1408 IEEE TRANEL\.CTIONS ON ELECTRON DEVICES, VOL. ED-31, NO. 10, OCTOBER 1984

Low-Frequency Noise irt Permeable Base Transistors

XICHEN C. ZHU, X. N. ZHANG, ALDERT VAN DER ZIEL, LIFE FELLOW, IEEE, AND CARL 0. BO;!:LER, SENIOR MEMBER, IEEE

Abstract-The predominant noise is l/f noise and consists of two Parts: a) Noise varying as I;, generated mostly with conducting chan- nel and predominating for normal values of the collector voltage I’,cE. b) Noise at low VCE and practically independent of VCE; it is gener- ated chiefly in the space charge region around the base grating and &;kes collector l/f noise at VCE= 0. The turnover frequency of the first noise source lies at about 20 MHz for VCE = 0.30 V, VBE = 0.20 V. At sufficiently high frequencies the PBT shows thermal noise of the OUI put conductance g,,, at zero bias. Generation-recombination noise is tab- served at large VBE and low VCE and comes mostly from the stlace charge region around the base grating.

I. INTRODUCTION

T HE PERMEABLE base transistor (PBT) is a high-spl:ed three-terminal device. Another name for the PBT coAd

be “vertical transistor.” It consists of a four-layer sandwi1:h: the n’ substrate, the n-type emitter layer, the patterned thin- film tungsten base, and the n-type collector layer. By proper choice of the device structure and the carrier concentration, a 200-GHz cut-off frequency can possibly be achieved; ap,ply- ing this device to logic circuits, the switching time delay in an inverter gate with a fan out of one could be as low as 2 ps. A number of papers by Bozler and his coworkers [1]-[6] h ~ v e been published. Low-frequency noise below 40 kHz of a GaAs PBT is studied in this paper. It will help one to undx- stand the operation of this device,

Three types of noise were observed: 1) flicker noise or llfnoise with l / fy spectrum withy 2~ 1, 2) generation-recombination noise (G-R noise), and 3) thermal noise, observed especially near zero collector bias.

11. EXPERIMENTAL RESULTS A. Characteristic

Fig. 1 shows the characteristics (IC, V&) with the base vo:;t- age V,, as a parameter. Except for V’E = -0.1 V, the chair- acteristic does not saturate well. We have no satisfactory e~ planation for the anomaly at VBE = -0.1 V. If we choo re VB,Q = 0.20 V, V& = 0.30 V as an operating point, then IBE z 6 X lo-’ mA, so that the low-frequency input conductance gb = aIBE/aVBE is negligible. If we now introduce the trans-

Manuscript received January 30, 1984; revised May 7, 1984. This work was supported under a National Science Foundation Grant.

X. C. Zhu was a Visiting Scientist with the Electrical Engineering Department, University of Minnesota, Minneapolis, MN 55455. He Is with the Kunming Institute of Physics, China.

X. N. Zhang and A. van der Ziel are with the Electrical Engineering Department, University of Minnesota, Minneapolis, MN 55455.

C. 0. Bozler is with the Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173-0073.

2 6 1 - 7 / /+o‘~v,+o.06Y H“

0 0.1 0.2 0.3 0.4 0.6 0.6 0.7 ’CE ( V I

Fig. 1. The IC-VCE characteristics of PBT.

conductance gm = aIC/aVBE, the output conductance g, = aIc/a VcE and the HF input conductance gb, which varies as w 2 because of transit time effects, then the maximum power gain Gmax is obtained by matching the signal source to the in- put and the output load to the output. This yields

At lower frequencies G,,, can be quite large since gb is quite small. Because of the frequency dependence of gb, there is an upper cut-off frequency fT of the device which is determined by G,,, = 1. The value of fT for the units under study here was estimated to be well above 10 GHz.

B. Flicker Noise or IlfNoise The predominant noise in PBT’s at low frequencies is flicker

noise. Fig. 2 shows a typical noise spectrum S z ( f ) of a PBT at V’E 0.35 V, V& = 12 mV, varying as l/f with y N 1 at low frequencies, and leveling off to a constant value of 3.2 X A2/Hz at higher frequencies. Also shown is the thermal noise level of 1.5 X A2/Hz of the device at VBE = 0.35 V. The difference of 1.7 X A2/Hz must be attributed to generation-recombination noise with a relatively short time constant.

