistor. (b) Resonant tunneling bipolar transistor with quantum well in the base. (c) Multistate resonant tunneling bipolar transistor with multiple quantum wells in the emitter.

ing, crossing over into the collector. This configuration has exhibited excellent peak- to-valley ratios and high-frequency operation in excess of 20 GHz. The design of this device is critical, for room temperature operation, since it relies on ballistic transport in the base. Hot electrons that scatter in the base may not be able to surmount the collector barrier, thus degrading the gain of the device.

Conceptually, the simplest way to build a resonant-tunneling transistor is to form a contact with the heavily doped quantum well of a double barrier, as originally sug- gested by Ricch and Solomon 191. This contact would serve as the control terminal. Unfortunately, this approach is fraught with major technical difficulties, and attempts to implement it have not yet succeeded. An approach that is conceptually similar to Riccb and Solomon’s but technologically much easier is to incorporate a double barrier into the base of a bipolar transistor, as first proposed by Capasso and Kiehl [ 101.

3. Transfer characteristics (a) and output characteristics (b) in

the common-emitter configuration of a multistate RTBT. The vertical

axis represents the collector current. The horizontal axis represents the base-emitter

voltage in (a) and the collector-emitter voltage in (6).

The collector-current scale in (b) is 20 mNdiv.

A resonant-tunneling bipolar transistor (RTBT) has a promising energy-band diagram (Fig. 2b). As the base-emitter voltage i s increased, resonant tunneling through each sub-band first reaches a max imum and is then suppressed as the bottom of the sub-band, (i.e., the energy level shown in the well in Fig. 2b) is lowered below the conduction-band edge in the emitter. This mechanism should produce multiple peaks in the collector current as a function of base-emitter voltage (i.e., negative transconductance).

Capasso and his coworkers at Bell Labs reported the first operation of a RTBT in 1986 [ 1 I] . We demonstrated room- temper- ature operation with a single peak in the I-V curve and a 3: 1 peak-to-valley ratio, because resonant tunneling through the first resonance was suppressed. But it turns out that the band structure of Fig. 2b makes it difficult to obtain multiple peaks of compa- rable height, which are desirable for circuit applications.

May 1991 21

4. Current gain (MI) as a function of

frequency for different bias points in the common-emitter

configuration, with the corresponding

collector-current densities JC. Curve (a)

refers to an operating point after the second peak in the common

emitter characteristics. The fTis 24 GHz.

Curve (b) refers to a bias point between the

two peaks. For curve (c), the base current of 40 mA is such that no

peak appears in the corresponding

common-emitter chatacteristic.

To solve this problem, we introduced a new structure (Fig. 2c). In this device, two or more double barriers are placed in the emitter rather than in the base. To obtain multiple peaks in the plot of collector current versus base-emitter voltage we exploit space-charge buildup in the quantum wells. The attendant electrostatic screen- ing makes the field across the two double barriers spatially nonuniform. The field is higher in the well closer to the base. Thus, as the bias is increased, resonant tunneling through the two wells is suppressed at two different voltages, which yields two peaks in the I-V transfer characteristic. (When resonant tunneling is suppressed at only one well, current continuity is ensured by the inelastic tunneling current.) The structure was grown by MBE using AlInAdGaInAs heterojunctions for the two double barriers in the emitter, and InGaAs for the p-type base and n-type collector. The barriers and wells had a thickness of 50 A.

How Multistate Transistors Perform The multistate RTBT's transfer chardcteris- tic at 300 K displays two peaks in accor- dance with the physical operation discussed above (Fig. 3a). The common-emitter direct characteristics show that at low base currents (IS), and therefore at low base-emitter voltages (VBE), the device behaves as a conventional bipolar transistor with a large differential current gain of 100 at 300 K (Fig. 3b). As IB(VBE) is increased beyond the flat-band condition, the excess applied volt-

I I I I I I l l 1 I I I 1 1 1 1 l ~ I I I I I I I I

a

b - C -

-

I I I 1 1 1 1 1 1

1 .o 10 F REQU ENCY ( GHz)

