Theoretical and numerical investigations of carriers transport in N-semi-insulating-N and P-semi-insulating-P diodes - A new approach

摘要

A new and simple theoretical model is developed for ambipolar transport in compensated N-semi-insu-lating (Si)-N or P-SI-P diodes and is validated with exact J-V_a characteristics obtained through numerical modelisation. The electrical parameters used correspond to SI GaAs layers, but these results are valid for other compounds such as SI InP and InGaP. A relation between the bulk non-equilibrium excess carrier concentrations, valid for low and intermediate applied voltage, is first established. For a deep donor (N_t) compensating a residual shallow acceptor (N_A): (n-n_e) ≈ (τ_(nt))/(τ_(pt))((N_t-N_A)/(N_A))(p-p_e), where n_e and p_e are the thermal equilibrium free carrier densities in the SI layer, τ_(nt), τ_(pt) and n_(1t), p_(1t) are the familiar Shock-ley-Read-Hall (SRH) parameters of the deep trap. This relation represents an extension of the well known quasi space charge neutrality condition: (n - n_e) ≈ (p - p_e) valid for extrinsic semiconductors. We show then that a linear J-V_a relationship is observed in N-SI-N diodes when M_t(= (N_A)/(N_t-N_A)/(τ_(nt) p_(1t)/(τ_(pt) (n_(1t)) < 1 and in P-SI-P diodes when: M_t > 1. The quasi totality of the applied voltage V_a is lost across the SI layer and the electric field is constant (E≈ V_a/L_(S1)). M_t < 1 characterizes a SI(N~-) layer where a strong hole depletion (p≈0) across the SI bulk is associated to an "ohmic" electron current where n is constant but such that n < n_e. M_t > 1 characterizes a SI(P~-) layer where forn ≈ 0, p_e. On the other hand, the J-V_a characteristics of N-SI(P~-)-N diodes and P-S1(N~-)-P diodes show a saturation effect. Most of the applied voltage is now lost across the reverse biased contact and the electric field is low across the SI layer. For M_t values close to 1, we switch from a linear to a saturation regime. Equivalent relations are given for a deep acceptor compensating a shallow donor. We present results for short SI layers having lengths L_(S1) in the micrometer range and of the order or inferior to the ambipolar diffusion length L_(Da), such layer are used as insulating layers in buried heterostructures for diode laser technology, as well as for long SI layers, L_(SI) L_(Da), as used in radiation detectors technology.