estigation of GaAs homojunctio transistor with delta doping emitter s

H.C.Wei Y. H . Wa n g M.P.Houng

Indexing terms: GaAs homojunction bipolar transistor, Delta dropping emitter

Abstract: A GaAs homojunction bipolar transistor with a delta doping emitter structure is proposed and demonstrated. The proposed device makes use of a delta doping structure inducing a triangular barrier for minority carrier confinement, resulting in a high emitter injection efficiency. Based on the minority carrier transport in the bulk emitter region with drift-diffusion mechanisms, and in the triangular barrier region with tunnelling and thermionic-emission mechanisms, an analytical derivation of current- voltage characteristics, including the effect of bandgap shrinkage, was obtained. The calculated results show that the triangular barrier is the key parameter in determining the electrical properties. In addition, due to the absence of heterojunction, the proposed device exhibits more nearly constant current gain with collector current than for the AlGaAdGaAs heterojunction bipolar transistor (HBT). The proposed device, grown by molecular beam epitaxy, shows a differential current gain of 13 and an offset voltage of 60mV at a base-to- emitter doping ratio of 10. The offset voltage is attributed mostly to the geometric limits. With a simple chemical treatment of Na2S-9H20, the differential current gain is enhanced to be 16 due to the reduced surface recombination. Theory and experiment indicate the potential application of the proposed device.

1 Introduction

High current gain and high cut-off frequency are com- mon figures of merit of a bipolar transistor. In a homo- junction transistor, the current gain is limited by the emitter injection efficiency, which is proportional to the ratio of the emitter and base Gummel numbers [l]. To obtain a high current gain, the emitter should be much more heavily doped than the base. As the emitter dop- ing becomes very high, so-called bandgap narrowing effect plays a crucial role in limiting the gain achievable in silicon bipolar transistors, which is attributed to the broadening of the impurity band and the formation of 0 IEE, 1995 IEE Proceedings online no. 19952201 Paper first received 6th March 1995 and in revised form 17th July 1995 The authors are with the Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan, Republic of China

band tails on the edge of the conduction and valence bands [2]. In 111-Vs bandgap shrinkage, the experimen- tal results by Harmon et al. [3] showed significant bandgap shrinkage in heavily doped p-GaAs and very little effective bandgap shrinkage in heavily doped n- GaAs because of its degeneracy. Heavily doping effects in the base of homojunction GaAs np+n transistors, play fortunately a favourable role, increase the collec- tor current and hence the current gain [4,5], which avoid base-collector punchthrough and achieve a low base resistance. This asymmetry of bandgap shrinkage in GaAs makes it particularly attractive for the applica- tion of the pseudo-heterojunction bipolar transistors (HBTs) concept.

With the recent interest in GaAs-based HBTs, it uti- lises the bandgap difference of the base and emitter materials to create a barrier for the minority carrier injection into the emitter. The advantages of HBTs are well known [6]. For example, the base is allowed to be heavily doped, leading to a decrease in base resistance. This not only improves the base conductance and the frequency response but also gives rise to a high Early voltage. Although HBTs offer many advantages over homojunction transistors, there remain several prob- lems of HBTs to overcome. For instance, to align pre- cisely the compositional junction to the doping junction is difficult because of the high diffusivity of the dopant, beryllium, commonly employed [7,8]. This out-diffusion of the dopant to the wide-gap emitter severely degrades the device performance [9]. Moreover, interface states located at the heterojunction trend to have a large rate of recombination and harm the current gain [lo]. Another important drawback of HBTs is the collector- emitter offset voltage which is undesirable in switching application because it increases the power consumption in saturation logic circuits. The cause of large offset voltage has been attributed to the conduction band dis- continuity at the base-emitter junction [I 1,121. This dis- continuity forms also a potential spike that acts as a tunnelling barrier and results in a reduction of electron current [ 13,141. These problems limit the improvement of HBT characteristics. The grading of A1 composition in the depletion region of the base-emitter junction has been proposed to reduce the discontinuity effect [15]. However, the precise parabolic grading is difficult to control and it results in the reduction of current gain by periphery recombination [ 161.

