Amplified current caused by barrier lowing of transistor

We can gain a basic understanding of the operation of the transistor and the relations between the various and voltages by considering simplified analysis. The minority carrier concentrations are shown in Fig.4.8 for an npn bipolar transistor biased on the forward active mode. The electrons diffuse across the base are swept into the collector by the electric field in the B-C space charge region. Assuming the ideal linear electron distribution in the base, the collector current can be written as a diffusion current given by The collector current controlled by the base-emitter voltage; that is, the current amplify from barrier lowing of a BJT was caused by applied bias.

FIG. 4.8 (a) Biasing and carrier distribution of an npn BJT in the forward-active mode. (b) The principle of operation of the phototransistor. The primary photocurrent acts as a base current and give a large photocurrent in the emitter-collector circuit.

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4.5.2 Phototransistor

The phototransistor [7] is a bipolar junction transistor (BJT) that operates as a photodetector with a photocurrent gain. The basic principle is illustrated in Fig.4.8.(b) In an ideal device, only the depletions, or the space charge layers (SCL),contain an electric field. The base is terminally open and there is a voltage applied between the collector and emitter terminals just as in the normal operation of a common emitter BJT. An incident photon is absorbed in the SCL between the base and collector to generate an electron hole pair (EHP). The field E in the SCL separates the electron hole pair and drifts them in opposite direction. This is the primary photocurrent.

When the drifting electron reaches the collector, it becomes collected (and thereby neutralized)by the battery. On the other hand, when the hole enters the neutral base region, it can only be neutralized by injecting a large number of electrons from the emitter into the base. These electrons diffuse across the base and reach the collector and thereby constitute an amplified phptocurrent.

Alternatively, one can argue that the photogeneration of EHPs in the collector SCL decrease the resistance of this region which decreases the voltage VBC across the base-collector junction. Consequently the base-emitter voltage VBE must increase inasmuch as VBE +VBC=VBC (Fig.4.8.(b)). This increase in VBE act as if it were a forward bias across the base-emitter junction and injects electrons into the base due to the transistor action, that is the emitter current

) / exp(eV kT

I_E ∝ _BE

The current amplify from barrier lowing was caused by the incident photon in the space charge region.

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4.5.3 Transistor-like mechanism

Based on the experimental evidence, we propose the following mechanism to explain why the detector efficiency could be more than 100% through this two-terminal detector with a nc-Si (or Ge )-embedded MS layer as the absorption medium. This is illustrated in Fig. 4.9.(a) Before illumination, the device operates like a normal MOS capacitor, i.e., a significant number of electrons are accumulated at the MS-Si interface forming a n-type inversion layer if a large enough reverse bias is applied. Only a few carriers could flow over the MS barrier layer, leading to a considerably low dark current. As seen in Fig. 4.9. (b), upon optical excitation, strong absorption occurs for incident photons with the energies matching the transition energies from the ground hole states (Eh0) to the ground electron states (Ee0) (the gap energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbital according to Ref 8, and the ground hole states (Eh0) to the excited states (Ee*) in connection to the Si–O interface states as well. With the applied electric field, the excited electrons in the Ee0 and Ee* states are driven towards the ITO contact layer via the tunneling process. The holes, however, will be trapped by the interface states [9] with the energy level somewhat above the ground hole states (Eh0). The immobile positive charge centers will subsequently lower the barrier height, causing the resonant injection of electrons from the inversion layer through the MS layer to the ITO contact. Therefore, the measured photocurrent is expected to be composed of two parts, i.e., the photoexcited electrons plus the injected electrons. The mechanism of current amplification observed in this work is thus similar to that of a conventional phototransistor.[10] In particular, the enhancement of photoresponsivity measured between 530 and 970 nm, can be explained by the two resonant states of the nc-Si/MS system[8,9,11].

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FIG. 4.9 Illustration of transistor like operation of an ITO/nc- Si-embedded MS/p-Si device under reveres bias. (a) A schematic band diagram before illumination and (b) under illumination. The symbols Ip and Iinv in the figure represent photoexcited and injected currents, respectively.

inversion layer

p‐Si

E _i

ITO V>0

I_inv

I

inversion layer

E

E

p-Si ITO

V>0

E

E

E

*

(b)

(a)

40

The situation is different when the gate bias is changed from positive to negative (FIG. 4.10). Although excited electrons in this case are expected to move to the side of the Si substrate, recombination with holes in the surface accumulation layer will significantly reduce the charge transport. At the same time, there will be little enhancement of hole transport from the MS layer to ITO, since the photogenerated holes are mostly trapped at the interface states. Therefore, the measured current would not differ too much from the case without photoexcitation.

FIG. 4.10 A schematic band diagram of an ITO/ nc-Si(Ge)-embedded MS/ p-Si device under forward bias.

accumulation layer

E

E

p-Si ITO

V<0

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4.6 References

1. A. T. Cho, J. M. Shieh, J. Shieh, Y. F. Lai, B. T. Dai, F. M. Pan, H. C. Ku, Y.C.Lin, K. J. Chao, and P. H. Liu, Electrochem. Solid-State Lett. 8, G143 (2005).

2. T. Z. Lu,a M. Alexe, R. Scholz, V. Talelaev, and M. Zacharias, APL .87, 202110 (2005)

3. T. A. Burr, A. A. Seraphin, E. Werwa, and K. D. Kolenbrander, Phys. Rev. B 56, 4818 (1997).

4. 16M. A. Rafiq, Y. Tsuchiya, H. Mizuta, S. Oda, Shigeyasu Uno, Z. A. K. Durrani, and W. I. Milne, Appl. Phys. Lett. 87, 182101 (2005).

5. H. YAMAGISHI, Y. SUZUKI, A. HIRAIDE. IEEE Transactions on Instrumentation and Measurement, VOL 38, NO 2, (1989)

6.C. K. Wang, T. K. Ko, C. S. Chang, S. J. Chang, Y. K. Su, T. C. Wen, C. H. Kuo, and Y. Z. Chiou, IEEE Photonics Technol. Lett. 17, 2161 (2005).

7. S.O.Kasap, Optoelectronics and photonics,p238

8. A. Puzder, A. J. Williamson, J. C. Grossman, and G. Galli, Phys. Rev. Lett. 88, 097401 (2002).

9. S. H. Choi and R. G. Elliman, Appl. Phys. Lett. 74, 3987 (1999).

10.A. Elfving, M. Larsson, G. V. Hansson, P.-O. Holtz, and W.-X. Ni, Mater. Res.

Soc. Symp. Proc. 770, I2.2 (2003).

11.M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and C. Delerue, Phys. Rev. Lett.

82, 197 (1999).

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