Open Circuit Decay and CV
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Transcript of Open Circuit Decay and CV
Part 1) Determination of the minority carrier lifetime using open circuit voltage decay
Part2) Determination of doping profile in a semiconductor from C‐V characteristics
Part (1) Determination of the minority carrier lifetime using open circuit voltage decay:
Back ground information:
Both these experiments use a PN junction. For simplicity we consider a P+N junction. We consider the n – doped semiconductor with a shallow donor concentration given by Nd donors /cm
3. We focus on the N region. Electrons are the majority carriers and holes are the minority carriers in the N region. The equilibrium minority carrier concentration is given by p0 = (ni
2)/ Nd. Since we are dealing with a P+N one sided junction, we will ignore the
depletion region in the P+ region. Fig. 1(a) shows only the N region in the dark. The N region consists of a depletion region of width d0 and a neutral region of width W‐d0 where W is the thickness of the N region.
If light of energy > Eg is incident on the semiconductor, light is absorbed and increases the carrier concentration of both carriers ‐holes and electrons. We choose the intensity of the light is such that the hole concentration in the N region under illumination is given by pL so that Nd <<pL <<p0.
Under illumination the N type region spontaneously breaks up into two parts – Fig. 1(b). Minority carriers (holes) generated within a diffusion length (L) of the depletion region (shown as dL in fig 2) can diffuse to the junction and give rise to a current in the external circuit. (Current generation region in addition to the current generation region of the depletion region). Carriers generated in the region W‐L‐dL are lost by recombination (Recombination region).
Under illumination, the diode develops forward bias. At a given forward bias, carriers are lost due to extraction and to recombination. We can assume that the depletion region (dL) is negligibly small at Voc as the cell is under forward bias. At Voc carriers can be lost only by recombination. Hence the decay of the open
circuit voltage when the light is turned off is a measure of the minority carrier lifetime τ.The minority carrier lifetime is hence given by,
τ = (kT/q) ({1/ (dVoc/dt)}.
This can be easily seen from the expression for Voc in terms of the short circuit current Isc.
[ try and see this for yourself‐ Hint Voc = (kT/q)ln (Isc/I0)
Isc is proportional to PL and the recombination rate is proportional to (PL‐ p0)/ τ)]
Figure 1: N region (a) in dark (b) under illumination
Deviations from the simple theory above: Some possibilities
If the cell is internally shunted, then external open circuit condition is not true open circuit. (How will you check this?) If the lifetime depends on the injected carrier density, then it can change during the decay. (How will you check this?) Non uniform carrier generation – either due to the light source or non uniform material. (How will you check this?) Enjoy the experiment.
Part 2: Depth profiling the semiconductor to get the depth distribution of the shallow dopant using capacitance –voltage profiling. Background: As before for simplicity, we consider a P+N one sided junction. The depletion is primarily in the N layer. The X axis is along the length of the N doped semiconductor.
Figure 2: pn junction. Source: wikipedia In the depletion region, there are no free carriers (clearly this is not valid at the edge of the depletion region – transition from depletion to neutral region) There is a built in electric field (in the depletion region). The dopant distribution determines the dc electric field through the solution of the Poisson equation. For instance if the dopant is uniformally distributed, the
dc electric field is linear in space and the dc voltage consequently has a quadratic dependence. (see Fig.2). A different dopant distribution will give rise to a different dc spatial electric field and voltage dependence. For simplicity, we assume that the dopant distribution varies only along one direction say X and is constant in the YZ plane. We do not know the dopant distribution with depth in the N type material. The aim of this experiment is to determine the dopant distribution along X. The idea is to use capacitance vs voltage and then convert this information to dopant density vs depth. It is easy to see why the voltage can be used to selectively probe any given X in the sample. When the PN junction is reverse biased, the depletion region extends into the N region as the reverse bias voltage is increased. So X can be controlled by a dc voltage. How is Capacitance related to the dopant concentration? The capacitance C is the ac small signal capacitance and is given by C= dQ/dV. It is the change in charge due to a change in voltage. (C is NOT Q/V). This is very important. To begin with, let the dc bias across the diode be zero V. The dc electric field depends on the details of the dopant distribution vs. x. However, when a small ac field is applied across the sample, the depletion region does not contribute to dq as there is no mobile charge. The mobile charge exists only at the edge of the depletion region. The response to the ac field takes place only at the edge of the depletion region– independent of the details of the dopant distribution in the depletion region. It is a parallel plate capacitor. So the capacitance is simply given by
C = εε0A/xd ……………………………………………..1 where ε is the relative permittivity and ε0 the permittivity of vacuum. A is the device area and xd the depletion layer thickness. If we assume that dV is small, then the donor concentration is approximately constant for the small modulation of the depletion width.
dQ = qNd(xd ) Δ xd ………………………… …….2 dV = (q xd Nd(xd) Δ xd ) /(εε0)………………… …3 ΔC /ΔV = (ΔC/ Δ xd ) (Δ xd/ΔV)………… ………4 Using eqns 1‐4 we can write
Nd(Xd) = ‐ C3 ( 1/qεε0 A2) (ΔC /ΔV)‐1 ………….5
A measurement of C and ΔC /ΔV vs V will determine the doping profile. Eq. 5 can also be written as
{Nd(Xd)}‐1 = ‐1/2 (qεε0 A2) (d/dV (1/C2)
If Nd is constant
1/C2 = 2(Vbi ± V)/ (qεε0 A2 Nd) ......................6 A plot of 1/C2 vs V is a straight line for a constant doping density. From the slope one can get the doping density and the built in voltage from the intercept. Dopant profiling using cap‐voltage is a very powerful technique. Some sources of errors: The diodes should not be leaky. This becomes an issue as the reverse bias increases. Measuring the dc current under reverse bias provides an independent estimate of the leakage. Another way to quantitatively check whether leakage is important is to look at the phase shift of the current through the capacitor with respect to the applied ac voltage. (Think about this.)