Fundamental concept of DLTS

1.1.1 Capacitance transient

To fully understand DLTS, it is worth to have a basic knowledge of capacitance transients arising from the SCR of Schottky contacts or p+-n/n+-p asymmetric junctions. If a pulse voltage is applied to one of these device structures that is originally reverse-biased, the SCR width decreases and the trap centers are filled with carriers (majority or minority depending on the structure). When the junction is returned to reverse bias condition, the traps that remains occupied with carriers are emptied by thermal emission and results in a transient decay. The capacitance transients provide information about these defect centers. Here, we restrain our description to a p+-n junction where the p-side is more much heavily doped than the n-side, which gives the SCR almost in the low doped side.

Подпись: C(t) = A image520 Подпись: (1)

The causes of change in capacitance depend on the nature of applied voltage. In case of reverse biased voltage, the junction capacitance, due to the change in SCR width, is dominant. However, when the applied voltage is forward biased, the diffusion capacitance, due to the contribution of minority carrier density, is dominant. The basic equation governing the capacitance transient in the p+-n junction is expressed by

where A is the contact area, Vb is the built-in potential, єє0 is the permittivity of the semiconductor material, and e is the elementary charge of an electron. C0, Nt, Nd, and r

Подпись: AC = C0 Подпись: 1 _ N± ND Подпись: (2)

denote the junction capacitance at reverse bias, the density of filled traps under steady state conditions, the ionized donor concentration, and the time constant that gives the emission rate, respectively. The change in capacitance after the recharging of traps is given by

In most cases of using transient capacitance, the trap centers form only a small fraction of the SCR impurity density, i. e., Nt << Nd. Hence, using a first-order expansion of Eq. (2) gives

|AC| = C0I1 _ Nt/2Nd| = C0 Nt/2Nd (3)

Подпись: NT Подпись: AC 2 ND C Подпись: (4)

Thus, the trap concentration calculates from the capacitance change AC is expressed by

image527 Подпись: AC W2 2—N R C D L2 _ L2 '■—•n •*-'1 •*-'0 Подпись: (5)

Note that Eq. (3) assumes that NT << ND and the traps are filled throughout the total depletion width. To be more accurate, NT should be adjusted to NTadj according to [30]

where WR is the total SCR at reverse bias voltage Vr, L1 = WR – X, L2 = Wp – X, and

Подпись:Подпись:X = (^(Ef _ Et))1/2

Є Nd

where Wp, Ef, and Et denote the SCR at Vp, the Fermi level, and the trap energy level.

Updated: August 20, 2015 — 4:17 pm