Fig. 3 shows Sz( f ) versus IC at constant V,,, with V,, as a parameter. At sufficiently large values of IC, Sz(f) varies as I&, but there are some deviations at low I C , especially for large VBE; we come back to that feature in a moment. The I & dependence of Sz(f) indicates that under normal operating conditions Sz ( f ) / I& at constant VBE is independent of I C , as expected for a resistance fluctuation.

0018-9383/84/1000-I 408$01.00 0 1984 IEEE

ZHU et al.: LOW-FREQUENCY NOISE IN TRANSISTORS 1409

Idt0 : - \\; x, \ x,

limit \ X

\

x - Thermal

noisr I Cjt1 , d

100 I K IOK 100 K

Fig. 2. The noise spectrum SI (f) of a PBT at VBE = 0.35 V, VCE = '12 mv.

f (He)

Bearing in mind the I & dependence of S z ( f ) , and assuming y = 1, we find for V'E = 0.20 V; VCE = 0.30 V, IC = 10.5 mA

Sz ( f )=2 .3 X 10-'4/fA2/H~. (2)

According to FET theory [7], the thermal noise is given by

SZ(f)th = P4k%0 (3) where g,, = @I& V&)vcE=,. The parameter is a slow function of V& and has a value of approximately unity for relatively small V&. The thermal noise at the operating point VBE = 0.20 V, V C - = 0.30 V is then estimated to be 1.05 X 1 02' A2/Hz.

The turnover frequency f, is defined by the relationship

SZ(fc) = SZ(f)th. (3 a) With the above values this yields f, = 20 MHz. Since transit time effects are only important in the gigahertz range, one can therefore study the thermal noise behavior at intermediate frequencies quite accurately, and so determine possible hot- electron effects (0 > 1) for relatively large V& (0.2-0.3 V).

We see from Fig. 3 that S z ( f ) / I & decreases strongly with in- creasing V B ~ . To illustrate this more clearly we have plotted Sz( f) at IC = 1 mA versus V& in Fig. 4. Except for an anom- aly for V B ~ > 0.40 V, which is due to the same effect that causes a deviation of Sz(f) from the I & dependence in Fig. 3, SI&) at IC = 1 mA decreases strongly with increasing V'&.

This can be understood as follows. According to Klaassen's theory of number fluctuations llfnoise [9] and also to Hooge's expression [lo] for mobility fluctuation l/f noise, if N is the number of carriers in the device and a a constant (a -N 1 OA3)

The strong decrease in Sz(f>/I& with increasing VBE can be understood in terms of a strong increase in N . Such an in-

I 0.01 a2 0.8 2 s

I mA I, (mal

Fig. 3. S@ versusIc at constant VBE.

Fig. 4. Szc(f ) versus VBE at IC = 1 mA.

6

1410 IEEE TRANB.kZTIONS ON ELECTRON DEVICES, VOL. ED-31, NO. 10, OCTOBER 1984

f = 1000 He

A VOI? 9 0.06V 0 VoE = 0.15 V X VOE = 0 . 3 V

-0.3 -0.2 -0.1 0 0.1 0.2

Fig. 5. SIc( f )/IC as a function of VBE at constant VCE.

crease would be expected because the current injection at the emitter increases strongly with increasing VBE.

To ,eliminate the anomaly for VBE > 0.40 V, we measured sI(f)/IE as a function of v'E at cqnstant collector vo1:age Vci (Fig. 5). We now see that SI( f)/I& decreases uniforrnly with increasing VBE and that the anomaly for VBE > 0.41 V has disappeared.

The number of carriers involved in the l/f noise process can be evaluated by assuming a value for CY. Taking a = we find that N varies between lo5 and lo' when VBE varies from -0.30 t.0 t0.30 V.

We finally note from Fig. 5 that SI( f)/I& at constant 1'& depends only weakly upon VCE.