100

5. (a) Frequency multiplier using the multiple-state RTBT. (b) Corresponding power-output spectral response. The vertical scale is - 10 db/div from the top of the horizontal line (0-dbm reference). The frequency span is 1.8 GHz (180 MHddiv). The device is at room temperature.


age starts appearing across the series of DBs in the emitter. As RT through them quenches sequentially (at threshold base currents Iethi and I ~ t h 2 ) , the electron current across the base-emitter junction drops abruptly. This produces sudden quenching of the current gain at these threshold base currents. As a result, the collector current IC is also quenched, giving rise to two NDR regions (Fig. 3 4 . The highest peak-to-valley ratio we’ve observed is 6: 1 at room temperature, and 22:l at 77 K. The small-signal current gain of the transistor at room temperature in its second (1.2 mA < IB< 1.6 mA) and third (IB > 1.6 mA) operating regions is reduced to 40 and 20, respectively. This result is expected because the hole current flowing from the base toward the emitter increases with increasing VBE, thus reducing the injection efficiency. The reduction of the current gain is less pronounced at 77 K because the ther- mionic flow of holes is much lower at that temperature. The flow of holes from the base to the emitter can be minimized by inserting an n+ A10.48In0.52As layer between the stack of DBs and the base, similar to using a wide-gap emitter in a heterojunction bipolar transistor.

The current gain (h21) can be displayed as a function of frequency for different bias conditions. This was ob- tained by S-parameter measurements in the range 0.5 to 26.5 GHz using an auto- matic network analyzer (HP 8510B) (Fig. 4).

Circuit Applications

Frequency Multipliers The transfer characteristic of Fig. 3a was used to design a frequency-multiplier circuit (Fig. 5 4 . As the input voltage is increased, the collector current increases. This produces a decrease in the collector voltage until the device reaches the negative transconductance regions where sudden drops in the collector current and consequent increases in the output voltage are observed. So, if VBB is adjusted to bias the base-emitter junction between the two peaks of the common-emitter transfer characteristic, the frequency of triangular input waves will be multiplied by a factor of three, and the frequency of sinewaves by a factor of five [12]. Unlike two-terminal multipliers, the output signal in this

May 1991

4 - B I T DIGITAL

INPUT

= 4’5v

h

case is both referenced to ground and iso- lated from the input. In applications where two-terminal devices cannot be used, conventional frequency-independent multipliers require the use of a phase-lock loop and a digital frequency divider. For frequency multiplication at high frequencies, we biased the multistate transistor described earlier in this article in the common-emitter configuration with VCE = 3.2 V and the characteristic impedance of the 5 0 4 line as the load. The base-emitter junction was dc

6. (a) Four-bit parity generator circuit using a multistate RTBT (Ro = 15 M2, 5 1 = 6.9 M 1 , h = 2.4 Mz, and Rc = 15R). Note that the same circuit, when used with only two input bits, will act as an exclusive NOR gate. (b) Collector (top trace) and base (bottom trace) waveforms in parity generator circuit at 77 K. (c) Collector (top trace) and base (bottom trace) waveforms at 300 K.

biased at 2.0 V via a bias tee, and a 350-MHz sinewave was applied to the base. The amplitude was adjusted to achieve a base- emitter voltage swing large enough to bring the device into the negative transconductance regions of the transfer characteristic. We displayed the output power versus frequency on a spectrum analyzer. The amplitude of the fifth harmonic is much larger than that of the fourth and the sixth (Fig. 5b). The efficiency of the multiplier, defined as the power ratio of the fifth

23

harmonic to the fundamental, is about 15 percent.

Parity Generators Parity generators are commonly employed in communications systems to detect transmission errors. An elegant 4-bit parity-generator c i rcui t can be built around a multiple-state RTBT (Fig. 6a) [ 121. The voltages of the digital word's four input bits are added at the transistor's base node by the resistive network to generate a step-like waveform. The quiescent bias of the transistor, which is adjusted by the resistance R B I and the values of the resistances Ro, is chosen so the operating points of the transistor are selected alternately at low and high collector current levels (valleys and peaks of the transfer characteristics) at the successive steps of the summed voltages. In our circuit, Ro = 15 kR, RBI = 6.9 kR, R B ~ = 2.4 kR, Rc = 15 R, and Vcc = 4.5 V. The output voltage at the collector, whether high or low, will depend on whether it is an even or odd number of input bits that are set high. The result is a 4-bit parity generator using only one transistor, instead of the 24 transistors needed in an optimized conventional circuit using three exclusive OR gates. The circuit converts the 4-bit binary data to a multistate signal before sending it to the device for processing. This method is equivalent to processing all 4 bits in parallel, which is faster than the conventional sequential processing of binary logic. Potentially, multistate processing elements could replace clusters of circuits in existing binary-logic systems.