We propose a GaAs homojunction bipolar transistor with a delta doping emitter structure. Although the bandgap narrowing in a heavily doped base can enhance the current gain, the goal for the proposed

I F F Pvnr -C'rvruitv npv i rpc Yuvt Vnl 142 Nn 6 Ibrpmhm 1995 406

device is to use an artificial barrier to block the minor- ity carrier injection into the emitter. In 1969, Esaki and Tsu proposed semiconductor superlattices, they did not only consider compositional superlattices but also dop- ing superlattices [17]. Such a delta doping structure may generate potential modulation to form a triangular barrier for carrier injection [ 181. Hence, the proposed device uses the delta doping structure in the emitter to form a barrier for the suppression of the minority car- rier. This effect yields a high emitter injection effi- ciency. Drawbacks in HBTs disappear in the absence of heterojunction structure. Here we present the theory and experiment of such a transistor. The effect of bandgap shrinkage is also considered in device model- ling.

emitter base collector

P n

I

E,” \

”, v \ --a

JPZ JTFE JPI

J I t I I

WE+Wp WE+‘ WE WE-r 0 Fig. 1 device

Energy band diagram for the base-emitter junction of the proposed

2 Theoretical model

Fig. 1 depicts the idealised band diagram for the emitter-base junction of the proposed transistor. The i-Z(n+)-i delta doping structure is located in the emit- ter. If the donor atoms are fully or partially ionised, the impurity gradient induces a triangular potential barrier in the emitter. Also illustrated in the Figure, electron injection into the base is the same as in a homojunction transistor. However, the delta doping structure induces a potential barrier for holes. The net hole flow in the transistor is thus no longer governed by the drift-diffusion process; instead, it is expressed according to the following two mechanisms: (i) drift diffusion in the bulk emitter and (ii) thermionic emis- sion and tunnelling for the barrier. The mechanisms of carrier transport are similar to that in silicon bipolar technology [19]. The theoretical model is hence estab- lished according to the model of a polysilicon emitter [19,20]. In addition, for the sake of bandgap shrinkage, the n?, where ni is intrinsic carrier concentration, is replaced by an effective pn product, ni2.fs, even in the presence of degeneracy [21]. To simplify the derivation in the theoretical model, we assume that the excess holes injected from the base do not alter the potential distribution in the emitter region. The applied voltage VBE then drops entirely across the base-emitter junc- tion. Efn and Efp are the electron and hole quasi-Fermi levels, respectively, and are separated by the applied voltage VBE V, is the height of the barrier for the

holes in the bulk emitter and is mainly determined by the delta doping concentration.

Neglecting drift diffusion in the barrier region, we first derive the thermionic emission and tunnelling cur- rent. The potential modulation in the barrier is given by

- ( r ) ‘ U -

where WE is the distance between the delta doping layer and the base, and r is the thickness of the undoped layer and the half width of the barrier. The probability z(E) that a hole penetrates the barrier is based on the WKB approximation [22].

where mi is the effective mass of the hole and h is Planck’s constant. Substituting eqn. 1 into eqn. 2 yields

The hole current through the barrier that includes ther- mionic emission and tunnelling currents is determined from the general tunnelling formalism [23].

J T F E = 4nqm;kT

h3

where

where k is Boltzmann’s constant, fR(@ and fL(@ are the Fermi-Dirac distribution functions of the energy level at the right and left sides of the barrier, respec- tively. E , is the valence band energy and qP is the hole quasi-Fermi level in the emitter. All energies are meas- ured with respect to the electron quasi-Fermi level in the emitter. We consider the hole energy larger than the barrier height to have unity transmission probability. Hence, eqn. 4 may be rewritten as

where

NE (9)

vp = (kTi2nm~)”* is the average thermal velocity of holes, niE is the effective intrinsic carrier concentration

IEE Proc.-Circuits Devices Syst., Vol. 142, No. 6, December 1995 407

for n-GaAs in the emitter, and NE is the electron con- centration in the emitter. Jtu, and Jth represent current components arising from tunnelling and thermionic emission, respectively.

We proceed to derive an expression for the drift-dif- fusion current inside the emitter region adjacent to the base. The hole current density is proportional to the gradient of the hole quasi-Fermi level,

where CL/, is the mobility of hole and p is the hole con- centration. As the length of this region is smaller than the hole diffusion length, the recombination in the emitter neutral region is negligible and the hole current is hence constant. Integrating eqn. 10 over this bulk emitter region and using the Einstein relation, we find

(11)

where D, is the hole diffusion coefficient. With the same derivation in J p l , the hole current density in the emitter region near the cap layer is given by

where Wp is the thickness of the emitter region near the cap layer. The three current components given by eqns. 7, 11 and 12 act in series, and must therefore be equal. Because of the continuity requirement of the hole cur- rent density (J,, = qI2 = JTFE), the component of QP in the derived process is eliminated. The analytical expres- sion for the hole current density is then

In addition to the hole current density injected from the base into the emitter, the effects of recombination, including both recombination in the space-charge region and at the surface, are considered. We assume that recombination In the space-charge region is mainly dominated by the capture process [24].