C. Thermal Noise at Zero Collector Bias Without collector current one would expect the output

noise of the collector to be equal to the thermal noise of the output conductance gc0 = (aIc/a V C E ) V ~ ~ = , , or

S k f ) = 4k~g,,. ((5)

We shall see, however, that this is not always the case. If we measure a spectrum SI(f) , , we can always expess

it in terms of an equivalent noise conductance g,, withL the help of the definition

SI(f )m = 4kT'cn. ( !j a) Fig. 6 shows SI(& in A2/Hz versus VBE. Also show:;[ is

the thermal noise calculated from (5). Fig. 7 shows gco xnd g,, versus VBE at 10 kHz. We see from these two figwes

16Lo I I

- 'II

o Experiment Vag - 0 f * IO KHc

- calculated thermal noise I

-

l l P -0.2 -0.1 0 0. I 0.2 0.3 0.4 0.1

Hu ( V I

Fig. 6 . S I c ( f ) versus VBE at VCE = 0 and f = 10 kHz.

2ool

that the noise is thermal noise at low VBE but that there is excess noise at the highest values of VBE. As Fig. 8 indicates this excess noise is l/f noise terminating into thermal noise at high frequencies. At VBE = 0.20 V the noise at V& = 0 is thermal noise throughout, but at VBE = 0.50 V the flicker noise is predominant below 50 kHz.

D. The Influence of Base Current on Flicker Noise at the Collector

We noted that there were some peculiarities in Figs. 3, 4, and 6-8 when the base voltage VBE was larger than 0.35 V. In Fig. 3 the noise spectra are not proportional to the collec- tor current squared at high values of VBE and low values of the collector current IC. In Fig. 4 SI(f) increases with in-

ZHU et al.: LOW-FREQUENCY NOISE IN TRANSISTORS

0 c d o

I I I I I I I

1 0 0 IK IO K

f (H,)

Fig. 8. The noise spectra S1&f) at VCE = 0.

u Fig. 9. Base current generated collector l/fnoise at VCE = 0.

creasing VBE for V',, > 0.4. In Figs. 6 and 7 there is excess noise at 10 kHz for the largest values of VBE and Fig. 8 shows that the noise is of the llftype.

Where is the excess noise generated? It cannot be generated in the base-collector region, since there is no current flow at V& = 0. It must therefore be generated in the base-emitter region and in the region between adjacent base grating ele- ments (see Fig. 9). There will, therefore, be a l/f noise voltage developed between the base grating elements and to this volt- age the collector couples resistively; hence, there can be collec- tor llfnoise even at zero collector bias. At sufficiently high collector voltages the llfnoise is generated in the base-collector region predominates and it varies as I & . In between the noise gradually switches from the one to the other mechanism and S,(f) varies slower than I & and may be even independent of IC (at VBE = 0.5 V).

The reason for all this is that there is a significant base cur- rent IB, flowing, even at zero collector bias VCE, and it pro- duces l/f noise, The system is now not in thermal equilibrium and there can then be llfnoise over and above thermal noise.

I I I I 5 0.4 0.5

141 1

v= ( v )

Fig. 10. The base IBE - VBE characteristics of PBT.

I

o v , ~ .0.45v, I,= 0.06 ma r D V,, = 0.5v, I, = 0.234 ma

-

- -

16'O -

t 1 0 - 2 1 1 1 I I I I

I I

100 IK f (H,)

10 K

Fig. 11. The base noise spectra SzB (f),

Fig. 10 shows log IBE versus VBE. The curve is a simple ex- perimental IBE = I o exp(qV~~/rnkT) with an ideality factor of 1.38. This is what one would expect for a Schottky-barrier diode with traps. It is then not unreasonable to expect gen- eration-recombination noise from discrete traps and l/f noise from a trap distribution.

Fig. 11 shows l/f noise in the base current at VBE = 0.45 V and at V B ~ = 0.50 V (and VCE = 0 in both cases). In addition there is generation-recombination noise at higher frequencies;

1412 IEEE TRANS.LCTIONS ON ELECTRON DEVICES, VOL. ED-31, NO. 10, OCTOBER 1984

f e 1000 He 1

I d I

10 -21 1 I I I I I I

am 0x)I 0.10 1.0 I,, (ma)

Fig. 12. The base noise spectra versus base current IBE.

both G-R bumps seems to have the same time constant T. The white noise above 10 kHz (V& = 0.45 V) is not shot noise, since I B ~ = 0.06 mA and hence S,(f) = 1.9 X A2/Xlz, which is two orders of magnitude too low; apparently the noise is the low-frequency part of a second G-R bump with a much shorter time constant.

