Characterization of bipolar devices by steady state and modulated electroluminescence

Mid-Srate Electronics Vol. 33, No. 5, pp. 561-563, Printed in Great Britain. All rights reserved

1990

0038-l lOl/90 $3.00 + 0.00 0 1990 Pergamon Press plc

copyright

CHARACTERIZATION OF BIPOLAR DEVICES BY STEADY STATE AND MODULATED ELECTROLUMINESCENCE K. MISIAKOS, J. S. PARK, A. NEUGRCXCHELand F. A. LINDHOLM Department of Electrical Engineering, University of Florida, Gainesville, FL 32611, U.S.A. (Received 24

August

1989)

Abstract-A new characterization method is proposed based on measurements of the electroluminescence signal as a function of the terminal bias and the modulation frequency. The method is not susceptible to space charge region and emitter recombination currents and provides a direct measure for the separation of Fermi levels at the edge of the principal quasi-neutral region. The extraction of parameters, such as the series resistance and the minority carrier lifetime, is experimentally demonstrated on a silicon diode. The method can be extended to heterojunction bipolar transistors to measure the Fermi level drop across the heterojunction.

1. INTRODUCTION Characterization of the carrier transport in the quasineutral base of a bipolar device requires methods that are immune to the emitter and space charge region recombination. Here, by emitter we mean the thin and usually heavily doped layer that is diffused or implanted or epitaxially grown at the top of a much thicker substrate that constitutes the base. When developing new experimental techniques that will be sensitive only to base transport parameters, a major configuration factor inherent in the fabrication

of a realistic diode should be considered: the base is usually one to three orders of magnitude thicker than the emitter and space charge region combined. Therefore, by measuring an observable that is roughly proportional to the active volume of the device we would effectively eliminate any emitter or space charge region parasitic contributions. Such an observable can be the photocurrent response to uniformly absorbed light[l, 21 or the terminal capacitance[3]. This paper proposes an alternative observable: the electroluminescence emitted by the small percentage of carriers that recombine through radiative recombination. A detector placed next to a forward biased p+/n junction will detect an electroluminescence signal that originates in all points where excess carriers are injected. Every point contributes to the total signal an amount proportional to the local pn product. If SF is the separation of Fermi levels across the junction space charge region, then the combined emitter and space charge region contributions will have a maximum value of Cn’[exp(GF /kT) - l] IV,. Here C is a proportionality constant, n, is the intrinsic carrier density and W, is the width of the emitter and the space charge region combined. At the same time, the base contribution in low injection is Cnf[exp(GF/kT) - l]W,, where W, is an effective active base width. In the worst case of a short base

diode with no back surface field, W, will be half of the base width, W, whereas in any other case W, will lie between, W, and the minority carrier diffusion length, L. Thus, it becomes obvious that the base contribution dominates the electroluminescence signal since most often W,>> W,. 2. THEORY Let EL be the electroluminescence signal. In accordance with the previous discussion, we can assume that the signal is entirely due to the base. Then W

EL = C

b(x)n(x)

-~o(x)n,(x)l

dx

(1)

s0 In eqn (I), and without loss of generality, p is the majority hole density in the base and pO, n,, are the equilibrium carrier densities. Equation (1) accounts for the integrated excess pn product in the base. In a uniform base under low injection p(x) is constant and the continuity equation can be solved in closed form, so that eqn (I) becomes EL = Cnf[exp(GF/kT) X

- l]

L[sinh( W/L) + (SLID) cosh( W/L) - SLID] cosh( W/L) + (SLID) sinh( W/L) ’

(2)

In eqn (2) S is the minority carrier recombination velocity at the base back contact and D the minority carrier diffusion coefficient. If V is the terminal voltage, then 6F = e (V - IR,), where I is the terminal current and R, is the series resistance. Therefore, the EL dependence on V and I becomes EL = EL,,[exp(e (V - ZR,)/kGT) - I],

(3)

where EL,, is the saturation value of EL. Note that eqn (3) holds even for a nonuniform base provided that low injection prevails. Equation (3) suggests that R, can be extracted from the EL dependence on V and I. 561

562

K.

MISIAKOSer

ideal exp(eV/kr) V - ZR, > 4kT/e V,, is given by

(a) 104 .** 7 ? 9

103 :

2

EL 102 r

;

,.'* - IO2 ;i E

.i"

s 2 To

G

&OO .' IO' 5 f

/

00

/

00

- 10'

I

00 o" 100 = 0' 0.50 0.55

I 0.60 Termind

0.65

I 0.70

I 0.75

0.60

voltageIV)

Cb) 0.03 r

Termind current ImAl

Fig. I. Electroluminescence (0) and current (0) as functions of the terminal voltage (a), and voltage drop as a function of the terminal current (b). In (a) the solid line is the ideal exp(eV/kr) plot. In (b) the solid line has a slope of 0.45 n.

