Current pulses and high-frequency oscillations in a-Si/Si(p)/Si(n) heterojunction device

Abstract

A three-terminal a-Si/Si(p)/Si(n) heterojunction device was found to generate pulses of the collector current with a rise time of approximately 10 −7 s and a fall time of approximately 4 × 10 −7 s. The voltage applied to the base (0.8-1.5 V) controlled the pulse repetition rate. When the voltage reaches ∼1.0 V, the pulses start to overlap, and the output signal is a high-frequency oscillation (10 6 -10 7 Hz) of the collector current.A simple mechanism explaining the pulse generation and the high-frequency oscillations has been proposed. It involves the extremely high recombination rate of the injected carriers in the thin a-Si layer of the a-Si/Si(p) interface.

Electrical properties of an a-Si/Si(p)/Si(n) heterojunction device

“…In regimes II and III the thickness of the a-Si region, where the electron-hole plasma is confined, is determined by the ambipolar diffusion length L a . This length can be estimated from the following formula [7][8][9]:…”

“…The voltage U a required to create electron-hole plasma in the a-Si region is given by the relation [8,9] U a = (3πkT /8q)e W/(2La) (7) where q is the electron charge, k is the Boltzmann constant, T is the absolute temperature and W is the width of the a-Si region. With L a ∼ 100 nm, we obtain from equation ( 7) a value for U a of about 1.6 V. This is consistent with the value U a = 1.5 V for which the characteristics enter regime II.…”

“…To explain the appearance of regime III we took into account both the modified thyristor model proposed in [1][2][3][4] and the formation of a narrow layer within the a-Si collector, adjacent to the interface J 2 exhibiting 'relaxation behaviour', very similar to that reported in [9]. Injection of carriers of one sign into a 'relaxation semiconductor' does not result in an increased concentration of carriers of the opposite sign, in contrast to a conventional 'lifetime semiconductor', but in their decrease.…”

On the influence of noise in the evaluation of the electrical oscillations occurring in the α-Si/Si(p)/Si(n) device

“…The a-Si/Si(p)/Si(n) device is promising as a suitable high frequency generator with quite strong signals needing no further amplification [1][2][3][4][5]. In particular, electrical oscillations occur in the negative differential resistance-region (NDR-region) of the current controlled I-U characteristic of this device [1,3].…”

“…The corresponding junction diagram is shown in Fig. 1b [1,2]. The voltage drop between the emitter E and the collector C and the corresponding current are denoted as U CE and I C , respectively.…”