J~~~ = 4+)0 5 ~ ~ 7 2 Z E ( ' l i , ~ t B { 4 ~ E ~ 2 v T 1 n ( ~ + , ~ ) - V U E : ] /t} -0.5 exp(VBE / 2 k T ) (14)

where CT is the capture cross section, v, ( = (XkTI 7cm,)O 5, is the mean electron thermal velocity, N f B is the trapping density in the space-charge region, and E is the GaAs dielectric permittivity. The current density due to surface recombination is given by [25]

where A, is the recombination velocity at surface, Ls is the surface diffusion length, niB is effective intrinsic car- rier concentration for the p-GaAs in the base, P is the base-emitter junction perimeter, and AE is the emitter area. The product soLs for GaAs is assumed to be 3cm2/s [25]. The base current density is then

JB = JP i- JSCR 4- Js (16)

4nx

The electron current controlled by the homojunction is expressed according to the following equation [26]:

where WB is the base thickness and NB is the base dop- ing concentration. The current gain is given by p = J,I JB. In the calculation, device parameters are based on Fig. 5 except for the altered height and width ( = 2r) of the barrier. The other relevant parameters given to match the actual device are rn; = 4.55 x 10-31kg, m,* = 6.09 x 10-32kg, h = 6.635 x 10-34.J s, k = 1.38 x

JIK, Dp = 6.15cm2/s, D, = 40cm2/s, E = 1.15 x F/cm, CT = 10-'4~m2, NfB = 1014~m-3, n,E = 2.89 x

106cn-3, nrB = 5.32 x 106~m-3, P / A E = 565.7cm-'. In eqn. 13, the hole current density contains three

terms in the denominator. The first and second terms correspond to the emitter Gummel numbers adjacent to the base and to the cap layer, respectively. The third teim is the contribution of thc potential barrier; if it is absent then eqn. 13 represents an amount of hole cur- rent density in a conventional homojunction transistor. Except for the factor of bandgap shrinkage, the poten- tial barrier is believed to be a limiting factor in the base current and the current gain. Fig. 2 shows the base cur- rent density and the current gain as a function of bar- rier height with a barrier width of 12nm. The base current is significantly decreased with a barrier of a few tenths of an electronvolt. This effect is due to the smaller transmission probability s(E) through the increased barrier height and to only holes with suffi- cient thermal energy being able to surmount the bar- rier, resulting in decreased base current. The current gain hence increases with the barrier height. The effects of the barrier width on the device performances are the same as that for the barrier height. Due to the small z(E) through the thick barrier, the base current is diminished, thus increasing the current gain, as shown in Fig. 3 in which the barrier height is assumed to be 200meV. When the barrier width is greatly increased, the base current diminishes to a constant value due to a neglected tunnelling component. The base current is dominated by the thermionic-emission component. The current gain gradually increases and reaches saturation with increasing barrier width. To suppress the minority carrier from flowing into the emitter, the large barrier height and the proper barrier width are required. The free GaAs surface possesses both a large surface recom- bination velocity (so - 106cm/s) and a high surface state density (N, - 1012cm-2 eV-') [27]. These characteristics restrict the GaAs device performance. Therefore, sur- face passivation, i.e. the reduction of minority carrier surface recombination, is critical for achieving efficient operation of GaAs bipolar transistors. To eliminate the carrier recombination through junction perimeters, sur- face chemical passivation by Na2S 9H20 treatment has been performed. In the best case, the surface recombi- nation velocity decreases from 5 x lo6 cmis to 103 cmis after passivation [28]. Current gains are enhanced after passivation (dashed lines shown in Figs. 2 and 3) com- pared to those without passivation (solid lines), Fig. 4 shows a comparison of calculated results between the proposed device, with a barrier height of 200meV and a barrier width of 12nm and the conventional homo- junction transistor. The structure of conventional bipo- lar transistor used in the calculation is the same as that of the proposed transistor except for the absence of the

IEE Proc -Circuits Devices Syst Vol 142 NO 6, December 1555

delta doping structure. Because of the barrier for the minority carriers, the proposed device presents better electrical characteristics than a conventional transistor. To compare the proposed transistor and the HBT, a thermionic-diffusion model for an AIGaAs/GaAs HBT with A1 composition of 0.3 was employed [29], taking into account the recombination effects at the interface and surface. The HBT device structure used in the cal- culation is the same as that of the proposed device except that the emitter is AlGaAs material of thickness lOOnm with A1 composition of 0.3. In the low collector regime the interface recombination plays a significant role in HBTs. However, this problem is absent from a homojunction transistor. According to the theoretical calculation, the proposed device exhibits improved elec- trical characteristics if the optimised structure can be achieved.