We tested the circuit with a pseudo- random sequence of 4-bit binary words. Experimental results at 77 and 300 K are shown in Figs. 6b and 6c, respectively. T h e top t r a c e s s h o w t h e o u t p u t waveforms and the bottom traces show the waveforms on the transistor's base. Assigning the dotted line in the upper trace as a logic threshold level, the output is low for the second and the fourth voltage levels at the base, and high for the others.

Analog-to-Digital Converter Among the other circuit applications of the multistate RTBT, the analog-to-digital converter is potentially one of the most significant (Fig. 7) [12]. The analog input is simultaneously applied to an array of RTBTs having different voltage-scaling networks. To understand the circuit's

7. Analog-to-digital converter circuit using multistate

RTBTs.

ANALOG INPUT

I

BINARY OUTPUT

"B

(4

TRUTH TABLE

I INPUT I OUTPUT

v2

8. Schematic operation of the analog-to-digital converter circuit of Fig. 7 involving only 2 bits: (a) the voltages at different points of the circuit for various input voltages, and (b) the truth table.


operation, consider the simple system comprising only two transistors, Q I and 42. The voltages at different points in this circuit for various input voltages, Vi, are shown in Fig. 8a. The resistances Ro, R I , and R2 are so chosen that the base voltages V B ~ and Vez of the transistors QI and Q2 vary with Vi, according to the curves VBI and V B ~ , respectively. With the input voltage at V I , the output of both of the transistors will be at the operating point P I (high state). When the input changes to V2, the output of Ql will become low (P2), while that of 4 2 will remain high (closer to Pi). When this logic is also applied to the input voltages V3 and V4, the circuit follows the truth table of Fig. 8b. Thus, the outputs of the RTBT-array constitute a binary code representing the quantized analog input level. The system can be ex- tended to more bits with a large number of peaks in the I-V.

This circuit is a flash converter that requires only n transistors for n-bit conversion, compared to 2n analog comparators in conventional flash converters. In addition, the RTBTs work not only as comparators but also give the digital output directly, eliminating the 2n-to-n bit decoder needed in conventional circuits. This further reduces circuit complexity and enhances the speed of operation. However, implementing this circuit with the present generation of RTBTs is difficult. Successful operation of the circuit relies on transfer characteristics where the current remains at a high or low level for a significant span of the base-emitter voltage. Unfortunately, in the multi-state RTBTs implemented so far, the current gradually increases with the input voltage, suddenly drops, and gradually rises again.

In all of these circuits, the minimum allowable collector voltage is determined by the maximum input signal voltage applied at the base terminal, which is higher than that in a normal bipolar transistor. In fact, in the multiple-state RTBTs demonstrated so far, the QWs are positioned between the base and the emitter contact so that the applied base-to-emitter voltage can bias the emitter- base p-n junction and the QW’s. The base potential is then elevated to a relatively high value under operating conditions in the common-emitter mode. As a result, the quiescent collector bias must be large to allow for sufficient swing of the output signal without forward biasing the collector-base junction.

This requires proper care on the part of the circuit designer, as well as careful device design to achieve sufficiently high break- down.

Where We Are Quantum-effect devices are exciting tools for investigating the physics of transport over short distances, and potentially interesting devices for digital and analog circuit applications. The RTBT is among the most promising of these devices because of its large negative t r ansconduc tance and its abi l i ty to operate on multiple states. The latter characteristic might be the most significant in light of its potentially far- reaching circuit implications, particularly for new computer architectures based on multiple-valued logic. But a word of cau- tion is necessary in this context.

