8.3.2
with the effective electron mass . A detailed derivation can be found in [24, 31]. Also
transport of minority carriers occurs, but it is orders of magnitude smaller than the
majority carrier transport. Hence, it can be neglected for our application.
Ohmic contacts
If the resistance of the metal-semiconductor junction is negligible with respect to the bulk
resistance of the semiconductor, the junction is called ohmic. In such a junction, the
voltage drop is small and hardly affects the device performance.
As a figure of merit, the specific contact resistance R C can be used. It is defined as
If the semiconductor has a low doping concentration, R C is given by
Hence, for reducing the contact resistance, the barrier height should be kept low.
The situation looks different if a junction is formed between a metal and a highlydoped semiconductor. The width of the depletion region W decreases if doping is
increased, as we can see from Eq. (8.48). As a result, quantum-mechanic tunnelling of the
charge carriers through the barrier becomes more probable. In this case, tunnelling can be
the major current transport mechanism. As a consequence, R C is also determined by
tunnelling. We find [31]
Hence, if the doping concentration is increased, the contact resistance decreases.
In solar cells, the contact resistance at a metal-semiconductor junction should be as
small as possible. Reducing the barrier height is often not possible as the materials that
determine the barrier cannot be optimized for the barrier height as they fulfil other
purposes. However, close to the metal-semiconductor junction a highly-doped region can
be introduced that enables tunnelling and hence reduces the contact resistance as seen in
Eq. (8.56).
with the effective electron mass . A detailed derivation can be found in [24, 31]. Also
transport of minority carriers occurs, but it is orders of magnitude smaller than the
majority carrier transport. Hence, it can be neglected for our application.
Ohmic contacts
If the resistance of the metal-semiconductor junction is negligible with respect to the bulk
resistance of the semiconductor, the junction is called ohmic. In such a junction, the
voltage drop is small and hardly affects the device performance.
As a figure of merit, the specific contact resistance R C can be used. It is defined as
If the semiconductor has a low doping concentration, R C is given by
Hence, for reducing the contact resistance, the barrier height should be kept low.
The situation looks different if a junction is formed between a metal and a highlydoped semiconductor. The width of the depletion region W decreases if doping is
increased, as we can see from Eq. (8.48). As a result, quantum-mechanic tunnelling of the
charge carriers through the barrier becomes more probable. In this case, tunnelling can be
the major current transport mechanism. As a consequence, R C is also determined by
tunnelling. We find [31]
Hence, if the doping concentration is increased, the contact resistance decreases.
In solar cells, the contact resistance at a metal-semiconductor junction should be as
small as possible. Reducing the barrier height is often not possible as the materials that
determine the barrier cannot be optimized for the barrier height as they fulfil other
purposes. However, close to the metal-semiconductor junction a highly-doped region can
be introduced that enables tunnelling and hence reduces the contact resistance as seen in
Eq. (8.56).