u

I n U

1I-l-1

c C

101 g U

1 0 0

" 0 100 200 300-

barrier heiaht. meV . , , Fig.2 rier width of 12nm

Calculated parameters as a functlon of barrler height with a bar-

a base current density b current gain ~ without passivation _ _ _ _ with passivation

.------r--__________I____________ IO0 1 0 20 30

0 0

barrier width, nm Fig.3 height of 2OOmeV a base current density b current gain

Calculatedparameters as a function of barrier width with a barrier

3 Device growth and fabrication

A cross-sectional view of the proposed device, grown by molecular beam epitaxy, is shown in Fig. 5. First, a 300nm n'-GaAs buffer layer with Si doped to 5 ~ 1 0 ' ~

cm-3 was grown on a (100) n+-GaAs substrate, fol- lowed by a 600nm n-GaAs collector layer with Si doped to Sx10l6 cm-3 and a 200nm p+-GaAs base layer with Be doped to Sx10'* CM-~. Next, a 40nm GaAs emitter layer with Si doped to 5 ~ 1 0 ' ~ cm-3 was grown. Then the delta doping structure, consisting of a Si delta doping layer with a sheet concentration of 1x10" cm-* and undoped GaAs layer of 6nm on both sides of the delta doping layer, was grown. To avoid the out-diffu- sion, the substrate temperature was altered from 580 to 510°C when the delta doping structure was being grown. Finally, a 50nm emitter with Si doped to 5 ~ 1 0 ' ~ ~ m - ~ and a 200nm n+-GaAs cap layer with Si doped to 5 ~ 1 0 ' ~ ~ m - ~ was deposited for good ohmic contact.

1 o3 1, I' A

collector current denslty, A/cmZ Calculated common-emitter current gain against collector current Fig.4

density a and a' Proposed device with barrier b and b' An AlGaAdGaAs HBT with c Conventtanal homojunction transistor a and a' represent the calculated results with passivation

barrier width 12nm tion of 0 3

emitter

n-GaAs 5~10'~cm-~209nm

6 doplng structure -N~d=1x10'3cm-2

base n-GaAs 5~10 '~cm-~f+O nm base

I 1

collector Fig.5 emitter structure

Schematic structure of U GaAs homojunction with U delta doping

During fabrication process, both photolithography and lift-off techniques were employed. Wet chemical etching with lH2S04 : 1H,02 : 24H20 was used to define the emitter pattern and to expose the base layer. AulGe and AulZn were evaporated on the cap layer and on the exposed base layer, respectively, to form ohmic contact. The active device was isolated by mesa etching in a lH2S04 : 8H202 : 18H20 solution. The base-emitter junction and the base-collector junction

IEE Proc -Circuits Devices Syst , Val 142, No 6, December 1995 409

are 5 ~ 1 0 - ~ c m ~ and 2.55x104cm2, respectively. The ratio of collector-to-emitter area is 5.1. Finally, the device was alloyed at 450°C in a forming nitrogen ambient and was then measured with Tektronics 370A curve tracer and HP4 1458 semiconductor parameter analyser.

Fig. 6 Common-emitter current-voltage characteristics Unpassivated device

Fig .7 Common-emitter current-voltage characteristics Passivated device at large base current injection of 200pAlstep

collector current, A Dependence of' current gain on collector current before and after Fig.8

surface passivation Nu# 9H20. __ before passivation _ _ _ - after passivation . . . . . . . . . . . calculated result with a barrier height of 15OmeV and soLs of 1.5cm2/s