Fig. 12 shows S,(f) as a function of IBE at f= 1000 Hz (:./f noise regime). We see that S,(f) is proportional to (IBE)’” 19. This exponent is typical for a p-n junction or Schottky-barr: er space charge region involving traps [ 11 I-[ 131. The nature of the excess l/f noise is therefore fully understood: as l/fnojse generated in the base-emitter region to which the floating col- lector (or the collector at zero bias) couples resistivily.

E. Generation-Recom bination Noise Fig. 2 already gave an indication of G-R noise present at a

particular bias (VBE = 0.35 V, VcE = 12 mV), and Fig. 1 1 showed the presence of G-R noise in the base-emitter Schottk,f- barrier diode. We now investigate this phenomenon in greatlx detail.

Can there be generation-recombination noise produced i n the main channel between base and collector? Of course it can, but it is usually masked by l/f noise generated in thc same channel. This cannot be remedied by going to a diffe-- ent collector current IC, since both processes give an I & depen- dence of the noise. If we operate the devices at large V’& an:\ small V& (or small IC), however, we come to a regime where the l/f noise is nearly independent of IC (and hence of V&;I

This might therefore be a suitable bias condition. Fig. 13 shows noise spectra at VBE = 0.35 V, IC = 0.3 mA at

room temperature and at 0°C. There are three G-R noiw “bumps” in the frequency range between 1 and 30 kHz; their corresponding time constants are 6.25 X s, 3.3 X s,

f (Hz)

Fig. 13. Noise spectra at = 0.35 V, IC = 0.3 mA, and at room tem- perature and 0°C. There are three G R noise “bumps.”

and 7.6 X s, respectively. These “bumps” shifted to lower frequencies (and hence larger time constants) at 0°C. If we write

T = ro exp(qE,/kT) ‘ (6)

where E, is the activation energy of the trap involved. The activation energies of the traps can be estimated as 0.035, 0.047, and 0.19 eV, respectively. We believe that these traps are located in the space charge region surrounding the base grating .

111. CONCLUSIONS The predominant low-frequency noise in PBT’s is l/f noise.

There are two forms of it: a) l/f noise varying as I & and generated mostly in the con-

ducting channel between the base gratings and in the base- collector region. It predominates at normal values of the cob lector voltage V&.

b) l/f noise practically independent of IC at low VCE; it is generated chiefly in the space charge region around the base grating and the collector couples to it by resistive coupling. It is responsible for the l/f collector noise observed at VCE = 0. At VBE = 0.20 V, VcE = 0.30 V the l/f noise of type a is masked by the thermal noise.

At zero collector bias the PBT shows thermal noise of the output conductance gco = (aIc/a VcE)vcEs at zero bias and sufficiently high frequencies. At low frequencies it is masked by llfnoise of the type just mentioned.

Generation-recombination bias is best observed at large V& and low VcE. We observed three traps and determined the activation energies to be 0.035,0.47, and 0.19 eV.

The l/f noise of PBT’s is comparable to that of good-quality MESFET’s. It is better than that for GaAs MODFET’s, but there are indications that the l/f noise of the latter can be re- duced considerably. We hope to report on these comparisons at a later date.

In a solid-state microwave oscillator the low-frequency noise at a frequency fi can beat with the carrier frequency fi

to produce sidebands of frequencies fi * fi . This is a very harmful effect, but it can be eliminated by proper circuit techniques [ 141.

REFERENCES [ l ] C. 0. Bozler, G. D. Alley, R. A. Murphy, D. C. Flanders, and

W. T. Lindley, “Permeable base transistor,” in Proc. 7th Bicen. Cornel1 Conf. Active Microwave Semiconductor Devices, p. 33, Aug. 1979.

[2] G. D. Alley, C. 0. Bozler, R. A. Murphy, and W. T. Lindley, “Twodimensional numerical simulation of the permeable base transistor,” in Proc. 7th Bien. Con5 Active Microwave Semicon- ductor Devices, p. 43, Aug. 1979.

[3] C. 0. Bozler, G. D. Alley, R. A. Murphy, D. C. Flanders, and W. T. Lindley, “Fabrication and microwave performance of the permeable base transistor,” in IEDM Tech. Dig., pp. 384-387, Dec. 1979.

[4] C. 0. Bozler and G. D. Alley, “Fabrication and numerical simu- lation of the permeable base transistor,” IEEE Trans. Electron Devices, vol. ED-27, pp. 1128-1141, June 1980.