It is hard to see how a technology based on quantum-effect devices alone could replace conventional architectures based on silicon two-state devices. Technologies based on 111-V materials are many years away from silicon technology’s level of in- tegration and reliability. Quantum-effect devices are likely to have large-scale impact if their operation can be demonstrated in silicon-based materials, such as silicon-ger- manium alloys, or if dramatic progress oc- curs in the next decade in AIGaAs/GaAs materials and technology. Work on RT in such heterostructures is in its infancy, and one cannot yet draw conclusions on the usefulness of such heterostructures for quantum-effect circuits and devices. A more realistic view is that quantum-effect devices will have impact in certain niche applications such as multipliers, parity generators, and oscillators in the millimeter-wave region.

Acknowledgment I t i s a p l e a s u r e t o a c k n o w l e d g e collaborations with R.A. Kiehl, P.R. Smith, H. French, D.L. Sivco, and A.C. Gossard . CD

Federico Capasso [F] is head of the Quantum Phenomena and Device Research Dept. at AT&T Bell Laboratories in Murray Hill, New Jersey.

Susanta Sen is a full professor in the Dept. of Electronic Science, University College of Science, Calcutta, India.

Leda M. Lunardi [MI is a member of t he t e c h n i c a l s t a f f a t A T & T B e l l Laboratories in Holmdel, New Jersey.

Al fred Y. Cho [F] is director of the A T & T Bell Labora to r i e s Mate r i a l s Processing Research Laboratory at Mur- ray Hill.

References 1 , F. Capasso, ed., Physics of Quantum

Electron Devices, Springer-Verlag, New York (1990).

2 . A. Y. C h o and J . R. A r t h u r , “Molecular Beam Epitaxy,” Progr. Solid State Chem., 10, p. 157, 1975.

3. J . A. Morton, “From Physics to Func- tion,” IEEE Spectrum, 62, 1965.

4. F. Capasso, “Band-Gap Engineering: From Physics and Materials to New Semi- conductor Devices,” Science, 235, p. 172, 1987.

5 . L. L. Chang, L. Esaki, and R. Tsu, “Resonant Tunneling in Semiconductor Double Barriers,” A p p l . Phys. Lett . , 24, p . 593, 1974.

6. E. R. Brown, C. D. Parker, L. J . Mahon- ey, J. Soderstrom, and T. C. McGill, “Room- Temperature Oscillations up to 675 GHz in Resonant Tunneling Diodes,” Abstract of the 48th Device Research Conference, Santa Barbara, Calif., June 25-27, 1990.

7. T. P. E. Broekart, W. Lee, and C. G. Fonstadt, “Pseudomorphic Ino.5-1Gao.47Ad AIAs/InAs Resonant Tunneling Diodes with Peak-to-Valley Current Ratios of 30 at Room Temperature,” Appl . Phys. Lett., 53, p. 1545, 1988.

8. B. Ricco and P. Solomon, “Tunable Resonant Tunneling Semiconductor Emit- ter Structure,” ZBM Tech. Dig. Bull . , 27, p. 3053, 1984.

9. N. Yokoyama, K . lmamura, S. Muto, S . Hiyamizu, and H. Nishi, “ A New Funct iona l Resonant Tunnel ing Hot Electron Transistor (RHET),” Japan. J . Appl . Phys. , 24, p. L853, 1985.

10. F. Capasso and R . A . Kiehl , “Resonant Tunneling Transistor with Quantum Well Base and High-Energy In- jection: A New Negative Differential Resistance Device,” J . Appl . Phys. , 58, p. 1366, 1985.

11. F. Capasso, S . Sen, A . C. Gossard, A. L . Hutchinson, and J . H. English, “Quantum Well Resonant Tunnel ing Bipolar Transistor Operating at Room Temperature,” IEEE Electron Device Lett . , EDL7, p. 573, 1986.

12. F. Capasso e t ul . , “Quantum Func- tional Devices: Resonant Tunneling Tran- sistors, Circuits with Reduced Complexity, and Multiple-valued Logic,” ZEEE Trans. Electron Devices, 36, p. 2065, 1989.

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quantum transistors and circuits break through the barriers

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