3.

dependence. From eqn (3) and for the series resistance voltage drop,

V, = IR, = (V - V,) - (kT/e)

- 103

z 5 0

al

RESULTS AND DISCUSSION

Figure l(a) shows how EL and Zdepend on the bias for a n +/p silicon diode. The diode had a 0.11 R ‘cm base (NA z 1.1 x 1017cm-3), a thickness of 311 pm, an area of 2.5 x 2.5 mm* and was cut from a 2 x 2 cm’ solar cell. The emitter thickness was about 0.5 pm and the base doping density corresponds to a space charge region thickness of nearly 0.1 pm. Thus the base thickness if W z 3 10 pm. The electroluminescence signal was detected by a 0.25 mm* InGas detector and was amplified by a Princeton Applied Research Model 184 current amplifier and displayed on a 7D20 Tektronix digital scope. To suppress the background noise, the bias across the diode was pulsed and the scope signal was averaged 256 times. The temperature was 24.2% As Fig. l(a) shows, for V < 650 mV the ideality factor of EL is 1 because the series resistance voltage drop is negligible. On the other hand, the ideality factor of I is much higher and between 2 and 1.5 because of the space charge region recombination. The series resistance effects become evident above 650 mV and cause EL and I to bend away from the

ln(EL/EL,)

(4)

where EL, is the electroluminescence at a voltage V, low enough to make the voltage drop negligible. Figure l(b) shows V, as a function of the current and, here, V, corresponds to a current of 1 mA. The solid line corresponds to R, = 0.45 R. Given the low resistivity of the base. this resistance is due to the lateral flow of majority carriers in the grid-contact emitter. The last experimental point near 50 mA is well above the solid line. This deviation could be a result of a non-uniform distribution of the base charge under excessive voltage drop in the two dimensional emitter. In the experimental case presented here, the voltage drop is due to series resistance. In a heterojunction device the voltage drop could have been caused by a discontinuity of the Fermi-levels at the heterojunction. Therefore, this method allows measuring the flatness of the Fermi level across the space charge region at a heterojunction interface. Information about the minority carrier transport can be extracted by extending the experiment in the frequency domain. This is done by superimposing a small signal voltage to the steady state voltage and then monitoring the amplitude and phase shift of the modulated electroluminescence as a function of the modulation frequency. Here, the reference signal is the sinusoidal component of the terminal voltage. The amplitude and the phase of EL can be identified with the amplitude and the phase of the expression in eqn (2) provided that L is replaced by the complex diffusion length L* = L(l + ~wT)~’ ‘. For W > 2L (long base diode), EL will be proportional to L*. Thus, the amplitude of EL is expected to decrease with frequency as A(w) = (1 + u%~)-‘~ while the phase shift should be 4(o) = 0.5 tan-’ (0~). The measurements were done by superimposing a sinusoidal signal with an amplitude of 7 mV on a steady state terminal voltage of 730 mV. The frequency was swept from 1 to 50 kHz. To account for the limited band width of the current amplifier, the amplitude and phase response of the amplifier were measured by taking the Fourier transform of its impulse response. The impulse excitation was realized by exciting the detector with a nanosecond light pulse of a Nd : YAG laser. Figure 2 shows the experimental amplitude A(w) and phase 4(w) for different values of T. The amplitude best fit for frequencies up to 60 kHz and the phase best fit for frequencies up to 20 kHz is obtained for T = 9 + 1 ps. The drop of the experimental phase and amplitude for higher frequencies could be due to the series resistance. Above the corner frequency the small signal current component increases and this induces an additional voltage drop across the series resistance. As a result, the

Characterization

of devices by electroluminescence

(a) 100 C-*-.-.-.1.

4

-

i g 6

-

-A 1.

\

‘\

\

2

B .J E b z

‘1.

3

1

_

563

to a limitation of the presented method: when measuring devices with substantial series resistances, reliable results can be obtained only in the vicinity of the corner frequency. The ho!e diffusion length corresponding to the measured 7” = 9 ps is L, = (D,r,)“* o 124 pm using D, x 17 cm/s[4]. Hence, W/L = 2.5, which justifies the assumption of a long base diode used to derive 7” from eqn (2).

4. 10-i

+ 103

105

104 Frequency

(Hz)

(b)

Frequency

(Hz)

Fig. 2. Amplitude (a) and phase (b) of the small signal electroluminescence. Solid lines 1,2 and 3 are the theoretical plots that correspond to 7” = 12, 10 and 8 ps, respectively. The amplitude is normalized with respect to the zero frequency response. Both the amplitude and the phase are referenced to the small signal component of the texminal voltage. Fermi potential separation across the space charge region is smaller in magnitude and lags in phase compared to the terminal voltage. This effect points

SUMMARY

In conclusion, a new experimental method based on electroluminescence measurements was presented. The steady state electroluminescence can be used to measure the voltage difference between the applied terminal bias and the separation of the quasi Fermi potentials at the space charge region edge of a quasi-neutral base. This voltage difference provides a measure of the series resistance or a measure of how flat the Fermi levels are across a heterojunction. In addition to the steady state measurements, the frequency dependence of the modulated electroluminescence can provide the minority carrier lifetime in a long base diode. Acknowledgement-This work was supported partially by DARPA under Grant MDA 972-88-J-1006.

REFERENCES 1.

D. M. Bielle-Daspet

and G. D. Gasset, Solid-3.

Elecfron. 21, 1219 (1978). 2. M. Di Giulio, S. Galassini, G. Micocci, A. Tepore and C. Manfredotti, J. appl. Phys. 52, 7219 (1981). 3. A. Neugroschel, P. J. Chen, S. C. Pao and F. A. Lindholm, IEEE Trans. Electron Devices ED25, 485 (1978). 4. K. Misiakos and A. Neugroschel, IEEE Electron Deuice

Lert.

EDL-8,

358 (1987).