[SI G. D. Alley, C. 0. Bozler, D. C. Flanders, R. A. Murphy, and W. T. Lindley, “Recent experimental results on permeable base transistors,” in IEDM Tech. Dig,, pp. 608-612, Dec. 1980.

[6] C. 0. Bozler and G. D. Alley, “The permeable base transistor and its application to logic circuits,” Proc. IEEE, vol. 70, pp. 46-52, Jan. 1982.

[7] A. van der Ziel, “Thermal noise in field effect transistors,”Boc.

[8] A. G. Jordan and N. A. Jordan, “Theory of noise in metal oxide semiconductor devices,” IEEE Trans. Electron Devices, vol. ED-12, pp. 148-156, Mar. 1965.

[9] F. M. Klaassen, “Characterization of low llfnoise in MOS transis- tors,” IEEE Trans. Electron Devices, vol. ED-18, pp. 887-891, Oct. 1971.

[ l o ] F. N. Hooge, “Discussion of recent experiments on l/fnoise,” Physica, vol. 60, pp. 130-144, July 1972.

[ l l ] S. T. Hsu, D. J. Fitzgerald, and A. S. Grove, “Surface state re- lated l/f noise in p-n junction and MOS transistors,” Appl. Phys. Lett., vol. 12, pp. 287-289, May 1968.

[12] A. van der Ziel, “Noise in solid-state devices and lasers,” Proc.

[ 131 J. Viner, M.Sc. thesis, University of Minnesota, Minneapolis, MN, 1969.

[14] H. B. Chen, A. van der Ziel, and K. Amberiadis, “Oscillator with odd-symmetrical characteristics eliminates low-frequency noise sidebands,” IEEE Dam. Circuits Syst., vol. CAS-31, no. 9, Sept. 1984.

IRE, V O ~ . 5 0 , ~ ~ . 1808-1811,Aug. 1962.

IEEE, Vol. 58, pp. 1178-1206,1970.

Characteristics and Reliability of the SEPROM Cell HIROSHI NOZAWA, YOUICHIRO NIITSU, NAOHIRO MATSUKAWA, JUNICHI MATSUNAGA,

AND SusUMu KOHYAMA, MEMBER, IEEE

Aktruct-A new EPROM named SEPROM, based on a modified SEPOX process, is proposed and evaluated. The SEPROM offers a process compatibility t o logic LSI’s with higher packing density, since the area of the second gate oxide is equal to that of the first gate oxide. To improve the coupling capacitance ratio, which relates t o write and read operations, a thin second gate oxide is required for the SEPROM cell at a risk of degradation in charge retention characteristics. A mea- sured test device, however, shows sufficiently good characteristics both in programming and charge retention, due to the desirable struc- ture of the cell. The SEPROM structure appears to be practical and promising for both EPROM and logic device applications.

I I. INTRODUCTION

N ADVANCED random logic LSI’s, an integration of EPROM on the same chip is often required and desirable.

For that purpose, a process-compatible EPROM structure should be developed, while also pursuing higher packing den- sity. Conventional EPROM cells can not satisfy the demands adequately due to their double-level poly-Si structure. There-

Manuscript received December 9, 1983; revised March 21, 1984. The authors are with the Semiconductor Device Engineering Labo-

ratory, Toshiba Corporation, Kawasaki 210, Japan.

fore, a new EPROM structure, called SEPROM [l] , [2], is proposed and fabricated by modified SEPOX process [3] .

Historically, the first floating-gate-type EPROM cell devel- oped, known as FAMOS [4], has a single-level poly-Si gate structure and employes a standard MOS LSI process for main- taining the process compatibility. However, most of the cur- rent floating-gate-type EPROM cells such as SAMOS [5] have stacked poly-Si gate structures. The SAMOS-type cell has had a large impact on packing density and writing speed. In the structure, the second-level poly-Si gate, called the control gate, selects the cell address and also enhances the electric field strength under the floating gate at programming. The high packing density was the result of eliminating the isolated select transistor from the memory cell, and the high writing speed was obtained by enhanced hot-carrier injections. In return for high performance and high packing density, process compatibility has been lost.

Furthermore, the dual-level poly-Si gate accompanies an undesirable control gate morphology at the floatinggate steps, often observed as an overhang [6] . The geometrical structure should increase poly-Si oxide conductivity, because the electric field concentrates at the step. The nature might

0018-9383/84/1000-1413$01 .OO 0 1984 IEEE