4 Results and discussion

Fig. 6 shows the common-emitter characteristics for an unpassivated device at a large base current injection (200p.4 per step). A differential current gain with 13 was measured at a collector current of 12.5mA. This value is higher than that of the transistor without delta- doped emitter structure (less than 1). It proves the effects of the induced emitter barrier. This delta doping profile results in a V-shaped energy band. According to the model by Schubert et al. [30], the barrier height for Nf = 1 x 1013cm-2 was calculated to be 367meV. The current gain is expected to be 35 in the theoretical cal- culation. The discrepancy between the experimental and theoretical results is that the calculated barrier height is an ideal case in which the dopants in the delta doping layer are incorporated into the single atomic plane. Under some growth conditions, dopants such as Si in GaAs can segregate or diffuse away from the ini- tial plane [31,32]. The measured SIMS profile indicates that the full width at half maximum (FWHM) of delta doping layer in the studied device is 9nm. This out-dif- fusion effect lowers the barrier height [33]. It should be pointed out that the length of bulk emitter adjacent to the base is critical to the suppression of the minority carrier. If this emitter is too thick, compared with the hole diffusion length, the transistor turns into the homojunction transistor without the suppression effect. To check this effect, a 65nm GaAs emitter layer adja- cent to the base instead of 40nm was grown. A differ- ential gain of only five was measured. Due to the delta doping layer located at the depletion region of base- emitter junction, the compensation between the delta doping layer and p+-base makes the induced barrier height of 40nm emitter length higher than that of 65nm emitter length. A 25nm change in emitter length drops the current gain by more than a factor of two. This length plays an important role in device performance and needs to be further optimised. In addition to the common-emitter current gain, the breakdown voltage V,--, was measured to be 14.5V. The flat output cur- rent-voltage curves indicate the high Early voltage due to the heavily doped base, which is difficult to achieve in a conventional homojunction transistor. Fig. 7 shows the common-emitter characteristics for the passi- vated transistor by Na2S . 9H20 chemical treatment. The differential current gain was enhanced to be 16. Fig. 8 shows the dependence of current gain on collec- tor current for both unpassivated and passivated tran- sistors. The unpassivated transistor exhibits a slightly flat current gain against collector current with a slope of 0.36. Relative to the AIGaAs/GaAs HBT, it exhibits a strong dependence of p on the collector current (p = 1c0.5) due to the effects of interface recombination [34]. To further examine the carrier transport, the Gummel plots are measured as shown in Fig. 9. The ideality factor of the collector current is 1.04. The IkT charac- teristics mean the p-n homojunction governs the emitter electron injection. The ideality factor for the base cur- rent is found to be 1.89 due to surface recombination which is quite close to the value expected for recombi- nation current in GaAs devices. After a simple chemi- cal treatment with Na2S . 9H20, the slope and ideality factor are improved to be 0.3 and 1.68, respectively. This is attributed to the reduced surface recombination. Fig. 8 shows also the calculated results with a barrier height of 15OmeV and a soLs of 1.5cm2/s. The barrier

n i n TEE Pror -Circuits Devices Svst.. Vol. 142. No. 6. December 1995

10-2[

6 I E J

5 Conclusion

the large offset voltages are observed at a large injec- tion of base current due to the increase of the third term in eqn. 18, as shown in Figs. 6 and 7.

Fig. 7 0 rent injection Offset voltage is about 60mV

Common-emitter current-voltage characteristics at low base cur-

Fig. 10 shows the common-emitter characteristics for an unpassivated device at a low base current injection (20pA per step). The transistor exhibits a low offset voltage of 60mV due to the homojunction structure in the device. The offset voltage is expressed as [l 13:

where Ac is the collector area, AE is the emitter area, and rE is the emitter resistance. Jcs and JEs are the magnitudes of the collector and emitter saturation cur- rents. aE is the common-base current gain. Since the base-emitter and base-collector junctions are homo- junctions, Jcs is approximately equal to JEP Jcs/adEs is then equal to the inverse of a,. For the studied device, Ac/A, = 5.1 contributes the offset voltage to 42mV. The second and third terms in AVcE lead to a 18mV higher than the geometric effect. Fig. 11 shows the offset voltage as a function of area ratio Ac/AE at a base current of 20p.A. The offset voltage can be further improved by reducing the area ratio. In general, the offset voltage increases with the base current. This is

We have proposed and demonstrated a new kind of GaAs homojunction bipolar transistor with a delta doping structure to suppress the minority carrier injec- tion into the emitter. An analytical expression for the current-voltage characteristics is derived, showing the contribution of the delta doping structure. Maximum height of the barrier is desirable to obtain high current gain. Experimental results show that a current gain of 13 is obtained with a base-to-emitter doping ratio of 10. Due to the absence of the conduction band discon- tinuity, the offset voltage of 60mV is achieved, which is mainly contributed by the geometric limits. The device exhibits improved characteristics of current gain with collector current. It is also shown that a simple chemi- cal treatment results in a slight improvement in the cur- rent gain of the device. The delta doping emitter device structure provides the prospect of a homojunction tran- sistor of high gain with a heavily doped base. With fur- ther optimisation of the delta doping structure for a large height and proper width of the barrier and with the best treatment to passivate the surface, the electri- cal characteristics can be further improved.

6 Acknowledgment

This work was supported in part by National Science Council of the Republic of China under contract NSC- 82-0404-E006-435. Helpful discussion and assistance of Prof. J.I. Ghyi are highly appreciated.

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41? IEE Proc,-Circuits Devzces Syst , Vol 142, No 6, December 1995