Effects of Hydrogenation on the Off-State of Polysilicon Thin Film Transistors
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EFFECTS OF HYDROGENATION ON THE OFF-STATE OF POLYSILICON THIN FILM TRANSISTORS, Babar A. Khan and Ranjana Pandya, Philips Laboratories, North American Philips Corporation, 345 Scarborough Road, Briarcliff Manor, NY 10510. ABSTRACT: We have carried out transient photoconductivity experiments on hydrogenated and unhydrogenated polysilicon Thin Film Transistors (TFTs) which indicate that the primary effect of plasma hydrogenation is the reduction by several orders of magnitude of the band tail trapping states. A smaller effect is a reduction of the dangling bond density by a factor of 2 or 3 as indicated by Electron Spin Resonance (ESR) measurements. Since the dangling bond is associated with the generation/recombination center, it may not be obvious how such a small reduction in generation/recombination center density leads to over 2 orders of magnitude reduction in off-state current of the TFTs. In this paper, we describe how the large reduction of band tail states can lead to significant reduction of carrier generation near the contacts despite the fact that the generation/recombination center density is not changed significantly. This reduction of carrier generation leads to the observed reduction in off-state currents. Introduction:
Polysilcon MOSFETs are finding new applications in 3D ICs and electro-optical devices because of recent improvements in their performance characteristics [1,2]. We had previously reported that higher mobilities and low off-state currents have been achieved by treating the devices in a hydrogen plasma [2]. These mobilities (>45cm 2/V-sec for holes and >65cm2/V-sec for electons ) and off-state currents ( kT so that the fermi function can be approximated by a step function. The excess trapped charge (Pt) is therefore given by 2
Pt =
f Ne- /krodc
(2)
E,-+qo
where ' is the electrostatic potential. We assume that the trapped charge dominates the total charge density so that we can ignore the free carriers. Of course, this assumption is no longer valid when the fermi level gets close to the band edge. Therefore, we can write Poisson's equation, 2
d4_ -qp, 2 dz
E,
(3)
356
where E, is the band gap (mobility gap). We can therefore write, (4)
where 6= qNkT C'0
-E,2kT(
(5)
and a=---kT
0
When -qOkTo) , Eq(4) can be written as 2,0 - - -° e
d
dx
2
(9)
Eq(9) can be solved by integration in the usual way by first solving for the electric field and the solution is O= -!-laIn[
(V'2ax+D))]
-- C2(-
(10)
where C & D are constants. It is clear (see Fig.6) that even when -qO>>kTo at the p+ intrinsic silicon junction (x=O), there is a region further away from the junction where -qO
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